A History of Science — Volume 3
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Title: A History of Science — Volume 3
Author: Henry Smith Williams
Edward Huntington Williams
Release date: April 1, 1999 [eBook #1707]
Most recently updated: November 17, 2009
Language: English
Other information and formats: www.gutenberg.org/ebooks/1707
Credits: Produced by Charles Keller, and David Widger
*** START OF THE PROJECT GUTENBERG EBOOK A HISTORY OF SCIENCE — VOLUME 3 ***
Produced by Charles Keller
A HISTORY OF SCIENCE
MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES
By Henry Smith Williams, M.D., Ll.D.
Assisted By Edward H. Williams, M.D.
In Five Volumes
Volume III.
CONTENTS
BOOK III
CHAPTER I. THE SUCCESSORS OF NEWTON IN ASTRONOMY
The work of Johannes Hevelius--Halley and Hevelius--Halley's observation
of the transit of Mercury, and his method of determining the parallax of
the planets--Halley's observation of meteors--His inability to
explain these bodies--The important work of James Bradley--Lacaille's
measurement of the arc of the meridian--The determination of the
question as to the exact shape of the earth--D'Alembert and his
influence upon science--Delambre's History of Astronomy--The
astronomical work of Euler.
CHAPTER II. THE PROGRESS OF MODERN ASTRONOMY
The work of William Herschel--His discovery of Uranus--His discovery
that the stars are suns--His conception of the universe--His deduction
that gravitation has caused the grouping of the heavenly bodies--The
nebula, hypothesis,--Immanuel Kant's conception of the formation of the
world--Defects in Kant's conception--Laplace's final solution of the
problem--His explanation in detail--Change in the mental attitude of the
world since Bruno--Asteroids and satellites--Discoveries of Olbersl--The
mathematical calculations of Adams and Leverrier--The discovery of the
inner ring of Saturn--Clerk Maxwell's paper on the stability of Saturn's
rings--Helmholtz's conception of the action of tidal friction--Professor
G. H. Darwin's estimate of the consequences of tidal action--Comets
and meteors--Bredichin's cometary theory--The final solution of the
structure of comets--Newcomb's estimate of the amount of cometary dust
swept up daily by the earth--The fixed stars--John Herschel's studies
of double stars--Fraunhofer's perfection of the refracting
telescope--Bessel's measurement of the parallax of a star,--Henderson's
measurements--Kirchhoff and Bunsen's perfection of the
spectroscope--Wonderful revelations of the spectroscope--Lord Kelvin's
estimate of the time that will be required for the earth to become
completely cooled--Alvan Clark's discovery of the companion star of
Sirius--The advent of the photographic film in astronomy--Dr. Huggins's
studies of nebulae--Sir Norman Lockyer's "cosmogonic guess,"--Croll's
pre-nebular theory.
CHAPTER III. THE NEW SCIENCE OF PALEONTOLOGY
William Smith and fossil shells--His discovery that fossil rocks are
arranged in regular systems--Smith's inquiries taken up by Cuvier--His
Ossements Fossiles containing the first description of hairy
elephant--His contention that fossils represent extinct species
only--Dr. Buckland's studies of English fossil-beds--Charles Lyell
combats catastrophism,--Elaboration of his ideas with reference to
the rotation of species--The establishment of the doctrine of
uniformitarianism,--Darwin's Origin of Species--Fossil man--Dr.
Falconer's visit to the fossil-beds in the valley of the
Somme--Investigations of Prestwich and Sir John Evans--Discovery of the
Neanderthal skull,--Cuvier's rejection of human fossils--The finding
of prehistoric carving on ivory--The fossil-beds of America--Professor
Marsh's paper on the fossil horses in America--The Warren mastodon,--The
Java fossil, Pithecanthropus Erectus.
CHAPTER IV. THE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY
James Hutton and the study of the rocks--His theory of the earth--His
belief in volcanic cataclysms in raising and forming the continents--His
famous paper before the Royal Society of Edinburgh, 1781---His
conclusions that all strata of the earth have their origin at the bottom
of the sea---His deduction that heated and expanded matter caused the
elevation of land above the sea-level--Indifference at first shown this
remarkable paper--Neptunists versus Plutonists--Scrope's classical work
on volcanoes--Final acceptance of Hutton's explanation of the origin
of granites--Lyell and uniformitarianism--Observations on the gradual
elevation of the coast-lines of Sweden and Patagonia--Observations on
the enormous amount of land erosion constantly taking place,--Agassiz
and the glacial theory--Perraudin the chamois-hunter, and his
explanation of perched bowlders--De Charpentier's acceptance of
Perraudin's explanation--Agassiz's paper on his Alpine studies--His
conclusion that the Alps were once covered with an ice-sheet--Final
acceptance of the glacial theory--The geological ages--The work of
Murchison and Sedgwick--Formation of the American continents--Past,
present, and future.
CHAPTER V. THE NEW SCIENCE OF METEOROLOGY
Biot's investigations of meteors--The observations of Brandes and
Benzenberg on the velocity of falling stars--Professor Olmstead's
observations on the meteoric shower of 1833--Confirmation of Chladni's
hypothesis of 1794--The aurora borealis--Franklin's suggestion that
it is of electrical origin--Its close association with terrestrial
magnetism--Evaporation, cloud-formation, and dew--Dalton's demonstration
that water exists in the air as an independent gas--Hutton's theory of
rain--Luke Howard's paper on clouds--Observations on dew, by Professor
Wilson and Mr. Six--Dr. Wells's essay on dew--His observations
on several appearances connected with dew--Isotherms and ocean
currents--Humboldt and the-science of comparative climatology--His
studies of ocean currents--Maury's theory that gravity is the cause
of ocean currents--Dr. Croll on Climate and Time--Cyclones and
anti-cyclones,--Dove's studies in climatology--Professor Ferrel's
mathematical law of the deflection of winds--Tyndall's estimate of
the amount of heat given off by the liberation of a pound of
vapor--Meteorological observations and weather predictions.
CHAPTER VI. MODERN THEORIES OF HEAT AND LIGHT
Josiah Wedgwood and the clay pyrometer--Count Rumford and the vibratory
theory of heat--His experiments with boring cannon to determine the
nature of heat--Causing water to boil by the friction of the borer--His
final determination that heat is a form of motion--Thomas Young and the
wave theory of light--His paper on the theory of light and colors--His
exposition of the colors of thin plates--Of the colors of thick
plates, and of striated surfaces,--Arago and Fresnel champion the wave
theory--opposition to the theory by Biot--The French Academy's tacit
acceptance of the correctness of the theory by its admission of Fresnel
as a member.
CHAPTER VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM
Galvani and the beginning of modern electricity--The construction of
the voltaic pile--Nicholson's and Carlisle's discovery that the galvanic
current decomposes water--Decomposition of various substances by Sir
Humphry Davy--His construction of an arc-light--The deflection of the
magnetic needle by electricity demonstrated by Oersted--Effect of
this important discovery--Ampere creates the science of
electro-dynamics--Joseph Henry's studies of electromagnets--Michael
Faraday begins his studies of electromagnetic induction--His famous
paper before the Royal Society, in 1831, in which he demonstrates
electro-magnetic induction--His explanation of Arago's
rotating disk--The search for a satisfactory method of storing
electricity--Roentgen rays, or X-rays.
CHAPTER VIII. THE CONSERVATION OF ENERGY
Faraday narrowly misses the discovery of the doctrine of
conservation--Carnot's belief that a definite quantity of work can be
transformed into a definite quantity of heat--The work of James Prescott
Joule--Investigations begun by Dr. Mayer--Mayer's paper of 1842--His
statement of the law of the conservation of energy--Mayer and
Helmholtz--Joule's paper of 1843--Joule or Mayer--Lord Kelvin and the
dissipation of energy-The final unification.
CHAPTER IX. THE ETHER AND PONDERABLE MATTER
James Clerk-Maxwell's conception of ether--Thomas Young and
"Luminiferous ether,"--Young's and Fresnel's conception of transverse
luminiferous undulations--Faraday's experiments pointing to the
existence of ether--Professor Lodge's suggestion of two ethers--Lord
Kelvin's calculation of the probable density of ether--The vortex theory
of atoms--Helmholtz's calculations in vortex motions--Professor
Tait's apparatus for creating vortex rings in the air---The ultimate
constitution of matter as conceived by Boscovich--Davy's speculations
as to the changes that occur in the substance of matter at different
temperatures--Clausius's and Maxwell's investigations of the
kinetic theory of gases--Lord Kelvin's estimate of the size of the
molecule--Studies of the potential energy of molecules--Action of gases
at low temperatures.
APPENDIX
A HISTORY OF SCIENCE
BOOK III. MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES
With the present book we enter the field of the distinctively modern.
There is no precise date at which we take up each of the successive
stories, but the main sweep of development has to do in each case with
the nineteenth century. We shall see at once that this is a time both
of rapid progress and of great differentiation. We have heard almost
nothing hitherto of such sciences as paleontology, geology, and
meteorology, each of which now demands full attention. Meantime,
astronomy and what the workers of the elder day called natural
philosophy become wonderfully diversified and present numerous
phases that would have been startling enough to the star-gazers and
philosophers of the earlier epoch.
Thus, for example, in the field of astronomy, Herschel is able, thanks
to his perfected telescope, to discover a new planet and then to reach
out into the depths of space and gain such knowledge of stars and
nebulae as hitherto no one had more than dreamed of. Then, in rapid
sequence, a whole coterie of hitherto unsuspected minor planets is
discovered, stellar distances are measured, some members of the starry
galaxy are timed in their flight, the direction of movement of the solar
system itself is investigated, the spectroscope reveals the chemical
composition even of suns that are unthinkably distant, and a tangible
theory is grasped of the universal cycle which includes the birth and
death of worlds.
Similarly the new studies of the earth's surface reveal secrets of
planetary formation hitherto quite inscrutable. It becomes known that
the strata of the earth's surface have been forming throughout untold
ages, and that successive populations differing utterly from one another
have peopled the earth in different geological epochs. The entire point
of view of thoughtful men becomes changed in contemplating the history
of the world in which we live--albeit the newest thought harks back to
some extent to those days when the inspired thinkers of early Greece
dreamed out the wonderful theories with which our earlier chapters have
made our readers familiar.
In the region of natural philosophy progress is no less pronounced and
no less striking. It suffices here, however, by way of anticipation,
simply to name the greatest generalization of the century in physical
science--the doctrine of the conservation of energy.
I. THE SUCCESSORS OF NEWTON IN ASTRONOMY
HEVELIUS AND HALLEY
STRANGELY enough, the decade immediately following Newton was one of
comparative barrenness in scientific progress, the early years of the
eighteenth century not being as productive of great astronomers as the
later years of the seventeenth, or, for that matter, as the later years
of the eighteenth century itself. Several of the prominent astronomers
of the later seventeenth century lived on into the opening years of the
following century, however, and the younger generation soon developed
a coterie of astronomers, among whom Euler, Lagrange, Laplace, and
Herschel, as we shall see, were to accomplish great things in this field
before the century closed.
One of the great seventeenth-century astronomers, who died just before
the close of the century, was Johannes Hevelius (1611-1687), of Dantzig,
who advanced astronomy by his accurate description of the face and
the spots of the moon. But he is remembered also for having retarded
progress by his influence in refusing to use telescopic sights in his
observations, preferring until his death the plain sights long before
discarded by most other astronomers. The advantages of these telescope
sights have been discussed under the article treating of Robert Hooke,
but no such advantages were ever recognized by Hevelius. So great was
Hevelius's reputation as an astronomer that his refusal to recognize the
advantage of the telescope sights caused many astronomers to hesitate
before accepting them as superior to the plain; and even the famous
Halley, of whom we shall speak further in a moment, was sufficiently
in doubt over the matter to pay the aged astronomer a visit to test his
skill in using the old-style sights. Side by side, Hevelius and Halley
made their observations, Hevelius with his old instrument and Halley
with the new. The results showed slightly in the younger man's favor,
but not enough to make it an entirely convincing demonstration. The
explanation of this, however, did not lie in the lack of superiority
of the telescopic instrument, but rather in the marvellous skill of the
aged Hevelius, whose dexterity almost compensated for the defect of his
instrument. What he might have accomplished could he have been induced
to adopt the telescope can only be surmised.
Halley himself was by no means a tyro in matters astronomical at that
time. As the only son of a wealthy soap-boiler living near London, he
had been given a liberal education, and even before leaving college
made such novel scientific observations as that of the change in the
variation of the compass. At nineteen years of age he discovered a new
method of determining the elements of the planetary orbits which was a
distinct improvement over the old. The year following he sailed for the
Island of St, Helena to make observations of the heavens in the southern
hemisphere.
It was while in St. Helena that Halley made his famous observation
of the transit of Mercury over the sun's disk, this observation being
connected, indirectly at least, with his discovery of a method of
determining the parallax of the planets. By parallax is meant the
apparent change in the position of an object, due really to a change in
the position of the observer. Thus, if we imagine two astronomers making
observations of the sun from opposite sides of the earth at the same
time, it is obvious that to these observers the sun will appear to be
at two different points in the sky. Half the angle measuring this
difference would be known as the sun's parallax. This would depend,
then, upon the distance of the earth from the sun and the length of
the earth's radius. Since the actual length of this radius has been
determined, the parallax of any heavenly body enables the astronomer to
determine its exact distance.
The parallaxes can be determined equally well, however, if two observers
are separated by exactly known distances, several hundreds or thousands
of miles apart. In the case of a transit of Venus across the sun's
disk, for example, an observer at New York notes the image of the planet
moving across the sun's disk, and notes also the exact time of this
observation. In the same manner an observer at London makes similar
observations. Knowing the distance between New York and London, and
the different time of the passage, it is thus possible to calculate the
difference of the parallaxes of the sun and a planet crossing its disk.
The idea of thus determining the parallax of the planets originated, or
at least was developed, by Halley, and from this phenomenon he thought
it possible to conclude the dimensions of all the planetary orbits. As
we shall see further on, his views were found to be correct by later
astronomers.
In 1721 Halley succeeded Flamsteed as astronomer royal at the Greenwich
Observatory. Although sixty-four years of age at that time his activity
in astronomy continued unabated for another score of years. At Greenwich
he undertook some tedious observations of the moon, and during those
observations was first to detect the acceleration of mean motion. He
was unable to explain this, however, and it remained for Laplace in the
closing years of the century to do so, as we shall see later.
Halley's book, the Synopsis Astronomiae Cometicae, is one of the most
valuable additions to astronomical literature since the time of Kepler.
He was first to attempt the calculation of the orbit of a comet, having
revived the ancient opinion that comets belong to the solar system,
moving in eccentric orbits round the sun, and his calculation of the
orbit of the comet of 1682 led him to predict correctly the return of
that comet in 1758. Halley's Study of Meteors.
Like other astronomers of his time he was greatly puzzled over the
well-known phenomena of shooting-stars, or meteors, making many
observations himself, and examining carefully the observations of other
astronomers. In 1714 he gave his views as to the origin and composition
of these mysterious visitors in the earth's atmosphere. As this
subject will be again referred to in a later chapter, Halley's views,
representing the most advanced views of his age, are of interest.
"The theory of the air seemeth at present," he says, "to be perfectly
well understood, and the differing densities thereof at all altitudes;
for supposing the same air to occupy spaces reciprocally proportional to
the quantity of the superior or incumbent air, I have elsewhere proved
that at forty miles high the air is rarer than at the surface of
the earth at three thousand times; and that the utmost height of the
atmosphere, which reflects light in the Crepusculum, is not fully
forty-five miles, notwithstanding which 'tis still manifest that some
sort of vapors, and those in no small quantity, arise nearly to that
height. An instance of this may be given in the great light the society
had an account of (vide Transact. Sep., 1676) from Dr. Wallis, which was
seen in very distant counties almost over all the south part of England.
Of which though the doctor could not get so particular a relation as was
requisite to determine the height thereof, yet from the distant places
it was seen in, it could not but be very many miles high.
"So likewise that meteor which was seen in 1708, on the 31st of July,
between nine and ten o'clock at night, was evidently between forty and
fifty miles perpendicularly high, and as near as I can gather, over
Shereness and the buoy on the Nore. For it was seen at London moving
horizontally from east by north to east by south at least fifty degrees
high, and at Redgrove, in Suffolk, on the Yarmouth road, about twenty
miles from the east coast of England, and at least forty miles to the
eastward of London, it appeared a little to the westward of the south,
suppose south by west, and was seen about thirty degrees high, sliding
obliquely downward. I was shown in both places the situation thereof,
which was as described, but could wish some person skilled in
astronomical matters bad seen it, that we might pronounce concerning its
height with more certainty. Yet, as it is, we may securely conclude
that it was not many more miles westerly than Redgrove, which, as I
said before, is about forty miles more easterly than London. Suppose it,
therefore, where perpendicular, to have been thirty-five miles east
from London, and by the altitude it appeared at in London--viz., fifty
degrees, its tangent will be forty-two miles, for the height of the
meteor above the surface of the earth; which also is rather of the
least, because the altitude of the place shown me is rather more than
less than fifty degrees; and the like may be concluded from the altitude
it appeared in at Redgrove, near seventy miles distant. Though at this
very great distance, it appeared to move with an incredible velocity,
darting, in a very few seconds of time, for about twelve degrees of
a great circle from north to south, being very bright at its first
appearance; and it died away at the east of its course, leaving for some
time a pale whiteness in the place, with some remains of it in the track
where it had gone; but no hissing sound as it passed, or bounce of an
explosion were heard.
"It may deserve the honorable society's thoughts, how so great a
quantity of vapor should be raised to the top of the atmosphere, and
there collected, so as upon its ascension or otherwise illumination, to
give a light to a circle of above one hundred miles diameter, not much
inferior to the light of the moon; so as one might see to take a pin
from the ground in the otherwise dark night. 'Tis hard to conceive what
sort of exhalations should rise from the earth, either by the action
of the sun or subterranean heat, so as to surmount the extreme cold
and rareness of the air in those upper regions: but the fact is
indisputable, and therefore requires a solution."
From this much of the paper it appears that there was a general belief
that this burning mass was heated vapor thrown off from the earth in
some mysterious manner, yet this is unsatisfactory to Halley, for after
citing various other meteors that have appeared within his knowledge, he
goes on to say:
"What sort of substance it must be, that could be so impelled and
ignited at the same time; there being no Vulcano or other Spiraculum of
subterraneous fire in the northeast parts of the world, that we ever yet
heard of, from whence it might be projected.
"I have much considered this appearance, and think it one of the hardest
things to account for that I have yet met with in the phenomena of
meteors, and I am induced to think that it must be some collection of
matter formed in the aether, as it were, by some fortuitous concourse
of atoms, and that the earth met with it as it passed along in its orb,
then but newly formed, and before it had conceived any great impetus of
descent towards the sun. For the direction of it was exactly opposite to
that of the earth, which made an angle with the meridian at that time
of sixty-seven gr., that is, its course was from west southwest to east
northeast, wherefore the meteor seemed to move the contrary way. And
besides falling into the power of the earth's gravity, and losing its
motion from the opposition of the medium, it seems that it descended
towards the earth, and was extinguished in the Tyrrhene Sea, to the
west southwest of Leghorn. The great blow being heard upon its first
immersion into the water, and the rattling like the driving of a cart
over stones being what succeeded upon its quenching; something like this
is always heard upon quenching a very hot iron in water. These facts
being past dispute, I would be glad to have the opinion of the learned
thereon, and what objection can be reasonably made against the above
hypothesis, which I humbly submit to their censure."(1)
These few paragraphs, coming as they do from a leading
eighteenth-century astronomer, convey more clearly than any comment the
actual state of the meteorological learning at that time. That this ball
of fire, rushing "at a greater velocity than the swiftest cannon-ball,"
was simply a mass of heated rock passing through our atmosphere, did not
occur to him, or at least was not credited. Nor is this surprising when
we reflect that at that time universal gravitation had been but recently
discovered; heat had not as yet been recognized as simply a form of
motion; and thunder and lightning were unexplained mysteries, not to
be explained for another three-quarters of a century. In the chapter on
meteorology we shall see how the solution of this mystery that puzzled
Halley and his associates all their lives was finally attained.
BRADLEY AND THE ABERRATION OF LIGHT
Halley was succeeded as astronomer royal by a man whose useful additions
to the science were not to be recognized or appreciated fully until
brought to light by the Prussian astronomer Bessel early in the
nineteenth century. This was Dr. James Bradley, an ecclesiastic, who
ranks as one of the most eminent astronomers of the eighteenth century.
His most remarkable discovery was the explanation of a peculiar motion
of the pole-star, first observed, but not explained, by Picard a
century before. For many years a satisfactory explanation was sought
unsuccessfully by Bradley and his fellow-astronomers, but at last he was
able to demonstrate that the stary Draconis, on which he was making his
observations, described, or appeared to describe, a small ellipse.
If this observation was correct, it afforded a means of computing the
aberration of any star at all times. The explanation of the physical
cause of this aberration, as Bradley thought, and afterwards
demonstrated, was the result of the combination of the motion of light
with the annual motion of the earth. Bradley first formulated this
theory in 1728, but it was not until 1748--twenty years of continuous
struggle and observation by him--that he was prepared to communicate the
results of his efforts to the Royal Society. This remarkable paper is
thought by the Frenchman, Delambre, to entitle its author to a place in
science beside such astronomers as Hipparcbus and Kepler.
Bradley's studies led him to discover also the libratory motion of the
earth's axis. "As this appearance of Draconis indicated a diminution
of the inclination of the earth's axis to the plane of the ecliptic,"
he says; "and as several astronomers have supposed THAT inclination to
diminish regularly; if this phenomenon depended upon such a cause, and
amounted to 18" in nine years, the obliquity of the ecliptic would, at
that rate, alter a whole minute in thirty years; which is much
faster than any observations, before made, would allow. I had reason,
therefore, to think that some part of this motion at the least, if not
the whole, was owing to the moon's action upon the equatorial parts of
the earth; which, I conceived, might cause a libratory motion of
the earth's axis. But as I was unable to judge, from only nine years
observations, whether the axis would entirely recover the same position
that it had in the year 1727, I found it necessary to continue my
observations through a whole period of the moon's nodes; at the end of
which I had the satisfaction to see, that the stars, returned into the
same position again; as if there had been no alteration at all in the
inclination of the earth's axis; which fully convinced me that I had
guessed rightly as to the cause of the phenomena. This circumstance
proves likewise, that if there be a gradual diminution of the obliquity
of the ecliptic, it does not arise only from an alteration in the
position of the earth's axis, but rather from some change in the plane
of the ecliptic itself; because the stars, at the end of the period
of the moon's nodes, appeared in the same places, with respect to the
equator, as they ought to have done, if the earth's axis had retained
the same inclination to an invariable plane."(2)
FRENCH ASTRONOMERS
Meanwhile, astronomers across the channel were by no means idle. In
France several successful observers were making many additions to the
already long list of observations of the first astronomer of the Royal
Observatory of Paris, Dominic Cassini (1625-1712), whose reputation
among his contemporaries was much greater than among succeeding
generations of astronomers. Perhaps the most deserving of these
successors was Nicolas Louis de Lacaille (1713-1762), a theologian who
had been educated at the expense of the Duke of Bourbon, and who, soon
after completing his clerical studies, came under the patronage of
Cassini, whose attention had been called to the young man's interest in
the sciences. One of Lacaille's first under-takings was the remeasuring
of the French are of the meridian, which had been incorrectly measured
by his patron in 1684. This was begun in 1739, and occupied him for
two years before successfully completed. As a reward, however, he was
admitted to the academy and appointed mathematical professor in Mazarin
College.
In 1751 he went to the Cape of Good Hope for the purpose of determining
the sun's parallax by observations of the parallaxes of Mars and Venus,
and incidentally to make observations on the other southern hemisphere
stars. The results of this undertaking were most successful, and were
given in his Coelum australe stelligerum, etc., published in 1763. In
this he shows that in the course of a single year he had observed some
ten thousand stars, and computed the places of one thousand nine hundred
and forty-two of them, measured a degree of the meridian, and made
many observations of the moon--productive industry seldom equalled in
a single year in any field. These observations were of great service to
the astronomers, as they afforded the opportunity of comparing the stars
of the southern hemisphere with those of the northern, which were being
observed simultaneously by Lelande at Berlin.
Lacaille's observations followed closely upon the determination of an
absorbing question which occupied the attention of the astronomers in
the early part of the century. This question was as to the shape of the
earth--whether it was actually flattened at the poles. To settle this
question once for all the Academy of Sciences decided to make the
actual measurement of the length of two degrees, one as near the pole
as possible, the other at the equator. Accordingly, three astronomers,
Godin, Bouguer, and La Condamine, made the journey to a spot on the
equator in Peru, while four astronomers, Camus, Clairaut, Maupertuis,
and Lemonnier, made a voyage to a place selected in Lapland. The result
of these expeditions was the determination that the globe is oblately
spheroidal.
A great contemporary and fellow-countryman of Lacaille was Jean Le Rond
d'Alembert (1717-1783), who, although not primarily an astronomer, did
so much with his mathematical calculations to aid that science that
his name is closely connected with its progress during the eighteenth
century. D'Alembert, who became one of the best-known men of science of
his day, and whose services were eagerly sought by the rulers of Europe,
began life as a foundling, having been exposed in one of the markets of
Paris. The sickly infant was adopted and cared for in the family of a
poor glazier, and treated as a member of the family. In later years,
however, after the foundling had become famous throughout Europe, his
mother, Madame Tencin, sent for him, and acknowledged her relationship.
It is more than likely that the great philosopher believed her story,
but if so he did not allow her the satisfaction of knowing his belief,
declaring always that Madame Tencin could "not be nearer than a
step-mother to him, since his mother was the wife of the glazier."
D'Alembert did much for the cause of science by his example as well
as by his discoveries. By living a plain but honest life, declining
magnificent offers of positions from royal patrons, at the same time
refusing to grovel before nobility, he set a worthy example to other
philosophers whose cringing and pusillanimous attitude towards persons
of wealth or position had hitherto earned them the contempt of the upper
classes.
His direct additions to astronomy are several, among others the
determination of the mutation of the axis of the earth. He also
determined the ratio of the attractive forces of the sun and moon,
which he found to be about as seven to three. From this he reached the
conclusion that the earth must be seventy times greater than the moon.
The first two volumes of his Researches on the Systems of the World,
published in 1754, are largely devoted to mathematical and astronomical
problems, many of them of little importance now, but of great interest
to astronomers at that time.
Another great contemporary of D'Alembert, whose name is closely
associated and frequently confounded with his, was Jean Baptiste Joseph
Delambre (1749-1822). More fortunate in birth as also in his educational
advantages, Delambre as a youth began his studies under the celebrated
poet Delille. Later he was obliged to struggle against poverty,
supporting himself for a time by making translations from Latin, Greek,
Italian, and English, and acting as tutor in private families. The
turning-point of his fortune came when the attention of Lalande was
called to the young man by his remarkable memory, and Lalande soon
showed his admiration by giving Delambre certain difficult astronomical
problems to solve. By performing these tasks successfully his future as
an astronomer became assured. At that time the planet Uranus had just
been discovered by Herschel, and the Academy of Sciences offered as the
subject for one of its prizes the determination of the planet's orbit.
Delambre made this determination and won the prize--a feat that brought
him at once into prominence.
By his writings he probably did as much towards perfecting modern
astronomy as any one man. His History of Astronomy is not merely a
narrative of progress of astronomy but a complete abstract of all the
celebrated works written on the subject. Thus he became famous as an
historian as well as an astronomer.
LEONARD EULER
Still another contemporary of D'Alembert and Delambre, and somewhat
older than either of them, was Leonard Euler (1707-1783), of Basel,
whose fame as a philosopher equals that of either of the great
Frenchmen. He is of particular interest here in his capacity of
astronomer, but astronomy was only one of the many fields of science in
which he shone. Surely something out of the ordinary was to be expected
of the man who could "repeat the AEneid of Virgil from the beginning
to the end without hesitation, and indicate the first and last line of
every page of the edition which he used." Something was expected, and he
fulfilled these expectations.
In early life he devoted himself to the study of theology and the
Oriental languages, at the request of his father, but his love of
mathematics proved too strong, and, with his father's consent, he
finally gave up his classical studies and turned to his favorite study,
geometry. In 1727 he was invited by Catharine I. to reside in St.
Petersburg, and on accepting this invitation he was made an associate
of the Academy of Sciences. A little later he was made professor of
physics, and in 1733 professor of mathematics. In 1735 he solved a
problem in three days which some of the eminent mathematicians would not
undertake under several months. In 1741 Frederick the Great invited him
to Berlin, where he soon became a member of the Academy of Sciences and
professor of mathematics; but in 1766 he returned to St. Petersburg.
Towards the close of his life he became virtually blind, being obliged
to dictate his thoughts, sometimes to persons entirely ignorant of the
subject in hand. Nevertheless, his remarkable memory, still further
heightened by his blindness, enabled him to carry out the elaborate
computations frequently involved.
Euler's first memoir, transmitted to the Academy of Sciences of Paris
in 1747, was on the planetary perturbations. This memoir carried off the
prize that had been offered for the analytical theory of the motions of
Jupiter and Saturn. Other memoirs followed, one in 1749 and another in
1750, with further expansions of the same subject. As some slight
errors were found in these, such as a mistake in some of the formulae
expressing the secular and periodic inequalities, the academy proposed
the same subject for the prize of 1752. Euler again competed, and won
this prize also. The contents of this memoir laid the foundation for
the subsequent demonstration of the permanent stability of the planetary
system by Laplace and Lagrange.
It was Euler also who demonstrated that within certain fixed limits
the eccentricities and places of the aphelia of Saturn and Jupiter are
subject to constant variation, and he calculated that after a lapse
of about thirty thousand years the elements of the orbits of these two
planets recover their original values.
II. THE PROGRESS OF MODERN ASTRONOMY
A NEW epoch in astronomy begins with the work of William Herschel, the
Hanoverian, whom England made hers by adoption. He was a man with a
positive genius for sidereal discovery. At first a mere amateur in
astronomy, he snatched time from his duties as music-teacher to grind
him a telescopic mirror, and began gazing at the stars. Not content with
his first telescope, he made another and another, and he had such genius
for the work that he soon possessed a better instrument than was ever
made before. His patience in grinding the curved reflective surface was
monumental. Sometimes for sixteen hours together he must walk steadily
about the mirror, polishing it, without once removing his hands.
Meantime his sister, always his chief lieutenant, cheered him with her
presence, and from time to time put food into his mouth. The telescope
completed, the astronomer turned night into day, and from sunset to
sunrise, year in and year out, swept the heavens unceasingly, unless
prevented by clouds or the brightness of the moon. His sister sat always
at his side, recording his observations. They were in the open air,
perched high at the mouth of the reflector, and sometimes it was so cold
that the ink froze in the bottle in Caroline Herschel's hand; but the
two enthusiasts hardly noticed a thing so common-place as terrestrial
weather. They were living in distant worlds.
The results? What could they be? Such enthusiasm would move mountains.
But, after all, the moving of mountains seems a liliputian task compared
with what Herschel really did with those wonderful telescopes. He moved
worlds, stars, a universe--even, if you please, a galaxy of universes;
at least he proved that they move, which seems scarcely less wonderful;
and he expanded the cosmos, as man conceives it, to thousands of times
the dimensions it had before. As a mere beginning, he doubled the
diameter of the solar system by observing the great outlying planet
which we now call Uranus, but which he christened Georgium Sidus,
in honor of his sovereign, and which his French contemporaries, not
relishing that name, preferred to call Herschel.
This discovery was but a trifle compared with what Herschel did later
on, but it gave him world-wide reputation none the less. Comets and
moons aside, this was the first addition to the solar system that had
been made within historic times, and it created a veritable furor of
popular interest and enthusiasm. Incidentally King George was flattered
at having a world named after him, and he smiled on the astronomer, and
came with his court to have a look at his namesake. The inspection
was highly satisfactory; and presently the royal favor enabled the
astronomer to escape the thraldom of teaching music and to devote his
entire time to the more congenial task of star-gazing.
Thus relieved from the burden of mundane embarrassments, he turned with
fresh enthusiasm to the skies, and his discoveries followed one another
in bewildering profusion. He found various hitherto unseen moons of our
sister planets; he made special studies of Saturn, and proved that this
planet, with its rings, revolves on its axis; he scanned the spots on
the sun, and suggested that they influence the weather of our earth; in
short, he extended the entire field of solar astronomy. But very soon
this field became too small for him, and his most important researches
carried him out into the regions of space compared with which the span
of our solar system is a mere point. With his perfected telescopes he
entered abysmal vistas which no human eve ever penetrated before, which
no human mind had hitherto more than vaguely imagined. He tells us that
his forty-foot reflector will bring him light from a distance of "at
least eleven and three-fourths millions of millions of millions of
miles"--light which left its source two million years ago. The smallest
stars visible to the unaided eye are those of the sixth magnitude; this
telescope, he thinks, has power to reveal stars of the 1342d magnitude.
But what did Herschel learn regarding these awful depths of space and
the stars that people them? That was what the world wished to know.
Copernicus, Galileo, Kepler, had given us a solar system, but the stars
had been a mystery. What says the great reflector--are the stars points
of light, as the ancients taught, and as more than one philosopher of
the eighteenth century has still contended, or are they suns, as others
hold? Herschel answers, they are suns, each and every one of all the
millions--suns, many of them, larger than the one that is the centre of
our tiny system. Not only so, but they are moving suns. Instead of
being fixed in space, as has been thought, they are whirling in gigantic
orbits about some common centre. Is our sun that centre? Far from it.
Our sun is only a star like all the rest, circling on with its attendant
satellites--our giant sun a star, no different from myriad other stars,
not even so large as some; a mere insignificant spark of matter in an
infinite shower of sparks.
Nor is this all. Looking beyond the few thousand stars that are visible
to the naked eye, Herschel sees series after series of more distant
stars, marshalled in galaxies of millions; but at last he reaches a
distance beyond which the galaxies no longer increase. And yet--so he
thinks--he has not reached the limits of his vision. What then? He has
come to the bounds of the sidereal system--seen to the confines of the
universe. He believes that he can outline this system, this universe,
and prove that it has the shape of an irregular globe, oblately
flattened to almost disklike proportions, and divided at one edge--a
bifurcation that is revealed even to the naked eye in the forking of the
Milky Way.
This, then, is our universe as Herschel conceives it--a vast galaxy
of suns, held to one centre, revolving, poised in space. But even
here those marvellous telescopes do not pause. Far, far out beyond the
confines of our universe, so far that the awful span of our own system
might serve as a unit of measure, are revealed other systems, other
universes, like our own, each composed, as he thinks, of myriads of
suns, clustered like our galaxy into an isolated system--mere islands of
matter in an infinite ocean of space. So distant from our universe are
these now universes of Herschel's discovery that their light reaches
us only as a dim, nebulous glow, in most cases invisible to the unaided
eye. About a hundred of these nebulae were known when Herschel began
his studies. Before the close of the century he had discovered about
two thousand more of them, and many of these had been resolved by his
largest telescopes into clusters of stars. He believed that the farthest
of these nebulae that he could see was at least three hundred thousand
times as distant from us as the nearest fixed star. Yet that nearest
star--so more recent studies prove--is so remote that its light,
travelling one hundred and eighty thousand miles a second, requires
three and one-half years to reach our planet.
As if to give the finishing touches to this novel scheme of cosmology,
Herschel, though in the main very little given to unsustained
theorizing, allows himself the privilege of one belief that he cannot
call upon his telescope to substantiate. He thinks that all the myriad
suns of his numberless systems are instinct with life in the human
sense. Giordano Bruno and a long line of his followers had held that
some of our sister planets may be inhabited, but Herschel extends
the thought to include the moon, the sun, the stars--all the heavenly
bodies. He believes that he can demonstrate the habitability of our own
sun, and, reasoning from analogy, he is firmly convinced that all the
suns of all the systems are "well supplied with inhabitants." In this,
as in some other inferences, Herschel is misled by the faulty physics
of his time. Future generations, working with perfected instruments, may
not sustain him all along the line of his observations, even, let alone
his inferences. But how one's egotism shrivels and shrinks as one grasps
the import of his sweeping thoughts!
Continuing his observations of the innumerable nebulae, Herschel is led
presently to another curious speculative inference. He notes that some
star groups are much more thickly clustered than others, and he is
led to infer that such varied clustering tells of varying ages of the
different nebulae. He thinks that at first all space may have been
evenly sprinkled with the stars and that the grouping has resulted from
the action of gravitation.
"That the Milky Way is a most extensive stratum of stars of various
sizes admits no longer of lasting doubt," he declares, "and that our sun
is actually one of the heavenly bodies belonging to it is as evident. I
have now viewed and gauged this shining zone in almost every direction
and find it composed of stars whose number... constantly increases and
decreases in proportion to its apparent brightness to the naked eye.
"Let us suppose numberless stars of various sizes, scattered over an
indefinite portion of space in such a manner as to be almost equally
distributed throughout the whole. The laws of attraction which no doubt
extend to the remotest regions of the fixed stars will operate in such a
manner as most probably to produce the following effects:
"In the first case, since we have supposed the stars to be of various
sizes, it will happen that a star, being considerably larger than its
neighboring ones, will attract them more than they will be attracted by
others that are immediately around them; by which means they will be,
in time, as it were, condensed about a centre, or, in other words, form
themselves into a cluster of stars of almost a globular figure, more
or less regular according to the size and distance of the surrounding
stars....
"The next case, which will also happen almost as frequently as the
former, is where a few stars, though not superior in size to the rest,
may chance to be rather nearer one another than the surrounding ones,...
and this construction admits of the utmost variety of shapes....
"From the composition and repeated conjunction of both the foregoing
formations, a third may be derived when many large stars, or combined
small ones, are spread in long, extended, regular, or crooked rows,
streaks, or branches; for they will also draw the surrounding stars, so
as to produce figures of condensed stars curiously similar to the former
which gave rise to these condensations.
"We may likewise admit still more extensive combinations; when, at the
same time that a cluster of stars is forming at the one part of
space, there may be another collection in a different but perhaps not
far-distant quarter, which may occasion a mutual approach towards their
own centre of gravity.
"In the last place, as a natural conclusion of the former cases, there
will be formed great cavities or vacancies by the retreating of the
stars towards the various centres which attract them."(1)
Looking forward, it appears that the time must come when all the suns
of a system will be drawn together and destroyed by impact at a common
centre. Already, it seems to Herschel, the thickest clusters have
"outlived their usefulness" and are verging towards their doom.
But again, other nebulae present an appearance suggestive of an opposite
condition. They are not resolvable into stars, but present an almost
uniform appearance throughout, and are hence believed to be composed of
a shining fluid, which in some instances is seen to be condensed at the
centre into a glowing mass. In such a nebula Herschel thinks he sees a
sun in process of formation.
THE NEBULAR HYPOTHESIS OF KANT
Taken together, these two conceptions outline a majestic cycle of world
formation and world destruction--a broad scheme of cosmogony, such as
had been vaguely adumbrated two centuries before by Kepler and in
more recent times by Wright and Swedenborg. This so-called "nebular
hypothesis" assumes that in the beginning all space was uniformly filled
with cosmic matter in a state of nebular or "fire-mist" diffusion,
"formless and void." It pictures the condensation--coagulation, if
you will--of portions of this mass to form segregated masses, and the
ultimate development out of these masses of the sidereal bodies that we
see.
Perhaps the first elaborate exposition of this idea was that given by
the great German philosopher Immanuel Kant (born at Konigsberg in 1724,
died in 1804), known to every one as the author of the Critique of Pure
Reason. Let us learn from his own words how the imaginative philosopher
conceived the world to have come into existence.
"I assume," says Kant, "that all the material of which the globes
belonging to our solar system--all the planets and comets--consist, at
the beginning of all things was decomposed into its primary elements,
and filled the whole space of the universe in which the bodies formed
out of it now revolve. This state of nature, when viewed in and by
itself without any reference to a system, seems to be the very simplest
that can follow upon nothing. At that time nothing has yet been formed.
The construction of heavenly bodies at a distance from one another,
their distances regulated by their attraction, their form arising out of
the equilibrium of their collected matter, exhibit a later state.... In
a region of space filled in this manner, a universal repose could last
only a moment. The elements have essential forces with which to put
each other in motion, and thus are themselves a source of life. Matter
immediately begins to strive to fashion itself. The scattered elements
of a denser kind, by means of their attraction, gather from a sphere
around them all the matter of less specific gravity; again, these
elements themselves, together with the material which they have united
with them, collect in those points where the particles of a still denser
kind are found; these in like manner join still denser particles, and
so on. If we follow in imagination this process by which nature fashions
itself into form through the whole extent of chaos, we easily perceive
that all the results of the process would consist in the formation of
divers masses which, when their formation was complete, would by the
equality of their attraction be at rest and be forever unmoved.
"But nature has other forces in store which are specially exerted when
matter is decomposed into fine particles. They are those forces by which
these particles repel one another, and which, by their conflict with
attractions, bring forth that movement which is, as it were, the lasting
life of nature. This force of repulsion is manifested in the elasticity
of vapors, the effluences of strong-smelling bodies, and the diffusion
of all spirituous matters. This force is an uncontestable phenomenon of
matter. It is by it that the elements, which may be falling to the point
attracting them, are turned sideways promiscuously from their movement
in a straight line; and their perpendicular fall thereby issues in
circular movements, which encompass the centre towards which they were
falling. In order to make the formation of the world more distinctly
conceivable, we will limit our view by withdrawing it from the infinite
universe of nature and directing it to a particular system, as the
one which belongs to our sun. Having considered the generation of this
system, we shall be able to advance to a similar consideration of the
origin of the great world-systems, and thus to embrace the infinitude of
the whole creation in one conception.
"From what has been said, it will appear that if a point is situated in
a very large space where the attraction of the elements there situated
acts more strongly than elsewhere, then the matter of the elementary
particles scattered throughout the whole region will fall to that point.
The first effect of this general fall is the formation of a body at this
centre of attraction, which, so to speak, grows from an infinitely
small nucleus by rapid strides; and in the proportion in which this mass
increases, it also draws with greater force the surrounding particles
to unite with it. When the mass of this central body has grown so great
that the velocity with which it draws the particles to itself with great
distances is bent sideways by the feeble degree of repulsion with which
they impede one another, and when it issues in lateral movements which
are capable by means of the centrifugal force of encompassing the
central body in an orbit, then there are produced whirls or vortices
of particles, each of which by itself describes a curved line by the
composition of the attracting force and the force of revolution that had
been bent sideways. These kinds of orbits all intersect one another,
for which their great dispersion in this space gives place. Yet these
movements are in many ways in conflict with one another, and they
naturally tend to bring one another to a uniformity--that is, into a
state in which one movement is as little obstructive to the other as
possible. This happens in two ways: first by the particles limiting
one another's movement till they all advance in one direction; and,
secondly, in this way, that the particles limit their vertical movements
in virtue of which they are approaching the centre of attraction, till
they all move horizontally--i. e., in parallel circles round the sun as
their centre, no longer intercept one another, and by the centrifugal
force becoming equal with the falling force they keep themselves
constantly in free circular orbits at the distance at which they move.
The result, finally, is that only those particles continue to move in
this region of space which have acquired by their fall a velocity, and
through the resistance of the other particles a direction, by which they
can continue to maintain a FREE CIRCULAR MOVEMENT....
"The view of the formation of the planets in this system has the
advantage over every other possible theory in holding that the origin
of the movements, and the position of the orbits in arising at that same
point of time--nay, more, in showing that even the deviations from the
greatest possible exactness in their determinations, as well as the
accordances themselves, become clear at a glance. The planets are formed
out of particles which, at the distance at which they move, have exact
movements in circular orbits; and therefore the masses composed out of
them will continue the same movements and at the same rate and in the
same direction."(2)
It must be admitted that this explanation leaves a good deal to be
desired. It is the explanation of a metaphysician rather than that of
an experimental scientist. Such phrases as "matter immediately begins to
strive to fashion itself," for example, have no place in the reasoning
of inductive science. Nevertheless, the hypothesis of Kant is a
remarkable conception; it attempts to explain along rational lines
something which hitherto had for the most part been considered
altogether inexplicable.
But there are various questions that at once suggest themselves which
the Kantian theory leaves unanswered. How happens it, for example, that
the cosmic mass which gave birth to our solar system was divided into
several planetary bodies instead of remaining a single mass? Were the
planets struck from the sun by the chance impact of comets, as Buffon
has suggested? or thrown out by explosive volcanic action, in accordance
with the theory of Dr. Darwin? or do they owe their origin to some
unknown law? In any event, how chanced it that all were projected in
nearly the same plane as we now find them?
LAPLACE AND THE NEBULAR HYPOTHESIS
It remained for a mathematical astronomer to solve these puzzles. The
man of all others competent to take the subject in hand was the French
astronomer Laplace. For a quarter of a century he had devoted his
transcendent mathematical abilities to the solution of problems of
motion of the heavenly bodies. Working in friendly rivalry with his
countryman Lagrange, his only peer among the mathematicians of the age,
he had taken up and solved one by one the problems that Newton left
obscure. Largely through the efforts of these two men the last lingering
doubts as to the solidarity of the Newtonian hypothesis of universal
gravitation had been removed. The share of Lagrange was hardly less than
that of his co-worker; but Laplace will longer be remembered, because
he ultimately brought his completed labors into a system, and,
incorporating with them the labors of his contemporaries, produced
in the Mecanique Celeste the undisputed mathematical monument of the
century, a fitting complement to the Principia of Newton, which it
supplements and in a sense completes.
In the closing years of the eighteenth century Laplace took up the
nebular hypothesis of cosmogony, to which we have just referred, and
gave it definite proportions; in fact, made it so thoroughly his own
that posterity will always link it with his name. Discarding the crude
notions of cometary impact and volcanic eruption, Laplace filled up the
gaps in the hypothesis with the aid of well-known laws of gravitation
and motion. He assumed that the primitive mass of cosmic matter which
was destined to form our solar system was revolving on its axis even at
a time when it was still nebular in character, and filled all space to
a distance far beyond the present limits of the system. As this vaporous
mass contracted through loss of heat, it revolved more and more swiftly,
and from time to time, through balance of forces at its periphery, rings
of its substance were whirled off and left revolving there, subsequently
to become condensed into planets, and in their turn whirl off minor
rings that became moons. The main body of the original mass remains in
the present as the still contracting and rotating body which we call the
sun.
Let us allow Laplace to explain all this in detail:
"In order to explain the prime movements of the planetary system,"
he says, "there are the five following phenomena: The movement of the
planets in the same direction and very nearly in the same plane; the
movement of the satellites in the same direction as that of the planets;
the rotation of these different bodies and the sun in the same
direction as their revolution, and in nearly the same plane; the slight
eccentricity of the orbits of the planets and of the satellites; and,
finally, the great eccentricity of the orbits of the comets, as if their
inclinations had been left to chance.
"Buffon is the only man I know who, since the discovery of the true
system of the world, has endeavored to show the origin of the planets
and their satellites. He supposes that a comet, in falling into the sun,
drove from it a mass of matter which was reassembled at a distance in
the form of various globes more or less large, and more or less removed
from the sun, and that these globes, becoming opaque and solid, are now
the planets and their satellites.
"This hypothesis satisfies the first of the five preceding phenomena;
for it is clear that all the bodies thus formed would move very nearly
in the plane which passed through the centre of the sun, and in the
direction of the torrent of matter which was produced; but the four
other phenomena appear to be inexplicable to me by this means. Indeed,
the absolute movement of the molecules of a planet ought then to be in
the direction of the movement of its centre of gravity; but it does not
at all follow that the motion of the rotation of the planets should be
in the same direction. Thus the earth should rotate from east to west,
but nevertheless the absolute movement of its molecules should be
from east to west; and this ought also to apply to the movement of the
revolution of the satellites, in which the direction, according to the
hypothesis which he offers, is not necessarily the same as that of the
progressive movement of the planets.
"A phenomenon not only very difficult to explain under this hypothesis,
but one which is even contrary to it, is the slight eccentricity of the
planetary orbits. We know, by the theory of central forces, that if
a body moves in a closed orbit around the sun and touches it, it also
always comes back to that point at every revolution; whence it follows
that if the planets were originally detached from the sun, they would
touch it at each return towards it, and their orbits, far from being
circular, would be very eccentric. It is true that a mass of matter
driven from the sun cannot be exactly compared to a globe which touches
its surface, for the impulse which the particles of this mass receive
from one another and the reciprocal attractions which they exert among
themselves, could, in changing the direction of their movements, remove
their perihelions from the sun; but their orbits would be always most
eccentric, or at least they would not have slight eccentricities except
by the most extraordinary chance. Thus we cannot see, according to
the hypothesis of Buffon, why the orbits of more than a hundred comets
already observed are so elliptical. This hypothesis is therefore
very far from satisfying the preceding phenomena. Let us see if it is
possible to trace them back to their true cause.
"Whatever may be its ultimate nature, seeing that it has caused or
modified the movements of the planets, it is necessary that this cause
should embrace every body, and, in view of the enormous distances which
separate them, it could only have been a fluid of immense extent.
In order to have given them an almost circular movement in the same
direction around the sun, it is necessary that this fluid should
have enveloped the sun as in an atmosphere. The consideration of the
planetary movements leads us then to think that, on account of excessive
heat, the atmosphere of the sun originally extended beyond the orbits of
all the planets, and that it was successively contracted to its present
limits.
"In the primitive condition in which we suppose the sun to have been, it
resembled a nebula such as the telescope shows is composed of a nucleus
more or less brilliant, surrounded by a nebulosity which, on condensing
itself towards the centre, forms a star. If it is conceived by analogy
that all the stars were formed in this manner, it is possible to imagine
their previous condition of nebulosity, itself preceded by other states
in which the nebulous matter was still more diffused, the nucleus being
less and less luminous. By going back as far as possible, we thus
arrive at a nebulosity so diffused that its existence could hardly be
suspected.
"For a long time the peculiar disposition of certain stars, visible
to the unaided eye, has struck philosophical observers. Mitchell
has already remarked how little probable it is that the stars in the
Pleiades, for example, could have been contracted into the small
space which encloses them by the fortuity of chance alone, and he has
concluded that this group of stars, and similar groups which the skies
present to us, are the necessary result of the condensation of a nebula,
with several nuclei, and it is evident that a nebula, by continually
contracting, towards these various nuclei, at length would form a group
of stars similar to the Pleiades. The condensation of a nebula with two
nuclei would form a system of stars close together, turning one upon
the other, such as those double stars of which we already know the
respective movements.
"But how did the solar atmosphere determine the movements of the
rotation and revolution of the planets and satellites? If these bodies
had penetrated very deeply into this atmosphere, its resistance would
have caused them to fall into the sun. We can therefore conjecture that
the planets were formed at their successive limits by the condensation
of a zone of vapors which the sun, on cooling, left behind, in the plane
of his equator.
"Let us recall the results which we have given in a preceding chapter.
The atmosphere of the sun could not have extended indefinitely. Its
limit was the point where the centrifugal force due to its movement
of rotation balanced its weight. But in proportion as the cooling
contracted the atmosphere, and those molecules which were near to them
condensed upon the surface of the body, the movement of the rotation
increased; for, on account of the Law of Areas, the sum of the areas
described by the vector of each molecule of the sun and its atmosphere
and projected in the plane of the equator being always the same, the
rotation should increase when these molecules approach the centre of the
sun. The centrifugal force due to this movement becoming thus larger,
the point where the weight is equal to it is nearer the sun. Supposing,
then, as it is natural to admit, that the atmosphere extended at some
period to its very limits, it should, on cooling, leave molecules behind
at this limit and at limits successively occasioned by the increased
rotation of the sun. The abandoned molecules would continue to revolve
around this body, since their centrifugal force was balanced by their
weight. But this equilibrium not arising in regard to the atmospheric
molecules parallel to the solar equator, the latter, on account of their
weight, approached the atmosphere as they condensed, and did not cease
to belong to it until by this motion they came upon the equator.
"Let us consider now the zones of vapor successively left behind. These
zones ought, according to appearance, by the condensation and mutual
attraction of their molecules, to form various concentric rings of vapor
revolving around the sun. The mutual gravitational friction of each ring
would accelerate some and retard others, until they had all acquired the
same angular velocity. Thus the actual velocity of the molecules most
removed from the sun would be the greatest. The following cause would
also operate to bring about this difference of speed. The molecules
farthest from the sun, and which by the effects of cooling and
condensation approached one another to form the outer part of the ring,
would have always described areas proportional to the time since the
central force by which they were controlled has been constantly directed
towards this body. But this constancy of areas necessitates an increase
of velocity proportional to the distance. It is thus seen that the same
cause would diminish the velocity of the molecules which form the inner
part of the ring.
"If all the molecules of the ring of vapor continued to condense without
disuniting, they would at length form a ring either solid or fluid. But
this formation would necessitate such a regularity in every part of the
ring, and in its cooling, that this phenomenon is extremely rare; and
the solar system affords us, indeed, but one example--namely, in the
ring of Saturn. In nearly every case the ring of vapor was broken into
several masses, each moving at similar velocities, and continuing to
rotate at the same distance around the sun. These masses would take
a spheroid form with a rotatory movement in the direction of the
revolution, because their inner molecules had less velocity than the
outer. Thus were formed so many planets in a condition of vapor. But
if one of them were powerful enough to reunite successively by its
attraction all the others around its centre of gravity, the ring of
vapor would be thus transformed into a single spheroidical mass of
vapor revolving around the sun with a rotation in the direction of its
revolution. The latter case has been that which is the most common, but
nevertheless the solar system affords us an instance of the first case
in the four small planets which move between Jupiter and Mars; at least,
if we do not suppose, as does M. Olbers, that they originally formed a
single planet which a mighty explosion broke up into several portions
each moving at different velocities.
"According to our hypothesis, the comets are strangers to our planetary
system. In considering them, as we have done, as minute nebulosities,
wandering from solar system to solar system, and formed by the
condensation of the nebulous matter everywhere existent in profusion in
the universe, we see that when they come into that part of the heavens
where the sun is all-powerful, he forces them to describe orbits either
elliptical or hyperbolic, their paths being equally possible in all
directions, and at all inclinations of the ecliptic, conformably to what
has been observed. Thus the condensation of nebulous matter, by which
we have at first explained the motions of the rotation and revolution
of the planets and their satellites in the same direction, and in nearly
approximate planes, explains also why the movements of the comets escape
this general law."(3)
The nebular hypothesis thus given detailed completion by Laplace is a
worthy complement of the grand cosmologic scheme of Herschel. Whether
true or false, the two conceptions stand as the final contributions
of the eighteenth century to the history of man's ceaseless efforts to
solve the mysteries of cosmic origin and cosmic structure. The world
listened eagerly and without prejudice to the new doctrines; and that
attitude tells of a marvellous intellectual growth of our race. Mark the
transition. In the year 1600, Bruno was burned at the stake for teaching
that our earth is not the centre of the universe. In 1700, Newton was
pronounced "impious and heretical" by a large school of philosophers
for declaring that the force which holds the planets in their orbits
is universal gravitation. In 1800, Laplace and Herschel are honored for
teaching that gravitation built up the system which it still controls;
that our universe is but a minor nebula, our sun but a minor star, our
earth a mere atom of matter, our race only one of myriad races peopling
an infinity of worlds. Doctrines which but the span of two human lives
before would have brought their enunciators to the stake were now
pronounced not impious, but sublime.
ASTEROIDS AND SATELLITES
The first day of the nineteenth century was fittingly signalized by the
discovery of a new world. On the evening of January 1, 1801, an Italian
astronomer, Piazzi, observed an apparent star of about the eighth
magnitude (hence, of course, quite invisible to the unaided eye), which
later on was seen to have moved, and was thus shown to be vastly nearer
the earth than any true star. He at first supposed, as Herschel had
done when he first saw Uranus, that the unfamiliar body was a comet; but
later observation proved it a tiny planet, occupying a position in space
between Mars and Jupiter. It was christened Ceres, after the tutelary
goddess of Sicily.
Though unpremeditated, this discovery was not unexpected, for
astronomers had long surmised the existence of a planet in the wide
gap between Mars and Jupiter. Indeed, they were even preparing to make
concerted search for it, despite the protests of philosophers, who
argued that the planets could not possibly exceed the magic number
seven, when Piazzi forestalled their efforts. But a surprise came
with the sequel; for the very next year Dr. Olbers, the wonderful
physician-astronomer of Bremen, while following up the course of Ceres,
happened on another tiny moving star, similarly located, which soon
revealed itself as planetary. Thus two planets were found where only one
was expected.
The existence of the supernumerary was a puzzle, but Olbers solved it
for the moment by suggesting that Ceres and Pallas, as he called his
captive, might be fragments of a quondam planet, shattered by internal
explosion or by the impact of a comet. Other similar fragments, he
ventured to predict, would be found when searched for. William Herschel
sanctioned this theory, and suggested the name asteroids for the tiny
planets. The explosion theory was supported by the discovery of another
asteroid, by Harding, of Lilienthal, in 1804, and it seemed clinched
when Olbers himself found a fourth in 1807. The new-comers were named
Juno and Vesta respectively.
There the case rested till 1845, when a Prussian amateur astronomer
named Hencke found another asteroid, after long searching, and opened a
new epoch of discovery. From then on the finding of asteroids became a
commonplace. Latterly, with the aid of photography, the list has been
extended to above four hundred, and as yet there seems no dearth in the
supply, though doubtless all the larger members have been revealed. Even
these are but a few hundreds of miles in diameter, while the smaller
ones are too tiny for measurement. The combined bulk of these minor
planets is believed to be but a fraction of that of the earth.
Olbers's explosion theory, long accepted by astronomers, has been
proven open to fatal objections. The minor planets are now believed to
represent a ring of cosmical matter, cast off from the solar nebula
like the rings that went to form the major planets, but prevented
from becoming aggregated into a single body by the perturbing mass of
Jupiter.
The Discovery of Neptune
As we have seen, the discovery of the first asteroid confirmed a
conjecture; the other important planetary discovery of the nineteenth
century fulfilled a prediction. Neptune was found through scientific
prophecy. No one suspected the existence of a trans-Uranian planet till
Uranus itself, by hair-breadth departures from its predicted orbit, gave
out the secret. No one saw the disturbing planet till the pencil of the
mathematician, with almost occult divination, had pointed out its place
in the heavens. The general predication of a trans-Uranian planet was
made by Bessel, the great Konigsberg astronomer, in 1840; the analysis
that revealed its exact location was undertaken, half a decade later,
by two independent workers--John Couch Adams, just graduated senior
wrangler at Cambridge, England, and U. J. J. Leverrier, the leading
French mathematician of his generation.
Adams's calculation was first begun and first completed. But it had one
radical defect--it was the work of a young and untried man. So it found
lodgment in a pigeon-hole of the desk of England's Astronomer Royal, and
an opportunity was lost which English astronomers have never ceased to
mourn. Had the search been made, an actual planet would have been seen
shining there, close to the spot where the pencil of the mathematician
had placed its hypothetical counterpart. But the search was not made,
and while the prophecy of Adams gathered dust in that regrettable
pigeon-hole, Leverrier's calculation was coming on, his tentative
results meeting full encouragement from Arago and other French savants.
At last the laborious calculations proved satisfactory, and, confident
of the result, Leverrier sent to the Berlin observatory, requesting that
search be made for the disturber of Uranus in a particular spot of the
heavens. Dr. Galle received the request September 23, 1846. That very
night he turned his telescope to the indicated region, and there, within
a single degree of the suggested spot, he saw a seeming star, invisible
to the unaided eye, which proved to be the long-sought planet,
henceforth to be known as Neptune. To the average mind, which finds
something altogether mystifying about abstract mathematics, this was a
feat savoring of the miraculous.
Stimulated by this success, Leverrier calculated an orbit for an
interior planet from perturbations of Mercury, but though prematurely
christened Vulcan, this hypothetical nursling of the sun still haunts
the realm of the undiscovered, along with certain equally hypothetical
trans-Neptunian planets whose existence has been suggested by "residual
perturbations" of Uranus, and by the movements of comets. No other
veritable additions of the sun's planetary family have been made in our
century, beyond the finding of seven small moons, which chiefly attest
the advance in telescopic powers. Of these, the tiny attendants of our
Martian neighbor, discovered by Professor Hall with the great Washington
refractor, are of greatest interest, because of their small size and
extremely rapid flight. One of them is poised only six thousand
miles from Mars, and whirls about him almost four times as fast as he
revolves, seeming thus, as viewed by the Martian, to rise in the west
and set in the east, and making the month only one-fourth as long as the
day.
The Rings of Saturn
The discovery of the inner or crape ring of Saturn, made simultaneously
in 1850 by William C. Bond, at the Harvard observatory, in America,
and the Rev. W. R. Dawes in England, was another interesting optical
achievement; but our most important advances in knowledge of Saturn's
unique system are due to the mathematician. Laplace, like his
predecessors, supposed these rings to be solid, and explained their
stability as due to certain irregularities of contour which Herschel
bad pointed out. But about 1851 Professor Peirce, of Harvard, showed
the untenability of this conclusion, proving that were the rings such as
Laplace thought them they must fall of their own weight. Then Professor
J. Clerk-Maxwell, of Cambridge, took the matter in hand, and his
analysis reduced the puzzling rings to a cloud of meteoric particles--a
"shower of brickbats"--each fragment of which circulates exactly as if
it were an independent planet, though of course perturbed and jostled
more or less by its fellows. Mutual perturbations, and the disturbing
pulls of Saturn's orthodox satellites, as investigated by Maxwell,
explain nearly all the phenomena of the rings in a manner highly
satisfactory.
After elaborate mathematical calculations covering many pages of his
paper entitled "On the Stability of Saturn's Rings," he summarizes his
deductions as follows:
"Let us now gather together the conclusions we have been able to draw
from the mathematical theory of various kinds of conceivable rings.
"We found that the stability of the motion of a solid ring depended
on so delicate an adjustment, and at the same time so unsymmetrical a
distribution of mass, that even if the exact conditions were fulfilled,
it could scarcely last long, and, if it did, the immense preponderance
of one side of the ring would be easily observed, contrary to
experience. These considerations, with others derived from the
mechanical structure of so vast a body, compel us to abandon any theory
of solid rings.
"We next examined the motion of a ring of equal satellites, and found
that if the mass of the planet is sufficient, any disturbances produced
in the arrangement of the ring will be propagated around it in the form
of waves, and will not introduce dangerous confusion. If the satellites
are unequal, the propagations of the waves will no longer be regular,
but disturbances of the ring will in this, as in the former case,
produce only waves, and not growing confusion. Supposing the ring to
consist, not of a single row of large satellites, but a cloud of evenly
distributed unconnected particles, we found that such a cloud must
have a very small density in order to be permanent, and that this is
inconsistent with its outer and inner parts moving with the same angular
velocity. Supposing the ring to be fluid and continuous, we found that
it will be necessarily broken up into small portions.
"We conclude, therefore, that the rings must consist of disconnected
particles; these must be either solid or liquid, but they must be
independent. The entire system of rings must, therefore, consist either
of a series of many concentric rings each moving with its own velocity
and having its own system of waves, or else of a confused multitude of
revolving particles not arranged in rings and continually coming into
collision with one another.
"Taking the first case, we found that in an indefinite number of
possible cases the mutual perturbations of two rings, stable in
themselves, might mount up in time to a destructive magnitude, and that
such cases must continually occur in an extensive system like that of
Saturn, the only retarding cause being the irregularity of the rings.
"The result of long-continued disturbance was found to be the
spreading-out of the rings in breadth, the outer rings pressing outward,
while the inner rings press inward.
"The final result, therefore, of the mechanical theory is that the only
system of rings which can exist is one composed of an indefinite number
of unconnected particles, revolving around the planet with different
velocities, according to their respective distances. These particles
may be arranged in series of narrow rings, or they may move through one
another irregularly. In the first case the destruction of the system
will be very slow, in the second case it will be more rapid, but there
may be a tendency towards arrangement in narrow rings which may retard
the process.
"We are not able to ascertain by observation the constitution of the two
outer divisions of the system of rings, but the inner ring is certainly
transparent, for the limb of Saturn has been observed through it. It is
also certain that though the space occupied by the ring is transparent,
it is not through the material parts of it that the limb of Saturn is
seen, for his limb was observed without distortion; which shows that
there was no refraction, and, therefore, that the rays did not pass
through a medium at all, but between the solar or liquid particles of
which the ring is composed. Here, then, we have an optical argument
in favor of the theory of independent particles as the material of
the rings. The two outer rings may be of the same nature, but not
so exceedingly rare that a ray of light can pass through their whole
thickness without encountering one of the particles.
"Finally, the two outer rings have been observed for two hundred years,
and it appears, from the careful analysis of all the observations of M.
Struve, that the second ring is broader than when first observed, and
that its inner edge is nearer the planet than formerly. The inner ring
also is suspected to be approaching the planet ever since its discovery
in 1850. These appearances seem to indicate the same slow progress of
the rings towards separation which we found to be the result of theory,
and the remark that the inner edge of the inner ring is more distinct
seems to indicate that the approach towards the planet is less rapid
near the edge, as we had reason to conjecture. As to the apparent
unchangeableness of the exterior diameter of the outer ring, we must
remember that the outer rings are certainly far more dense than the
inner one, and that a small change in the outer rings must balance a
great change in the inner one. It is possible, however, that some of the
observed changes may be due to the existence of a resisting medium.
If the changes already suspected should be confirmed by repeated
observations with the same instruments, it will be worth while to
investigate more carefully whether Saturn's rings are permanent or
transitory elements of the solar system, and whether in that part of
the heavens we see celestial immutability or terrestrial corruption
and generation, and the old order giving place to the new before our
eyes."(4)
Studies of the Moon
But perhaps the most interesting accomplishments of mathematical
astronomy--from a mundane standpoint, at any rate--are those that refer
to the earth's own satellite. That seemingly staid body was long ago
discovered to have a propensity to gain a little on the earth, appearing
at eclipses an infinitesimal moment ahead of time. Astronomers were
sorely puzzled by this act of insubordination; but at last Laplace and
Lagrange explained it as due to an oscillatory change in the earth's
orbit, thus fully exonerating the moon, and seeming to demonstrate the
absolute stability of our planetary system, which the moon's misbehavior
had appeared to threaten.
This highly satisfactory conclusion was an orthodox belief of celestial
mechanics until 1853, when Professor Adams of Neptunian fame, with whom
complex analyses were a pastime, reviewed Laplace's calculation, and
discovered an error which, when corrected, left about half the moon's
acceleration unaccounted for. This was a momentous discrepancy, which at
first no one could explain. But presently Professor Helmholtz, the great
German physicist, suggested that a key might be found in tidal friction,
which, acting as a perpetual brake on the earth's rotation, and
affecting not merely the waters but the entire substance of our planet,
must in the long sweep of time have changed its rate of rotation. Thus
the seeming acceleration of the moon might be accounted for as actual
retardation of the earth's rotation--a lengthening of the day instead of
a shortening of the month.
Again the earth was shown to be at fault, but this time the moon could
not be exonerated, while the estimated stability of our system, instead
of being re-established, was quite upset. For the tidal retardation is
not an oscillatory change which will presently correct itself, like the
orbital wobble, but a perpetual change, acting always in one direction.
Unless fully counteracted by some opposing reaction, therefore (as
it seems not to be), the effect must be cumulative, the ultimate
consequences disastrous. The exact character of these consequences was
first estimated by Professor G. H. Darwin in 1879. He showed that tidal
friction, in retarding the earth, must also push the moon out from the
parent planet on a spiral orbit. Plainly, then, the moon must formerly
have been nearer the earth than at present. At some very remote period
it must have actually touched the earth; must, in other words, have been
thrown off from the then plastic mass of the earth, as a polyp buds out
from its parent polyp. At that time the earth was spinning about in a
day of from two to four hours.
Now the day has been lengthened to twenty-four hours, and the moon has
been thrust out to a distance of a quarter-million miles; but the end is
not yet. The same progress of events must continue, till, at some remote
period in the future, the day has come to equal the month, lunar tidal
action has ceased, and one face of the earth looks out always at the
moon with that same fixed stare which even now the moon has been brought
to assume towards her parent orb. Should we choose to take even greater
liberties with the future, it may be made to appear (though some
astronomers dissent from this prediction) that, as solar tidal action
still continues, the day must finally exceed the month, and lengthen out
little by little towards coincidence with the year; and that the moon
meantime must pause in its outward flight, and come swinging back on a
descending spiral, until finally, after the lapse of untold aeons, it
ploughs and ricochets along the surface of the earth, and plunges to
catastrophic destruction.
But even though imagination pause far short of this direful culmination,
it still is clear that modern calculations, based on inexorable tidal
friction, suffice to revolutionize the views formerly current as to the
stability of the planetary system. The eighteenth-century mathematician
looked upon this system as a vast celestial machine which had been in
existence about six thousand years, and which was destined to run on
forever. The analyst of to-day computes both the past and the future of
this system in millions instead of thousands of years, yet feels well
assured that the solar system offers no contradiction to those laws of
growth and decay which seem everywhere to represent the immutable order
of nature.
COMETS AND METEORS
Until the mathematician ferreted out the secret, it surely never could
have been suspected by any one that the earth's serene attendant,
"That orbed maiden, with white fire laden,
Whom mortals call the moon,"
could be plotting injury to her parent orb. But there is another
inhabitant of the skies whose purposes have not been similarly free from
popular suspicion. Needless to say I refer to the black sheep of the
sidereal family, that "celestial vagabond" the comet.
Time out of mind these wanderers have been supposed to presage war,
famine, pestilence, perhaps the destruction of the world. And little
wonder. Here is a body which comes flashing out of boundless space into
our system, shooting out a pyrotechnic tail some hundreds of millions of
miles in length; whirling, perhaps, through the very atmosphere of the
sun at a speed of three or four hundred miles a second; then darting off
on a hyperbolic orbit that forbids it ever to return, or an elliptical
one that cannot be closed for hundreds or thousands of years; the tail
meantime pointing always away from the sun, and fading to nothingness as
the weird voyager recedes into the spatial void whence it came. Not many
times need the advent of such an apparition coincide with the outbreak
of a pestilence or the death of a Caesar to stamp the race of comets as
an ominous clan in the minds of all superstitious generations.
It is true, a hard blow was struck at the prestige of these alleged
supernatural agents when Newton proved that the great comet of 1680
obeyed Kepler's laws in its flight about the sun; and an even harder
one when the same visitant came back in 1758, obedient to Halley's
prediction, after its three-quarters of a century of voyaging but in
the abyss of space. Proved thus to bow to natural law, the celestial
messenger could no longer fully, sustain its role. But long-standing
notoriety cannot be lived down in a day, and the comet, though proved a
"natural" object, was still regarded as a very menacing one for
another hundred years or so. It remained for the nineteenth century to
completely unmask the pretender and show how egregiously our forebears
had been deceived.
The unmasking began early in the century, when Dr. Olbers, then the
highest authority on the subject, expressed the opinion that
the spectacular tail, which had all along been the comet's chief
stock-in-trade as an earth-threatener, is in reality composed of
the most filmy vapors, repelled from the cometary body by the sun,
presumably through electrical action, with a velocity comparable to that
of light. This luminous suggestion was held more or less in abeyance for
half a century. Then it was elaborated by Zollner, and particularly by
Bredichin, of the Moscow observatory, into what has since been regarded
as the most plausible of cometary theories. It is held that comets
and the sun are similarly electrified, and hence mutually repulsive.
Gravitation vastly outmatches this repulsion in the body of the comet,
but yields to it in the case of gases, because electrical force varies
with the surface, while gravitation varies only with the mass. From
study of atomic weights and estimates of the velocity of thrust of
cometary tails, Bredichin concluded that the chief components of the
various kinds of tails are hydrogen, hydrocarbons, and the vapor of
iron; and spectroscopic analysis goes far towards sustaining these
assumptions.
But, theories aside, the unsubstantialness of the comet's tail has been
put to a conclusive test. Twice during the nineteenth century the
earth has actually plunged directly through one of these threatening
appendages--in 1819, and again in 1861, once being immersed to a depth
of some three hundred thousand miles in its substance. Yet nothing
dreadful happened to us. There was a peculiar glow in the atmosphere,
so the more imaginative observers thought, and that was all. After such
fiascos the cometary train could never again pose as a world-destroyer.
But the full measure of the comet's humiliation is not yet told. The
pyrotechnic tail, composed as it is of portions of the comet's actual
substance, is tribute paid the sun, and can never be recovered. Should
the obeisance to the sun be many times repeated, the train-forming
material will be exhausted, and the comet's chiefest glory will have
departed. Such a fate has actually befallen a multitude of comets which
Jupiter and the other outlying planets have dragged into our system and
helped the sun to hold captive here. Many of these tailless comets were
known to the eighteenth-century astronomers, but no one at that time
suspected the true meaning of their condition. It was not even known how
closely some of them are enchained until the German astronomer Encke,
in 1822, showed that one which he had rediscovered, and which has
since borne his name, was moving in an orbit so contracted that it must
complete its circuit in about three and a half years. Shortly afterwards
another comet, revolving in a period of about six years, was discovered
by Biela, and given his name. Only two more of these short-period comets
were discovered during the first half of last century, but latterly they
have been shown to be a numerous family. Nearly twenty are known
which the giant Jupiter holds so close that the utmost reach of their
elliptical tether does not let them go beyond the orbit of Saturn. These
aforetime wanderers have adapted themselves wonderfully to planetary
customs, for all of them revolve in the same direction with the planets,
and in planes not wide of the ecliptic.
Checked in their proud hyperbolic sweep, made captive in a planetary
net, deprived of their trains, these quondam free-lances of the heavens
are now mere shadows of their former selves. Considered as to mere
bulk, they are very substantial shadows, their extent being measured in
hundreds of thousands of miles; but their actual mass is so slight that
they are quite at the mercy of the gravitation pulls of their captors.
And worse is in store for them. So persistently do sun and planets tug
at them that they are doomed presently to be torn into shreds.
Such a fate has already overtaken one of them, under the very eyes of
the astronomers, within the relatively short period during which these
ill-fated comets have been observed. In 1832 Biela's comet passed quite
near the earth, as astronomers measure distance, and in doing so created
a panic on our planet. It did no greater harm than that, of course, and
passed on its way as usual. The very next time it came within telescopic
hail it was seen to have broken into two fragments. Six years later
these fragments were separated by many millions of miles; and in 1852,
when the comet was due again, astronomers looked for it in vain. It had
been completely shattered.
What had become of the fragments? At that time no one positively knew.
But the question was to be answered presently. It chanced that just at
this period astronomers were paying much attention to a class of bodies
which they had hitherto somewhat neglected, the familiar shooting-stars,
or meteors. The studies of Professor Newton, of Yale, and Professor
Adams, of Cambridge, with particular reference to the great
meteor-shower of November, 1866, which Professor Newton had predicted
and shown to be recurrent at intervals of thirty-three years, showed
that meteors are not mere sporadic swarms of matter flying at random,
but exist in isolated swarms, and sweep about the sun in regular
elliptical orbits.
Presently it was shown by the Italian astronomer Schiaparelli that
one of these meteor swarms moves in the orbit of a previously observed
comet, and other coincidences of the kind were soon forthcoming. The
conviction grew that meteor swarms are really the debris of comets; and
this conviction became a practical certainty when, in November, 1872,
the earth crossed the orbit of the ill-starred Biela, and a shower of
meteors came whizzing into our atmosphere in lieu of the lost comet.
And so at last the full secret was out. The awe-inspiring comet, instead
of being the planetary body it had all along been regarded, is really
nothing more nor less than a great aggregation of meteoric particles,
which have become clustered together out in space somewhere, and which
by jostling one another or through electrical action become luminous. So
widely are the individual particles separated that the cometary body as
a whole has been estimated to be thousands of times less dense than the
earth's atmosphere at sea-level. Hence the ease with which the comet may
be dismembered and its particles strung out into streaming swarms.
So thickly is the space we traverse strewn with this cometary dust
that the earth sweeps up, according to Professor Newcomb's estimate, a
million tons of it each day. Each individual particle, perhaps no larger
than a millet seed, becomes a shooting-star, or meteor, as it burns to
vapor in the earth's upper atmosphere. And if one tiny planet sweeps
up such masses of this cosmic matter, the amount of it in the entire
stretch of our system must be beyond all estimate. What a story it tells
of the myriads of cometary victims that have fallen prey to the sun
since first he stretched his planetary net across the heavens!
THE FIXED STARS
When Biela's comet gave the inhabitants of the earth such a fright in
1832, it really did not come within fifty millions of miles of us. Even
the great comet through whose filmy tail the earth passed in 1861 was
itself fourteen millions of miles away. The ordinary mind, schooled to
measure space by the tiny stretches of a pygmy planet, cannot grasp the
import of such distances; yet these are mere units of measure compared
with the vast stretches of sidereal space. Were the comet which hurtles
past us at a speed of, say, a hundred miles a second to continue its
mad flight unchecked straight into the void of space, it must fly on its
frigid way eight thousand years before it could reach the very nearest
of our neighbor stars; and even then it would have penetrated but a
mere arm's-length into the vistas where lie the dozen or so of sidereal
residents that are next beyond. Even to the trained mind such distances
are only vaguely imaginable. Yet the astronomer of our century has
reached out across this unthinkable void and brought back many a secret
which our predecessors thought forever beyond human grasp.
A tentative assault upon this stronghold of the stars was being made
by Herschel at the beginning of the century. In 1802 that greatest of
observing astronomers announced to the Royal Society his discovery that
certain double stars had changed their relative positions towards one
another since he first carefully charted them twenty years before.
Hitherto it had been supposed that double stars were mere optical
effects. Now it became clear that some of them, at any rate, are
true "binary systems," linked together presumably by gravitation and
revolving about one another. Halley had shown, three-quarters of a
century before, that the stars have an actual or "proper" motion in
space; Herschel himself had proved that the sun shares this motion
with the other stars. Here was another shift of place, hitherto quite
unsuspected, to be reckoned with by the astronomer in fathoming sidereal
secrets.
Double Stars
When John Herschel, the only son and the worthy successor of the great
astronomer, began star-gazing in earnest, after graduating senior
wrangler at Cambridge, and making two or three tentative professional
starts in other directions to which his versatile genius impelled him,
his first extended work was the observation of his father's double
stars. His studies, in which at first he had the collaboration of Mr.
James South, brought to light scores of hitherto unrecognized pairs, and
gave fresh data for the calculation of the orbits of those longer
known. So also did the independent researches of F. G. W. Struve,
the enthusiastic observer of the famous Russian observatory at the
university of Dorpat, and subsequently at Pulkowa. Utilizing data
gathered by these observers, M. Savary, of Paris, showed, in 1827, that
the observed elliptical orbits of the double stars are explicable by
the ordinary laws of gravitation, thus confirming the assumption that
Newton's laws apply to these sidereal bodies. Henceforth there could be
no reason to doubt that the same force which holds terrestrial objects
on our globe pulls at each and every particle of matter throughout the
visible universe.
The pioneer explorers of the double stars early found that the systems
into which the stars are linked are by no means confined to single
pairs. Often three or four stars are found thus closely connected into
gravitation systems; indeed, there are all gradations between binary
systems and great clusters containing hundreds or even thousands of
members. It is known, for example, that the familiar cluster of the
Pleiades is not merely an optical grouping, as was formerly supposed,
but an actual federation of associated stars, some two thousand five
hundred in number, only a few of which are visible to the unaided eve.
And the more carefully the motions of the stars are studied, the more
evident it becomes that widely separated stars are linked together into
infinitely complex systems, as yet but little understood. At the same
time, all instrumental advances tend to resolve more and more seemingly
single stars into close pairs and minor clusters. The two Herschels
between them discovered some thousands of these close multiple systems;
Struve and others increased the list to above ten thousand; and Mr.
S. W. Burnham, of late years the most enthusiastic and successful of
double-star pursuers, added a thousand new discoveries while he was
still an amateur in astronomy, and by profession the stenographer of a
Chicago court. Clearly the actual number of multiple stars is beyond all
present estimate.
The elder Herschel's early studies of double stars were undertaken in
the hope that these objects might aid him in ascertaining the actual
distance of a star, through measurement of its annual parallax--that
is to say, of the angle which the diameter of the earth's orbit would
subtend as seen from the star. The expectation was not fulfilled. The
apparent shift of position of a star as viewed from opposite sides of
the earth's orbit, from which the parallax might be estimated, is so
extremely minute that it proved utterly inappreciable, even to the
almost preternaturally acute vision of Herschel, with the aid of any
instrumental means then at command. So the problem of star distance
allured and eluded him to the end, and he died in 1822 without seeing
it even in prospect of solution. His estimate of the minimum distance of
the nearest star, based though it was on the fallacious test of apparent
brilliancy, was a singularly sagacious one, but it was at best a
scientific guess, not a scientific measurement.
The Distance of the Stars
Just about this time, however, a great optician came to the aid of the
astronomers. Joseph Fraunhofer perfected the refracting telescope,
as Herschel had perfected the reflector, and invented a wonderfully
accurate "heliometer," or sun-measurer. With the aid of these
instruments the old and almost infinitely difficult problem of star
distance was solved. In 1838 Bessel announced from the Konigsberg
observatory that he had succeeded, after months of effort, in detecting
and measuring the parallax of a star. Similar claims had been made often
enough before, always to prove fallacious when put to further test; but
this time the announcement carried the authority of one of the greatest
astronomers of the age, and scepticism was silenced.
Nor did Bessel's achievement long await corroboration. Indeed, as so
often happens in fields of discovery, two other workers had almost
simultaneously solved the same problem--Struve at Pulkowa, where the
great Russian observatory, which so long held the palm over all others,
had now been established; and Thomas Henderson, then working at the
Cape of Good Hope, but afterwards the Astronomer Royal of Scotland.
Henderson's observations had actual precedence in point of time, but
Bessel's measurements were so much more numerous and authoritative that
he has been uniformly considered as deserving the chief credit of the
discovery, which priority of publication secured him.
By an odd chance, the star on which Henderson's observations were made,
and consequently the first star the parallax of which was ever measured,
is our nearest neighbor in sidereal space, being, indeed, some ten
billions of miles nearer than the one next beyond. Yet even this nearest
star is more than two hundred thousand times as remote from us as the
sun. The sun's light flashes to the earth in eight minutes, and to
Neptune in about three and a half hours, but it requires three and a
half years to signal Alpha Centauri. And as for the great majority of
the stars, had they been blotted out of existence before the Christian
era, we of to-day should still receive their light and seem to see them
just as we do. When we look up to the sky, we study ancient history;
we do not see the stars as they ARE, but as they WERE years, centuries,
even millennia ago.
The information derived from the parallax of a star by no means halts
with the disclosure of the distance of that body. Distance known, the
proper motion of the star, hitherto only to be reckoned as so many
seconds of arc, may readily be translated into actual speed of progress;
relative brightness becomes absolute lustre, as compared with the sun;
and in the case of the double stars the absolute mass of the components
may be computed from the laws of gravitation. It is found that stars
differ enormously among themselves in all these regards. As to speed,
some, like our sun, barely creep through space--compassing ten or twenty
miles a second, it is true, yet even at that rate only passing through
the equivalent of their own diameter in a day. At the other extreme,
among measured stars, is one that moves two hundred miles a second; yet
even this "flying star," as seen from the earth, seems to change its
place by only about three and a half lunar diameters in a thousand
years. In brightness, some stars yield to the sun, while others surpass
him as the arc-light surpasses a candle. Arcturus, the brightest
measured star, shines like two hundred suns; and even this giant orb is
dim beside those other stars which are so distant that their parallax
cannot be measured, yet which greet our eyes at first magnitude. As to
actual bulk, of which apparent lustre furnishes no adequate test, some
stars are smaller than the sun, while others exceed him hundreds or
perhaps thousands of times. Yet one and all, so distant are they, remain
mere disklike points of light before the utmost powers of the modern
telescope.
Revelations of the Spectroscope
All this seems wonderful enough, but even greater things were in store.
In 1859 the spectroscope came upon the scene, perfected by Kirchhoff
and Bunsen, along lines pointed out by Fraunhofer almost half a century
before. That marvellous instrument, by revealing the telltale lines
sprinkled across a prismatic spectrum, discloses the chemical nature
and physical condition of any substance whose light is submitted to it,
telling its story equally well, provided the light be strong enough,
whether the luminous substance be near or far--in the same room or at
the confines of space. Clearly such an instrument must prove a veritable
magic wand in the hands of the astronomer.
Very soon eager astronomers all over the world were putting the
spectroscope to the test. Kirchhoff himself led the way, and Donati and
Father Secchi in Italy, Huggins and Miller in England, and Rutherfurd in
America, were the chief of his immediate followers. The results exceeded
the dreams of the most visionary. At the very outset, in 1860, it was
shown that such common terrestrial substances as sodium, iron, calcium,
magnesium, nickel, barium, copper, and zinc exist in the form of glowing
vapors in the sun, and very soon the stars gave up a corresponding
secret. Since then the work of solar and sidereal analysis has gone on
steadily in the hands of a multitude of workers (prominent among whom,
in this country, are Professor Young of Princeton, Professor Langley of
Washington, and Professor Pickering of Harvard), and more than half
the known terrestrial elements have been definitely located in the sun,
while fresh discoveries are in prospect.
It is true the sun also contains some seeming elements that are unknown
on the earth, but this is no matter for surprise. The modern chemist
makes no claim for his elements except that they have thus far resisted
all human efforts to dissociate them; it would be nothing strange if
some of them, when subjected to the crucible of the sun, which is seen
to vaporize iron, nickel, silicon, should fail to withstand the test.
But again, chemistry has by no means exhausted the resources of the
earth's supply of raw material, and the substance which sends its
message from a star may exist undiscovered in the dust we tread or in
the air we breathe. In the year 1895 two new terrestrial elements were
discovered; but one of these had for years been known to the astronomer
as a solar and suspected as a stellar element, and named helium because
of its abundance in the sun. The spectroscope had reached out millions
of miles into space and brought back this new element, and it took the
chemist a score of years to discover that he had all along had samples
of the same substance unrecognized in his sublunary laboratory. There
is hardly a more picturesque fact than that in the entire history of
science.
But the identity in substance of earth and sun and stars was not more
clearly shown than the diversity of their existing physical conditions.
It was seen that sun and stars, far from being the cool, earthlike,
habitable bodies that Herschel thought them (surrounded by glowing
clouds, and protected from undue heat by other clouds), are in truth
seething caldrons of fiery liquid, or gas made viscid by condensation,
with lurid envelopes of belching flames. It was soon made clear, also,
particularly by the studies of Rutherfurd and of Secchi, that stars
differ among themselves in exact constitution or condition. There are
white or Sirian stars, whose spectrum revels in the lines of hydrogen;
yellow or solar stars (our sun being the type), showing various metallic
vapors; and sundry red stars, with banded spectra indicative of carbon
compounds; besides the purely gaseous stars of more recent discovery,
which Professor Pickering had specially studied. Zollner's famous
interpretation of these diversities, as indicative of varying stages
of cooling, has been called in question as to the exact sequence it
postulates, but the general proposition that stars exist under widely
varying conditions of temperature is hardly in dispute.
The assumption that different star types mark varying stages of cooling
has the further support of modern physics, which has been unable to
demonstrate any way in which the sun's radiated energy may be restored,
or otherwise made perpetual, since meteoric impact has been shown to
be--under existing conditions, at any rate--inadequate. In accordance
with the theory of Helmholtz, the chief supply of solar energy is held
to be contraction of the solar mass itself; and plainly this must
have its limits. Therefore, unless some means as yet unrecognized is
restoring the lost energy to the stellar bodies, each of them must
gradually lose its lustre, and come to a condition of solidification,
seeming sterility, and frigid darkness. In the case of our own
particular star, according to the estimate of Lord Kelvin, such a
culmination appears likely to occur within a period of five or six
million years.
The Astronomy of the Invisible
But by far the strongest support of such a forecast as this is furnished
by those stellar bodies which even now appear to have cooled to the
final stage of star development and ceased to shine. Of this class
examples in miniature are furnished by the earth and the smaller of its
companion planets. But there are larger bodies of the same type out
in stellar space--veritable "dark stars"--invisible, of course, yet
nowadays clearly recognized.
The opening up of this "astronomy of the invisible" is another of the
great achievements of the nineteenth century, and again it is Bessel
to whom the honor of discovery is due. While testing his stars
for parallax; that astute observer was led to infer, from certain
unexplained aberrations of motion, that various stars, Sirius himself
among the number, are accompanied by invisible companions, and in
1840 he definitely predicated the existence of such "dark stars." The
correctness of the inference was shown twenty years later, when Alvan
Clark, Jr., the American optician, while testing a new lens, discovered
the companion of Sirius, which proved thus to be faintly luminous. Since
then the existence of other and quite invisible star companions has been
proved incontestably, not merely by renewed telescopic observations, but
by the curious testimony of the ubiquitous spectroscope.
One of the most surprising accomplishments of that instrument is the
power to record the flight of a luminous object directly in the line of
vision. If the luminous body approaches swiftly, its Fraunhofer lines
are shifted from their normal position towards the violet end of the
spectrum; if it recedes, the lines shift in the opposite direction. The
actual motion of stars whose distance is unknown may be measured in this
way. But in certain cases the light lines are seen to oscillate on the
spectrum at regular intervals. Obviously the star sending such light
is alternately approaching and receding, and the inference that it is
revolving about a companion is unavoidable. From this extraordinary test
the orbital distance, relative mass, and actual speed of revolution of
the absolutely invisible body may be determined. Thus the spectroscope,
which deals only with light, makes paradoxical excursions into the
realm of the invisible. What secrets may the stars hope to conceal when
questioned by an instrument of such necromantic power?
But the spectroscope is not alone in this audacious assault upon the
strongholds of nature. It has a worthy companion and assistant in
the photographic film, whose efficient aid has been invoked by the
astronomer even more recently. Pioneer work in celestial photography
was, indeed, done by Arago in France and by the elder Draper in America
in 1839, but the results then achieved were only tentative, and it was
not till forty years later that the method assumed really important
proportions. In 1880, Dr. Henry Draper, at Hastings-on-the-Hudson, made
the first successful photograph of a nebula. Soon after, Dr. David
Gill, at the Cape observatory, made fine photographs of a comet, and the
flecks of starlight on his plates first suggested the possibilities of
this method in charting the heavens.
Since then star-charting with the film has come virtually to supersede
the old method. A concerted effort is being made by astronomers in
various parts of the world to make a complete chart of the heavens, and
before the close of our century this work will be accomplished, some
fifty or sixty millions of visible stars being placed on record with a
degree of accuracy hitherto unapproachable. Moreover, other millions of
stars are brought to light by the negative, which are too distant or
dim to be visible with any telescopic powers yet attained--a fact
which wholly discredits all previous inferences as to the limits of
our sidereal system. Hence, notwithstanding the wonderful instrumental
advances of the nineteenth century, knowledge of the exact form and
extent of our universe seems more unattainable than it seemed a century
ago.
The Structure of Nebulae
Yet the new instruments, while leaving so much untold, have revealed
some vastly important secrets of cosmic structure. In particular, they
have set at rest the long-standing doubts as to the real structure and
position of the mysterious nebulae--those lazy masses, only two or
three of them visible to the unaided eye, which the telescope reveals
in almost limitless abundance, scattered everywhere among the stars,
but grouped in particular about the poles of the stellar stream or disk
which we call the Milky Way.
Herschel's later view, which held that some at least of the nebulae are
composed of a "shining fluid," in process of condensation to form stars,
was generally accepted for almost half a century. But in 1844, when
Lord Rosse's great six-foot reflector--the largest telescope ever yet
constructed--was turned on the nebulae, it made this hypothesis seem
very doubtful. Just as Galileo's first lens had resolved the Milky Way
into stars, just as Herschel had resolved nebulae that resisted all
instruments but his own, so Lord Rosse's even greater reflector resolved
others that would not yield to Herschel's largest mirror. It seemed
a fair inference that with sufficient power, perhaps some day to be
attained, all nebulae would yield, hence that all are in reality what
Herschel had at first thought them--vastly distant "island universes,"
composed of aggregations of stars, comparable to our own galactic
system.
But the inference was wrong; for when the spectroscope was first applied
to a nebula in 1864, by Dr. Huggins, it clearly showed the spectrum not
of discrete stars, but of a great mass of glowing gases, hydrogen among
others. More extended studies showed, it is true, that some nebulae give
the continuous spectrum of solids or liquids, but the different types
intermingle and grade into one another. Also, the closest affinity
is shown between nebulae and stars. Some nebulae are found to contain
stars, singly or in groups, in their actual midst; certain condensed
"planetary" nebulae are scarcely to be distinguished from stars of the
gaseous type; and recently the photographic film has shown the presence
of nebulous matter about stars that to telescopic vision differ in no
respect from the generality of their fellows in the galaxy. The familiar
stars of the Pleiades cluster, for example, appear on the negative
immersed in a hazy blur of light. All in all, the accumulated
impressions of the photographic film reveal a prodigality of nebulous
matter in the stellar system not hitherto even conjectured.
And so, of course, all question of "island universes" vanishes, and the
nebulae are relegated to their true position as component parts of the
one stellar system--the one universe--that is open to present human
inspection. And these vast clouds of world-stuff have been found by
Professor Keeler, of the Lick observatory, to be floating through space
at the starlike speed of from ten to thirty-eight miles per second.
The linking of nebulae with stars, so clearly evidenced by all these
modern observations, is, after all, only the scientific corroboration of
what the elder Herschel's later theories affirmed. But the nebulae have
other affinities not until recently suspected; for the spectra of some
of them are practically identical with the spectra of certain comets.
The conclusion seems warranted that comets are in point of fact minor
nebulae that are drawn into our system; or, putting it otherwise, that
the telescopic nebulae are simply gigantic distant comets.
Lockyer's Meteoric Hypothesis
Following up the surprising clews thus suggested, Sir Norman Lockyer,
of London, has in recent years elaborated what is perhaps the most
comprehensive cosmogonic guess that has ever been attempted. His theory,
known as the "meteoric hypothesis," probably bears the same relation
to the speculative thought of our time that the nebular hypothesis of
Laplace bore to that of the eighteenth century. Outlined in a few words,
it is an attempt to explain all the major phenomena of the universe
as due, directly or indirectly, to the gravitational impact of such
meteoric particles, or specks of cosmic dust, as comets are composed
of. Nebulae are vast cometary clouds, with particles more or less widely
separated, giving off gases through meteoric collisions, internal or
external, and perhaps glowing also with electrical or phosphorescent
light. Gravity eventually brings the nebular particles into closer
aggregations, and increased collisions finally vaporize the entire mass,
forming planetary nebulae and gaseous stars. Continued condensation
may make the stellar mass hotter and more luminous for a time, but
eventually leads to its liquefaction, and ultimate consolidation--the
aforetime nebulae becoming in the end a dark or planetary star.
The exact correlation which Lockyer attempts to point out between
successive stages of meteoric condensation and the various types of
observed stellar bodies does not meet with unanimous acceptance. Mr.
Ranyard, for example, suggests that the visible nebulae may not be
nascent stars, but emanations from stars, and that the true pre-stellar
nebulae are invisible until condensed to stellar proportions. But such
details aside, the broad general hypothesis that all the bodies of the
universe are, so to speak, of a single species--that nebulae (including
comets), stars of all types, and planets, are but varying stages in the
life history of a single race or type of cosmic organisms--is accepted
by the dominant thought of our time as having the highest warrant of
scientific probability.
All this, clearly, is but an amplification of that nebular hypothesis
which, long before the spectroscope gave us warrant to accurately judge
our sidereal neighbors, had boldly imagined the development of stars out
of nebulae and of planets out of stars. But Lockyer's hypothesis does
not stop with this. Having traced the developmental process from the
nebular to the dark star, it sees no cause to abandon this dark star to
its fate by assuming, as the original speculation assumed, that this is
a culminating and final stage of cosmic existence. For the dark star,
though its molecular activities have come to relative stability and
impotence, still retains the enormous potentialities of molar motion;
and clearly, where motion is, stasis is not. Sooner or later, in its
ceaseless flight through space, the dark star must collide with some
other stellar body, as Dr. Croll imagines of the dark bodies which his
"pre-nebular theory" postulates. Such collision may be long delayed; the
dark star may be drawn in comet-like circuit about thousands of other
stellar masses, and be hurtled on thousands of diverse parabolic or
elliptical orbits, before it chances to collide--but that matters not:
"billions are the units in the arithmetic of eternity," and sooner
or later, we can hardly doubt, a collision must occur. Then without
question the mutual impact must shatter both colliding bodies into
vapor, or vapor combined with meteoric fragments; in short, into a
veritable nebula, the matrix of future worlds. Thus the dark star, which
is the last term of one series of cosmic changes, becomes the first term
of another series--at once a post-nebular and a pre-nebular condition;
and the nebular hypothesis, thus amplified, ceases to be a mere linear
scale, and is rounded out to connote an unending series of cosmic
cycles, more nearly satisfying the imagination.
In this extended view, nebulae and luminous stars are but the infantile
and adolescent stages of the life history of the cosmic individual; the
dark star, its adult stage, or time of true virility. Or we may think of
the shrunken dark star as the germ-cell, the pollen-grain, of the cosmic
organism. Reduced in size, as becomes a germ-cell, to a mere fraction
of the nebular body from which it sprang, it yet retains within
its seemingly non-vital body all the potentialities of the original
organism, and requires only to blend with a fellow-cell to bring a new
generation into being. Thus may the cosmic race, whose aggregate census
makes up the stellar universe, be perpetuated--individual solar systems,
such as ours, being born, and growing old, and dying to live again in
their descendants, while the universe as a whole maintains its unified
integrity throughout all these internal mutations--passing on, it may
be, by infinitesimal stages, to a culmination hopelessly beyond human
comprehension.
III. THE NEW SCIENCE OF PALEONTOLOGY
WILLIAM SMITH AND FOSSIL SHELLS
Ever since Leonardo da Vinci first recognized the true character of
fossils, there had been here and there a man who realized that the
earth's rocky crust is one gigantic mausoleum. Here and there a
dilettante had filled his cabinets with relics from this monster crypt;
here and there a philosopher had pondered over them--questioning whether
perchance they had once been alive, or whether they were not mere
abortive souvenirs of that time when the fertile matrix of the earth was
supposed to have
"teemed at a birth
Innumerous living creatures, perfect forms,
Limbed and full grown."
Some few of these philosophers--as Robert Hooke and Steno in the
seventeenth century, and Moro, Leibnitz, Buffon, Whitehurst, Werner,
Hutton, and others in the eighteenth--had vaguely conceived the
importance of fossils as records of the earth's ancient history, but the
wisest of them no more suspected the full import of the story written
in the rocks than the average stroller in a modern museum suspects the
meaning of the hieroglyphs on the case of a mummy.
It was not that the rudiments of this story are so very hard to
decipher--though in truth they are hard enough--but rather that the
men who made the attempt had all along viewed the subject through an
atmosphere of preconception, which gave a distorted image. Before this
image could be corrected it was necessary that a man should appear who
could see without prejudice, and apply sound common-sense to what he
saw. And such a man did appear towards the close of the century, in the
person of William Smith, the English surveyor. He was a self-taught man,
and perhaps the more independent for that, and he had the gift, besides
his sharp eyes and receptive mind, of a most tenacious memory. By
exercising these faculties, rare as they are homely, he led the way to
a science which was destined, in its later developments, to shake the
structure of established thought to its foundations.
Little enough did William Smith suspect, however, that any such dire
consequences were to come of his act when he first began noticing the
fossil shells that here and there are to be found in the stratified
rocks and soils of the regions over which his surveyor's duties led him.
Nor, indeed, was there anything of such apparent revolutionary character
in the facts which he unearthed; yet in their implications these facts
were the most disconcerting of any that had been revealed since the days
of Copernicus and Galileo. In its bald essence, Smith's discovery was
simply this: that the fossils in the rocks, instead of being scattered
haphazard, are arranged in regular systems, so that any given stratum
of rock is labelled by its fossil population; and that the order of
succession of such groups of fossils is always the same in any vertical
series of strata in which they occur. That is to say, if fossil A
underlies fossil B in any given region, it never overlies it in any
other series; though a kind of fossils found in one set of strata may
be quite omitted in another. Moreover, a fossil once having disappeared
never reappears in any later stratum.
From these novel facts Smith drew the commonsense inference that the
earth had had successive populations of creatures, each of which in
its turn had become extinct. He partially verified this inference by
comparing the fossil shells with existing species of similar orders,
and found that such as occur in older strata of the rocks had no
counterparts among living species. But, on the whole, being eminently
a practical man, Smith troubled himself but little about the inferences
that might be drawn from his facts. He was chiefly concerned in using
the key he had discovered as an aid to the construction of the first
geological map of England ever attempted, and he left to others the
untangling of any snarls of thought that might seem to arise from his
discovery of the succession of varying forms of life on the globe.
He disseminated his views far and wide, however, in the course of his
journeyings--quite disregarding the fact that peripatetics went out of
fashion when the printing-press came in--and by the beginning of the
nineteenth century he had begun to have a following among the geologists
of England. It must not for a moment be supposed, however, that his
contention regarding the succession of strata met with immediate or
general acceptance. On the contrary, it was most bitterly antagonized.
For a long generation after the discovery was made, the generality of
men, prone as always to strain at gnats and swallow camels, preferred to
believe that the fossils, instead of being deposited in successive ages,
had been swept all at once into their present positions by the current
of a mighty flood--and that flood, needless to say, the Noachian deluge.
Just how the numberless successive strata could have been laid down
in orderly sequence to the depth of several miles in one such fell
cataclysm was indeed puzzling, especially after it came to be admitted
that the heaviest fossils were not found always at the bottom; but to
doubt that this had been done in some way was rank heresy in the early
days of the nineteenth century.
CUVIER AND FOSSIL VERTEBRATES
But once discovered, William Smith's unique facts as to the succession
of forms in the rocks would not down. There was one most vital point,
however, regarding which the inferences that seem to follow from
these facts needed verification--the question, namely, whether the
disappearance of a fauna from the register in the rocks really implies
the extinction of that fauna. Everything really depended upon the answer
to that question, and none but an accomplished naturalist could answer
it with authority. Fortunately, the most authoritative naturalist of the
time, George Cuvier, took the question in hand--not, indeed, with the
idea of verifying any suggestion of Smith's, but in the course of his
own original studies--at the very beginning of the century, when Smith's
views were attracting general attention.
Cuvier and Smith were exact contemporaries, both men having been born in
1769, that "fertile year" which gave the world also Chateaubriand, Von
Humboldt, Wellington, and Napoleon. But the French naturalist was of
very different antecedents from the English surveyor. He was brilliantly
educated, had early gained recognition as a scientist, and while yet a
young man had come to be known as the foremost comparative anatomist of
his time. It was the anatomical studies that led him into the realm of
fossils. Some bones dug out of the rocks by workmen in a quarry were
brought to his notice, and at once his trained eye told him that they
were different from anything he had seen before. Hitherto such bones,
when not entirely ignored, had been for the most part ascribed to
giants of former days, or even to fallen angels. Cuvier soon showed
that neither giants nor angels were in question, but elephants of an
unrecognized species. Continuing his studies, particularly with material
gathered from gypsum beds near Paris, he had accumulated, by the
beginning of the nineteenth century, bones of about twenty-five species
of animals that he believed to be different from any now living on the
globe.
The fame of these studies went abroad, and presently fossil bones poured
in from all sides, and Cuvier's conviction that extinct forms of animals
are represented among the fossils was sustained by the evidence of many
strange and anomalous forms, some of them of gigantic size. In 1816
the famous Ossements Fossiles, describing these novel objects, was
published, and vertebrate paleontology became a science. Among
other things of great popular interest the book contained the first
authoritative description of the hairy elephant, named by Cuvier the
mammoth, the remains of which bad been found embedded in a mass of
ice in Siberia in 1802, so wonderfully preserved that the dogs of the
Tungusian fishermen actually ate its flesh. Bones of the same species
had been found in Siberia several years before by the naturalist Pallas,
who had also found the carcass of a rhinoceros there, frozen in a
mud-bank; but no one then suspected that these were members of an
extinct population--they were supposed to be merely transported relics
of the flood.
Cuvier, on the other hand, asserted that these and the other creatures
he described had lived and died in the region where their remains were
found, and that most of them have no living representatives upon the
globe. This, to be sure, was nothing more than William Smith had tried
all along to establish regarding lower forms of life; but flesh and
blood monsters appeal to the imagination in a way quite beyond the power
of mere shells; so the announcement of Cuvier's discoveries aroused the
interest of the entire world, and the Ossements Fossiles was accorded a
popular reception seldom given a work of technical science--a reception
in which the enthusiastic approval of progressive geologists was mingled
with the bitter protests of the conservatives.
"Naturalists certainly have neither explored all the continents," said
Cuvier, "nor do they as yet even know all the quadrupeds of those parts
which have been explored. New species of this class are discovered from
time to time; and those who have not examined with attention all the
circumstances belonging to these discoveries may allege also that the
unknown quadrupeds, whose fossil bones have been found in the strata
of the earth, have hitherto remained concealed in some islands not yet
discovered by navigators, or in some of the vast deserts which occupy
the middle of Africa, Asia, the two Americas, and New Holland.
"But if we carefully attend to the kind of quadrupeds that have been
recently discovered, and to the circumstances of their discovery, we
shall easily perceive that there is very little chance indeed of our
ever finding alive those which have only been seen in a fossil state.
"Islands of moderate size, and at a considerable distance from the large
continents, have very few quadrupeds. These must have been carried
to them from other countries. Cook and Bougainville found no other
quadrupeds besides hogs and dogs in the South Sea Islands; and the
largest quadruped of the West India Islands, when first discovered, was
the agouti, a species of the cavy, an animal apparently between the rat
and the rabbit.
"It is true that the great continents, as Asia, Africa, the two
Americas, and New Holland, have large quadrupeds, and, generally
speaking, contain species common to each; insomuch, that upon
discovering countries which are isolated from the rest of the world,
the animals they contain of the class of quadruped were found entirely
different from those which existed in other countries. Thus, when the
Spaniards first penetrated into South America, they did not find it to
contain a single quadruped exactly the same with those of Europe, Asia,
and Africa. The puma, the jaguar, the tapir, the capybara, the llama,
or glama, and vicuna, and the whole tribe of sapajous, were to them
entirely new animals, of which they had not the smallest idea....
"If there still remained any great continent to be discovered, we
might perhaps expect to be made acquainted with new species of large
quadrupeds, among which some might be found more or less similar to
those of which we find the exuviae in the bowels of the earth. But it
is merely sufficient to glance the eye over the maps of the world and
observe the innumerable directions in which navigators have traversed
the ocean, in order to be satisfied that there does not remain any large
land to be discovered, unless it may be situated towards the Antarctic
Pole, where eternal ice necessarily forbids the existence of animal
life."(1)
Cuvier then points out that the ancients were well acquainted with
practically all the animals on the continents of Europe, Asia, and
Africa now known to scientists. He finds little grounds, therefore, for
belief in the theory that at one time there were monstrous animals on
the earth which it was necessary to destroy in order that the present
fauna and men might flourish. After reviewing these theories and beliefs
in detail, he takes up his Inquiry Respecting the Fabulous Animals
of the Ancients. "It is easy," he says, "to reply to the foregoing
objections, by examining the descriptions that are left us by the
ancients of those unknown animals, and by inquiring into their origins.
Now that the greater number of these animals have an origin, the
descriptions given of them bear the most unequivocal marks; as in almost
all of them we see merely the different parts of known animals united by
an unbridled imagination, and in contradiction to every established law
of nature."(2)
Having shown how the fabulous monsters of ancient times and of foreign
nations, such as the Chinese, were simply products of the imagination,
having no prototypes in nature, Cuvier takes up the consideration of the
difficulty of distinguishing the fossil bones of quadrupeds.
We shall have occasion to revert to this part of Cuvier's paper in
another connection. Here it suffices to pass at once to the final
conclusion that the fossil bones in question are the remains of an
extinct fauna, the like of which has no present-day representation on
the earth. Whatever its implications, this conclusion now seemed to
Cuvier to be fully established.
In England the interest thus aroused was sent to fever-heat in 1821 by
the discovery of abundant beds of fossil bones in the stalagmite-covered
floor of a cave at Kirkdale, Yorkshire which went to show that England,
too, had once had her share of gigantic beasts. Dr. Buckland, the
incumbent of the chair of geology at Oxford, and the most authoritative
English geologist of his day, took these finds in hand and showed that
the bones belonged to a number of species, including such alien forms as
elephants, rhinoceroses, hippopotami, and hyenas. He maintained that all
of these creatures had actually lived in Britain, and that the caves in
which their bones were found had been the dens of hyenas.
The claim was hotly disputed, as a matter of course. As late as 1827
books were published denouncing Buckland, doctor of divinity though he
was, as one who had joined in an "unhallowed cause," and reiterating the
old cry that the fossils were only remains of tropical species washed
thither by the deluge. That they were found in solid rocks or in caves
offered no difficulty, at least not to the fertile imagination of
Granville Penn, the leader of the conservatives, who clung to the old
idea of Woodward and Cattcut that the deluge had dissolved the entire
crust of the earth to a paste, into which the relics now called fossils
had settled. The caves, said Mr. Penn, are merely the result of gases
given off by the carcasses during decomposition--great air-bubbles, so
to speak, in the pasty mass, becoming caverns when the waters receded
and the paste hardened to rocky consistency.
But these and such-like fanciful views were doomed even in the day of
their utterance. Already in 1823 other gigantic creatures, christened
ichthyosaurus and plesiosaurus by Conybeare, had been found in deeper
strata of British rocks; and these, as well as other monsters whose
remains were unearthed in various parts of the world, bore such strange
forms that even the most sceptical could scarcely hope to find their
counterparts among living creatures. Cuvier's contention that all the
larger vertebrates of the existing age are known to naturalists was
borne out by recent explorations, and there seemed no refuge from the
conclusion that the fossil records tell of populations actually extinct.
But if this were admitted, then Smith's view that there have been
successive rotations of population could no longer be denied. Nor could
it be in doubt that the successive faunas, whose individual remains have
been preserved in myriads, representing extinct species by thousands
and tens of thousands, must have required vast periods of time for the
production and growth of their countless generations.
As these facts came to be generally known, and as it came to be
understood in addition that the very matrix of the rock in which fossils
are imbedded is in many cases one gigantic fossil, composed of the
remains of microscopic forms of life, common-sense, which, after all,
is the final tribunal, came to the aid of belabored science. It was
conceded that the only tenable interpretation of the record in the rocks
is that numerous populations of creatures, distinct from one another and
from present forms, have risen and passed away; and that the geologic
ages in which these creatures lived were of inconceivable length. The
rank and file came thus, with the aid of fossil records, to realize
the import of an idea which James Hutton, and here and there another
thinker, had conceived with the swift intuition of genius long
before the science of paleontology came into existence. The Huttonian
proposition that time is long had been abundantly established, and by
about the close of the first third of the last century geologists had
begun to speak of "ages" and "untold aeons of time" with a familiarity
which their predecessors had reserved for days and decades.
CHARLES LYELL COMBATS CATASTROPHISM
And now a new question pressed for solution. If the earth has been
inhabited by successive populations of beings now extinct, how have
all these creatures been destroyed? That question, however, seemed to
present no difficulties. It was answered out of hand by the application
of an old idea. All down the centuries, whatever their varying phases of
cosmogonic thought, there had been ever present the idea that past times
were not as recent times; that in remote epochs the earth had been the
scene of awful catastrophes that have no parallel in "these degenerate
days." Naturally enough, this thought, embalmed in every cosmogonic
speculation of whatever origin, was appealed to in explanation of the
destruction of these hitherto unimagined hosts, which now, thanks to
science, rose from their abysmal slumber as incontestable, but also as
silent and as thought-provocative, as Sphinx or pyramid. These ancient
hosts, it was said, have been exterminated at intervals of odd millions
of years by the recurrence of catastrophes of which the Mosaic deluge is
the latest, but perhaps not the last.
This explanation had fullest warrant of scientific authority. Cuvier had
prefaced his classical work with a speculative disquisition whose
very title (Discours sur les Revolutions du Globe) is ominous of
catastrophism, and whose text fully sustains the augury. And Buckland,
Cuvier's foremost follower across the Channel, had gone even beyond
the master, naming the work in which he described the Kirkdale fossils,
Reliquiae Diluvianae, or Proofs of a Universal Deluge.
Both these authorities supposed the creatures whose remains they studied
to have perished suddenly in the mighty flood whose awful current, as
they supposed, gouged out the modern valleys and hurled great blocks of
granite broadcast over the land. And they invoked similar floods for the
extermination of previous populations.
It is true these scientific citations had met with only qualified
approval at the time of their utterance, because then the conservative
majority of mankind did not concede that there had been a plurality of
populations or revolutions; but now that the belief in past geologic
ages had ceased to be a heresy, the recurring catastrophes of the great
paleontologists were accepted with acclaim. For the moment science and
tradition were at one, and there was a truce to controversy, except
indeed in those outlying skirmish-lines of thought whither news from
headquarters does not permeate till it has become ancient history at its
source.
The truce, however, was not for long. Hardly had contemporary
thought begun to adjust itself to the conception of past ages of
incomprehensible extent, each terminated by a catastrophe of the
Noachian type, when a man appeared who made the utterly bewildering
assertion that the geological record, instead of proving numerous
catastrophic revolutions in the earth's past history, gives no warrant
to the pretensions of any universal catastrophe whatever, near or
remote.
This iconoclast was Charles Lyell, the Scotchman, who was soon to be
famous as the greatest geologist of his time. As a young man he had
become imbued with the force of the Huttonian proposition, that present
causes are one with those that produced the past changes of the
globe, and he carried that idea to what he conceived to be its logical
conclusion. To his mind this excluded the thought of catastrophic
changes in either inorganic or organic worlds.
But to deny catastrophism was to suggest a revolution in current
thought. Needless to say, such revolution could not be effected without
a long contest. For a score of years the matter was argued pro and con.,
often with most unscientific ardor. A mere outline of the controversy
would fill a volume; yet the essential facts with which Lyell at last
established his proposition, in its bearings on the organic world, may
be epitomized in a few words. The evidence which seems to tell of past
revolutions is the apparently sudden change of fossils from one stratum
to another of the rocks. But Lyell showed that this change is not always
complete. Some species live on from one alleged epoch into the next. By
no means all the contemporaries of the mammoth are extinct, and numerous
marine forms vastly more ancient still have living representatives.
Moreover, the blanks between strata in any particular vertical series
are amply filled in with records in the form of thick strata in some
geographically distant series. For example, in some regions Silurian
rocks are directly overlaid by the coal measures; but elsewhere this
sudden break is filled in with the Devonian rocks that tell of a great
"age of fishes." So commonly are breaks in the strata in one region
filled up in another that we are forced to conclude that the
record shown by any single vertical series is of but local
significance--telling, perhaps, of a time when that particular sea-bed
oscillated above the water-line, and so ceased to receive sediment until
some future age when it had oscillated back again. But if this be
the real significance of the seemingly sudden change from stratum to
stratum, then the whole case for catastrophism is hopelessly lost; for
such breaks in the strata furnish the only suggestion geology can offer
of sudden and catastrophic changes of wide extent.
Let us see how Lyell elaborates these ideas, particularly with reference
to the rotation of species.(2)
"I have deduced as a corollary," he says, "that the species existing at
any particular period must, in the course of ages, become extinct, one
after the other. 'They must die out,' to borrow an emphatic expression
from Buffon, 'because Time fights against them.' If the views which I
have taken are just, there will be no difficulty in explaining why
the habitations of so many species are now restrained within exceeding
narrow limits. Every local revolution tends to circumscribe the range
of some species, while it enlarges that of others; and if we are led
to infer that new species originate in one spot only, each must require
time to diffuse itself over a wide area. It will follow, therefore, from
the adoption of our hypothesis that the recent origin of some species
and the high antiquity of others are equally consistent with the general
fact of their limited distribution, some being local because they have
not existed long enough to admit of their wide dissemination; others,
because circumstances in the animate or inanimate world have occurred to
restrict the range within which they may once have obtained....
"If the reader should infer, from the facts laid before him, that the
successive extinction of animals and plants may be part of the constant
and regular course of nature, he will naturally inquire whether there
are any means provided for the repair of these losses? Is it possible as
a part of the economy of our system that the habitable globe should to a
certain extent become depopulated, both in the ocean and on the land, or
that the variety of species should diminish until some new era arrives
when a new and extraordinary effort of creative energy is to be
displayed? Or is it possible that new species can be called into being
from time to time, and yet that so astonishing a phenomenon can escape
the naturalist?
"In the first place, it is obviously more easy to prove that a species
once numerously represented in a given district has ceased to be
than that some other which did not pre-exist had made its
appearance--assuming always, for reasons before stated, that single
stocks only of each animal and plant are originally created, and that
individuals of new species did not suddenly start up in many different
places at once.
"So imperfect has the science of natural history remained down to our
own times that, within the memory of persons now living, the numbers
of known animals and plants have doubled, or even quadrupled, in many
classes. New and often conspicuous species are annually discovered in
parts of the old continent long inhabited by the most civilized nations.
Conscious, therefore, of the limited extent of our information, we
always infer, when such discoveries are made, that the beings in
question bad previously eluded our research, or had at least existed
elsewhere, and only migrated at a recent period into the territories
where we now find them.
"What kind of proofs, therefore, could we reasonably expect to find of
the origin at a particular period of a new species?
"Perhaps, it may be said in reply, that within the last two or three
centuries some forest tree or new quadruped might have been observed to
appear suddenly in those parts of England or France which had been most
thoroughly investigated--that naturalists might have been able to show
that no such being inhabited any other region of the globe, and that
there was no tradition of anything similar having been observed in the
district where it had made its appearance.
"Now, although this objection may seem plausible, yet its force will be
found to depend entirely on the rate of fluctuation which we suppose
to prevail in the animal world, and on the proportions which such
conspicuous subjects of the animal and vegetable kingdoms bear to those
which are less known and escape our observation. There are perhaps
more than a million species of plants and animals, exclusive of the
microscopic and infusory animalcules, now inhabiting the terraqueous
globe, so that if only one of these were to become extinct annually, and
one new one were to be every year called into being, much more than a
million of years might be required to bring about a complete revolution
of organic life.
"I am not hazarding at present any hypothesis as to the probable rate
of change, but none will deny that when the annual birth and the annual
death of one species on the globe is proposed as a mere speculation,
this, at least, is to imagine no slight degree of instability in the
animate creation. If we divide the surface of the earth into twenty
regions of equal area, one of these might comprehend a space of land and
water about equal in dimensions to Europe, and might contain a twentieth
part of the million of species which may be assumed to exist in the
animal kingdom. In this region one species only could, according to the
rate of mortality before assumed, perish in twenty years, or only five
out of fifty thousand in the course of a century. But as a considerable
portion of the whole world belongs to the aquatic classes, with which
we have a very imperfect acquaintance, we must exclude them from our
consideration, and, if they constitute half of the entire number, then
one species only might be lost in forty years among the terrestrial
tribes. Now the mammalia, whether terrestrial or aquatic, bear so small
a proportion to other classes of animals, forming less, perhaps, than
a thousandth part of a whole, that, if the longevity of species in the
different orders were equal, a vast period must elapse before it would
come to the turn of this conspicuous class to lose one of their number.
If one species only of the whole animal kingdom died out in forty years,
no more than one mammifer might disappear in forty thousand years, in a
region of the dimensions of Europe.
"It is easy, therefore, to see that in a small portion of such an area,
in countries, for example, of the size of England and France, periods
of much greater duration must elapse before it would be possible
to authenticate the first appearance of one of the larger plants or
animals, assuming the annual birth and death of one species to be the
rate of vicissitude in the animal creation throughout the world."(3)
In a word, then, said Lyell, it becomes clear that the numberless
species that have been exterminated in the past have died out one by
one, just as individuals of a species die, not in vast shoals; if
whole populations have passed away, it has been not by instantaneous
extermination, but by the elimination of a species now here, now there,
much as one generation succeeds another in the life history of any
single species. The causes which have brought about such gradual
exterminations, and in the long lapse of ages have resulted in rotations
of population, are the same natural causes that are still in operation.
Species have died out in the past as they are dying out in the present,
under influence of changed surroundings, such as altered climate, or
the migration into their territory of more masterful species. Past and
present causes are one--natural law is changeless and eternal.
Such was the essence of the Huttonian doctrine, which Lyell adopted and
extended, and with which his name will always be associated. Largely
through his efforts, though of course not without the aid of many other
workers after a time, this idea--the doctrine of uniformitarianism, it
came to be called--became the accepted dogma of the geologic world not
long after the middle of the nineteenth century. The catastrophists,
after clinging madly to their phantom for a generation, at last
capitulated without terms: the old heresy became the new orthodoxy, and
the way was paved for a fresh controversy.
THE ORIGIN OF SPECIES
The fresh controversy followed quite as a matter of course. For the idea
of catastrophism had not concerned the destruction of species merely,
but their introduction as well. If whole faunas had been extirpated
suddenly, new faunas had presumably been introduced with equal
suddenness by special creation; but if species die out gradually,
the introduction of new species may be presumed to be correspondingly
gradual. Then may not the new species of a later geological epoch be
the modified lineal descendants of the extinct population of an earlier
epoch?
The idea that such might be the case was not new. It had been suggested
when fossils first began to attract conspicuous attention; and such
sagacious thinkers as Buffon and Kant and Goethe and Erasmus Darwin
had been disposed to accept it in the closing days of the eighteenth
century. Then, in 1809, it had been contended for by one of the early
workers in systematic paleontology--Jean Baptiste Lamarck, who
had studied the fossil shells about Paris while Cuvier studied the
vertebrates, and who had been led by these studies to conclude that
there had been not merely a rotation but a progression of life on the
globe. He found the fossil shells--the fossils of invertebrates, as he
himself had christened them--in deeper strata than Cuvier's vertebrates;
and he believed that there had been long ages when no higher forms than
these were in existence, and that in successive ages fishes, and then
reptiles, had been the highest of animate creatures, before mammals,
including man, appeared. Looking beyond the pale of his bare facts,
as genius sometimes will, he had insisted that these progressive
populations had developed one from another, under influence of changed
surroundings, in unbroken series.
Of course such a thought as this was hopelessly misplaced in a
generation that doubted the existence of extinct species, and hardly
less so in the generation that accepted catastrophism; but it had been
kept alive by here and there an advocate like Geoffrey Saint-Hilaire,
and now the banishment of catastrophism opened the way for its more
respectful consideration. Respectful consideration was given it by Lyell
in each recurring edition of his Principles, but such consideration led
to its unqualified rejection. In its place Lyell put forward a modified
hypothesis of special creation. He assumed that from time to time,
as the extirpation of a species had left room, so to speak, for a new
species, such new species had been created de novo; and he supposed that
such intermittent, spasmodic impulses of creation manifest themselves
nowadays quite as frequently as at any time in the past. He did not say
in so many words that no one need be surprised to-day were he to see a
new species of deer, for example, come up out of the ground before him,
"pawing to get free," like Milton's lion, but his theory implied as
much. And that theory, let it be noted, was not the theory of Lyell
alone, but of nearly all his associates in the geologic world. There is
perhaps no other fact that will bring home to one so vividly the advance
in thought of our own generation as the recollection that so crude, so
almost unthinkable a conception could have been the current doctrine of
science less than half a century ago.
This theory of special creation, moreover, excluded the current doctrine
of uniformitarianism as night excludes day, though most thinkers of the
time did not seem to be aware of the incompatibility of the two ideas.
It may be doubted whether even Lyell himself fully realized it. If he
did, he saw no escape from the dilemma, for it seemed to him that
the record in the rocks clearly disproved the alternative Lamarckian
hypothesis. And almost with one accord the paleontologists of the
time sustained the verdict. Owen, Agassiz, Falconer, Barrande, Pictet,
Forbes, repudiated the idea as unqualifiedly as their great predecessor
Cuvier had done in the earlier generation. Some of them did, indeed,
come to believe that there is evidence of a progressive development of
life in the successive ages, but no such graded series of fossils had
been discovered as would give countenance to the idea that one species
had ever been transformed into another. And to nearly every one this
objection seemed insuperable.
But in 1859 appeared a book which, though not dealing primarily with
paleontology, yet contained a chapter that revealed the geological
record in an altogether new light. The book was Charles Darwin's Origin
of Species, the chapter that wonderful citation of the "Imperfections of
the Geological Record." In this epoch-making chapter Darwin shows what
conditions must prevail in any given place in order that fossils shall
be formed, how unusual such conditions are, and how probable it is that
fossils once imbedded in sediment of a sea-bed will be destroyed by
metamorphosis of the rocks, or by denudation when the strata are raised
above the water-level. Add to this the fact that only small territories
of the earth have been explored geologically, he says, and it becomes
clear that the paleontological record as we now possess it shows but
a mere fragment of the past history of organisms on the earth. It is
a history "imperfectly kept and written in a changing dialect. Of this
history we possess the last volume alone, relating only to two or three
countries. Of this volume only here and there a short chapter has been
preserved, and of each page only here and there a few lines." For a
paleontologist to dogmatize from such a record would be as rash, he
thinks, as "for a naturalist to land for five minutes on a barren point
of Australia and then discuss the number and range of its productions."
This citation of observations, which when once pointed out seemed almost
self-evident, came as a revelation to the geological world. In the
clarified view now possible old facts took on a new meaning. It was
recalled that Cuvier had been obliged to establish a new order for some
of the first fossil creatures he examined, and that Buckland had noted
that the nondescript forms were intermediate in structure between
allied existing orders. More recently such intermediate forms had been
discovered over and over; so that, to name but one example, Owen had
been able, with the aid of extinct species, to "dissolve by gradations
the apparently wide interval between the pig and the camel." Owen,
moreover, had been led to speak repeatedly of the "generalized forms"
of extinct animals, and Agassiz had called them "synthetic or prophetic
types," these terms clearly implying "that such forms are in fact
intermediate or connecting links." Darwin himself had shown some years
before that the fossil animals of any continent are closely related to
the existing animals of that continent--edentates predominating, for
example, in South America, and marsupials in Australia. Many observers
had noted that recent strata everywhere show a fossil fauna more nearly
like the existing one than do more ancient strata; and that fossils from
any two consecutive strata are far more closely related to each other
than are the fossils of two remote formations, the fauna of each
geological formation being, indeed, in a wide view, intermediate between
preceding and succeeding faunas.
So suggestive were all these observations that Lyell, the admitted
leader of the geological world, after reading Darwin's citations, felt
able to drop his own crass explanation of the introduction of species
and adopt the transmutation hypothesis, thus rounding out the doctrine
of uniformitarianism to the full proportions in which Lamarck had
conceived it half a century before. Not all paleontologists could follow
him at once, of course; the proof was not yet sufficiently demonstrative
for that; but all were shaken in the seeming security of their former
position, which is always a necessary stage in the progress of thought.
And popular interest in the matter was raised to white heat in a
twinkling.
So, for the third time in this first century of its existence,
paleontology was called upon to play a leading role in a controversy
whose interest extended far beyond the bounds of staid truth-seeking
science. And the controversy waged over the age of the earth had not
been more bitter, that over catastrophism not more acrimonious, than
that which now raged over the question of the transmutation of species.
The question had implications far beyond the bounds of paleontology, of
course. The main evidence yet presented had been drawn from quite other
fields, but by common consent the record in the rocks might furnish a
crucial test of the truth or falsity of the hypothesis. "He who rejects
this view of the imperfections of the geological record," said Darwin,
"will rightly reject the whole theory."
With something more than mere scientific zeal, therefore,
paleontologists turned anew to the records in the rocks, to inquire what
evidence in proof or refutation might be found in unread pages of the
"great stone book." And, as might have been expected, many minds being
thus prepared to receive new evidence, such evidence was not long
withheld.
FOSSIL MAN
Indeed, at the moment of Darwin's writing a new and very instructive
chapter of the geologic record was being presented to the public--a
chapter which for the first time brought man into the story. In 1859
Dr. Falconer, the distinguished British paleontologist, made a visit
to Abbeville, in the valley of the Somme, incited by reports that for
a decade before bad been sent out from there by M. Boucher de Perthes.
These reports had to do with the alleged finding of flint implements,
clearly the work of man, in undisturbed gravel-beds, in the midst of
fossil remains of the mammoth and other extinct animals. What Falconer
saw there and what came of his visit may best be told in his own words:
"In September of 1856 I made the acquaintance of my distinguished friend
M. Boucher de Perthes," wrote Dr. Falconer, "on the introduction of M.
Desnoyers at Paris, when he presented to me the earlier volume of his
Antiquites celtiques, etc., with which I thus became acquainted for the
first time. I was then fresh from the examination of the Indian fossil
remains of the valley of the Jumna; and the antiquity of the human race
being a subject of interest to both, we conversed freely about it,
each from a different point of view. M. de Perthes invited me to visit
Abbeville, in order to examine his antediluvian collection, fossil and
geological, gleaned from the valley of the Somme. This I was unable to
accomplish then, but I reserved it for a future occasion.
"In October, 1856, having determined to proceed to Sicily, I arranged
by correspondence with M. Boucher de Perthes to visit Abbeville on my
journey through France. I was at the time in constant communication
with Mr. Prestwich about the proofs of the antiquity of the human race
yielded by the Broxham Cave, in which he took a lively interest; and
I engaged to communicate to him the opinions at which I should arrive,
after my examination of the Abbeville collection. M. de Perthes gave me
the freest access to his materials, with unreserved explanations of all
the facts of the case that had come under his observation; and having
considered his Menchecourt Section, taken with such scrupulous care, and
identified the molars of elephas primigenius, which he had exhumed with
his own hands deep in that section, along with flint weapons, presenting
the same character as some of those found in the Broxham Cave, I arrived
at the conviction that they were of contemporaneous age, although I
was not prepared to go along with M. de Perthes in all his inferences
regarding the hieroglyphics and in an industrial interpretation of the
various other objects which he had met with."(4)
That Dr. Falconer was much impressed by the collection of M. de
Perthes is shown in a communication which he sent at once to his friend
Prestwich:
"I have been richly rewarded," he exclaims. "His collection of wrought
flint implements, and of the objects of every description associated
with them, far exceeds everything I expected to have seen, especially
from a single locality. He has made great additions, since the
publication of his first volume, in the second, which I now have by
me. He showed me flint hatchets which HE HAD DUG UP with his own hands,
mixed INDISCRIMINATELY with molars of elephas primigenius. I examined
and identified plates of the molars and the flint objects which were
got along with them. Abbeville is an out-of-the-way place, very little
visited; and the French savants who meet him in Paris laugh at Monsieur
de Perthes and his researches. But after devoting the greater part of
a day to his vast collection, I am perfectly satisfied that there is
a great deal of fair presumptive evidence in favor of many of his
speculations regarding the remote antiquity of these industrial objects
and their association with animals now extinct. M. Boucher's hotel
is, from the ground floor to garret, a continued museum, filled with
pictures, mediaeval art, and Gaulish antiquities, including antediluvian
flint-knives, fossil-bones, etc. If, during next summer, you should
happen to be paying a visit to France, let me strongly recommend you to
come to Abbeville. I am sure you would be richly rewarded."(5)
This letter aroused the interest of the English geologists, and in the
spring of 1859 Prestwich and Mr. (afterwards Sir John) Evans made a
visit to Abbeville to see the specimens and examine at first hand the
evidences as pointed out by Dr. Falconer. "The evidence yielded by the
valley of the Somme," continues Falconer, in speaking of this visit,
"was gone into with the scrupulous care and severe and exhaustive
analysis which are characteristic of Mr. Prestwich's researches. The
conclusions to which he was conducted were communicated to the Royal
Society on May 12, 1859, in his celebrated memoir, read on May 26th and
published in the Philosophical Transactions of 1860, which, in addition
to researches made in the valley of the Somme, contained an account of
similar phenomena presented by the valley of the Waveney, near Hoxne, in
Suffolk. Mr. Evans communicated to the Society of Antiquaries a memoir
on the character and geological position of the 'Flint Implements in the
Drift,' which appeared in the Archaeologia for 1860. The results arrived
at by Mr. Prestwich were expressed as follows:
"First. That the flint implements are the result of design and the work
of man.
"Second. That they are found in beds of gravel, sand, and clay, which
have never been artificially disturbed.
"Third. That they occur associated with the remains of land,
fresh-water, and marine testacea, of species now living, and most of
them still common in the same neighborhood, and also with the remains of
various mammalia--a few species now living, but more of extinct forms.
"Fourth. That the period at which their entombment took place was
subsequent to the bowlder-clay period, and to that extent post-glacial;
and also that it was among the latest in geological time--one apparently
anterior to the surface assuming its present form, so far as it regards
some of the minor features."(6)
These reports brought the subject of the very significant human fossils
at Abbeville prominently before the public; whereas the publications of
the original discoverer, Boucher de Perthes, bearing date of 1847, had
been altogether ignored. A new aspect was thus given to the current
controversy.
As Dr. Falconer remarked, geology was now passing through the same
ordeal that astronomy passed in the age of Galileo. But the times were
changed since the day when the author of the Dialogues was humbled
before the Congregation of the Index, and now no Index Librorum
Prohibitorum could avail to hide from eager human eyes such pages of
the geologic story as Nature herself had spared. Eager searchers were
turning the leaves with renewed zeal everywhere, and with no small
measure of success. In particular, interest attached just at this
time to a human skull which Dr. Fuhlrott had discovered in a cave at
Neanderthal two or three years before--a cranium which has ever since
been famous as the Neanderthal skull, the type specimen of what modern
zoologists are disposed to regard as a distinct species of man, Homo
neanderthalensis. Like others of the same type since discovered at Spy,
it is singularly simian in character--low-arched, with receding forehead
and enormous, protuberant eyebrows. When it was first exhibited to the
scientists at Berlin by Dr. Fuhlrott, in 1857, its human character was
doubted by some of the witnesses; of that, however, there is no present
question.
This interesting find served to recall with fresh significance some
observations that had been made in France and Belgium a long generation
earlier, but whose bearings had hitherto been ignored. In 1826 MM.
Tournal and Christol had made independent discoveries of what they
believed to be human fossils in the caves of the south of France; and
in 1827 Dr. Schmerling had found in the cave of Engis, in Westphalia,
fossil bones of even greater significance. Schmerling's explorations
had been made with the utmost care, and patience. At Engis he had
found human bones, including skulls, intermingled with those of extinct
mammals of the mammoth period in a way that left no doubt in his mind
that all dated from the same geological epoch. He bad published a full
account of his discoveries in an elaborate monograph issued in 1833.
But at that time, as it chanced, human fossils were under a ban as
effectual as any ever pronounced by canonical index, though of far
different origin. The oracular voice of Cuvier had declared against the
authenticity of all human fossils. Some of the bones brought him for
examination the great anatomist had pettishly pitched out of the window,
declaring them fit only for a cemetery, and that had settled the matter
for a generation: the evidence gathered by lesser workers could avail
nothing against the decision rendered at the Delphi of Science. But no
ban, scientific or canonical, can longer resist the germinative power of
a fact, and so now, after three decades of suppression, the truth which
Cuvier had buried beneath the weight of his ridicule burst its bonds,
and fossil man stood revealed, if not as a flesh-and-blood, at least as
a skeletal entity.
The reception now accorded our prehistoric ancestor by the progressive
portion of the scientific world amounted to an ovation; but the
unscientific masses, on the other hand, notwithstanding their usual
fondness for tracing remote genealogies, still gave the men of Engis
and Neanderthal the cold shoulder. Nor were all of the geologists quite
agreed that the contemporaneity of these human fossils with the animals
whose remains had been mingled with them had been fully established. The
bare possibility that the bones of man and of animals that long preceded
him had been swept together into the eaves in successive ages, and
in some mysterious way intermingled there, was clung to by the
conservatives as a last refuge. But even this small measure of security
was soon to be denied them, for in 1865 two associated workers,
M. Edouard Lartet and Mr. Henry Christy, in exploring the caves of
Dordogne, unearthed a bit of evidence against which no such objection
could be urged. This momentous exhibit was a bit of ivory, a fragment
of the tusk of a mammoth, on which was scratched a rude but unmistakable
outline portrait of the mammoth itself. If all the evidence as to man's
antiquity before presented was suggestive merely, here at last was
demonstration; for the cave-dwelling man could not well have drawn the
picture of the mammoth unless he had seen that animal, and to admit that
man and the mammoth had been contemporaries was to concede the entire
case. So soon, therefore, as the full import of this most instructive
work of art came to be realized, scepticism as to man's antiquity was
silenced for all time to come.
In the generation that has elapsed since the first drawing of the
cave-dweller artist was discovered, evidences of the wide-spread
existence of man in an early epoch have multiplied indefinitely, and
to-day the paleontologist traces the history of our race back beyond the
iron and bronze ages, through a neolithic or polished-stone age, to
a paleolithic or rough-stone age, with confidence born of unequivocal
knowledge. And he looks confidently to the future explorer of the
earth's fossil records to extend the history back into vastly more
remote epochs, for it is little doubted that paleolithic man, the most
ancient of our recognized progenitors, is a modern compared to those
generations that represented the real childhood of our race.
THE FOSSIL-BEDS OF AMERICA
Coincidently with the discovery of these highly suggestive pages of the
geologic story, other still more instructive chapters were being brought
to light in America. It was found that in the Rocky Mountain region, in
strata found in ancient lake beds, records of the tertiary period, or
age of mammals, had been made and preserved with fulness not approached
in any other region hitherto geologically explored. These records were
made known mainly by Professors Joseph Leidy, O. C. Marsh, and E. D.
Cope, working independently, and more recently by numerous younger
paleontologists.
The profusion of vertebrate remains thus brought to light quite beggars
all previous exhibits in point of mere numbers. Professor Marsh, for
example, who was first in the field, found three hundred new tertiary
species between the years 1870 and 1876. Meanwhile, in cretaceous
strata, he unearthed remains of about two hundred birds with teeth, six
hundred pterodactyls, or flying dragons, some with a spread of wings
of twenty-five feet, and one thousand five hundred mosasaurs of the
sea-serpent type, some of them sixty feet or more in length. In a single
bed of Jurassic rock, not larger than a good-sized lecture-room, he
found the remains of one hundred and sixty individuals of mammals,
representing twenty species and nine genera; while beds of the same age
have yielded three hundred reptiles, varying from the size of a rabbit
to sixty or eighty feet in length.
But the chief interest of these fossils from the West is not their
number but their nature; for among them are numerous illustrations of
just such intermediate types of organisms as must have existed in the
past if the succession of life on the globe has been an unbroken lineal
succession. Here are reptiles with bat-like wings, and others with
bird-like pelves and legs adapted for bipedal locomotion. Here are
birds with teeth, and other reptilian characters. In short, what with
reptilian birds and birdlike reptiles, the gap between modern reptiles
and birds is quite bridged over. In a similar way, various diverse
mammalian forms, as the tapir, the rhinoceros, and the horse, are linked
together by fossil progenitors. And, most important of all, Professor
Marsh has discovered a series of mammalian remains, occurring in
successive geological epochs, which are held to represent beyond cavil
the actual line of descent of the modern horse; tracing the lineage
of our one-toed species back through two and three toed forms, to an
ancestor in the eocene or early tertiary that had four functional toes
and the rudiment of a fifth. This discovery is too interesting and too
important not to be detailed at length in the words of the discoverer.
Marsh Describes the Fossil Horse
"It is a well-known fact," says Professor Marsh, "that the Spanish
discoverers of America discovered no horses on this continent, and that
the modern horse (Equus caballus, Linn.) was subsequently introduced
from the Old World. It is, however, not so generally known that these
animals had formerly been abundant here, and that long before, in
tertiary time, near relatives of the horse, and probably his ancestors,
existed in the far West in countless numbers and in a marvellous variety
of forms. The remains of equine mammals, now known from the tertiary and
quaternary deposits of this country, already represent more than double
the number of genera and species hitherto found in the strata of the
eastern hemisphere, and hence afford most important aid in tracing out
the genealogy of the horses still existing.
"The animals of this group which lived in America during the three
diversions of the tertiary period were especially numerous in the Rocky
Mountain regions, and their remains are well preserved in the old lake
basins which then covered so much of that country. The most ancient
of these lakes--which extended over a considerable part of the present
territories of Wyoming and Utah--remained so long in eocene times that
the mud and sand, slowly deposited in it, accumulated to more than a
mile in vertical thickness. In these deposits vast numbers of tropical
animals were entombed, and here the oldest equine remains occur,
four species of which have been described. These belong to the genus
Orohippus (Marsh), and are all of a diminutive size, hardly bigger than
a fox. The skeletons of these animals resemble that of the horse in many
respects, much more indeed than any other existing species, but, instead
of the single toe on each foot, so characteristic of all modern equines,
the various species of Orohippus had four toes before and three behind,
all of which reached the ground. The skull, too, was proportionately
shorter, and the orbit was not enclosed behind by a bridge of bone.
There were fifty four teeth in all, and the premolars were larger than
the molars. The crowns of these teeth were very short. The canine teeth
were developed in both sexes, and the incisors did not have the "mark"
which indicates the age of the modern horse. The radius and ulna were
separate, and the latter was entire through the whole length. The tibia
and fibula were distinct. In the forefoot all the digits except the
pollex, or first, were well developed. The third digit is the largest,
and its close resemblance to that of the horse is clearly marked. The
terminal phalanx, or coffin-bone, has a shallow median bone in front,
as in many species of this group in the later tertiary. The fourth digit
exceeds the second in size, and the second is much the shortest of all.
Its metacarpal bone is considerably curved outward. In the hind-foot
of this genus there are but three digits. The fourth metatarsal is much
larger than the second.
"The larger number of equine mammals now known from the tertiary
deposits of this country, and their regular distributions through the
subdivisions of this formation, afford a good opportunity to ascertain
the probable descent of the modern horse. The American representative of
the latter is the extinct Equus fraternus (Leidy), a species almost, if
not wholly, identical with the Old World Equus caballus (Linnaeus), to
which our recent horse belongs. Huxley has traced successfully the later
genealogy of the horse through European extinct forms, but the line in
America was probably a more direct one, and the record is more complete.
Taking, then, as the extreme of a series, Orohippus agilis (Marsh),
from the eocene, and Equus fraternus (Leidy), from the quaternary,
intermediate forms may be intercalated with considerable certainty
from thirty or more well-marked species that lived in the intervening
periods. The natural line of descent would seem to be through the
following genera: Orohippus, of the eocene; Miohippus and Anchitherium,
of the miocene; Anchippus, Hipparion, Protohippus, Phohippus, of the
pliocene; and Equus, quaternary and recent.
"The most marked changes undergone by the successive equine genera are
as follows: First, increase in size; second, increase in speed, through
concentration of limb bones; third, elongation of head and neck, and
modifications of skull. The eocene Orohippus was the size of a fox.
Miohippus and Anchitherium, from the miocene, were about as large as a
sheep. Hipparion and Pliohippus, of the pliocene, equalled the ass in
height; while the size of the quaternary Equus was fully up to that of a
modern horse.
"The increase of speed was equally well marked, and was a direct
result of the gradual formation of the limbs. The latter were slowly
concentrated by the reduction of their lateral elements and enlargement
of the axial bone, until the force exerted by each limb came to act
directly through its axis in the line of motion. This concentration is
well seen--e.g., in the fore-limb. There was, first, a change in the
scapula and humerus, especially in the latter, which facilitated motion
in one line only; second, an expansion of the radius and reduction of
the ulna, until the former alone remained entire and effective; third,
a shortening of all the carpal bones and enlargement of the median ones,
insuring a firmer wrist; fourth, an increase of size of the third digit,
at the expense of those of each side, until the former alone supported
the limb.
"Such is, in brief, a general outline of the more marked changes that
seemed to have produced in America the highly specialized modern Equus
from his diminutive four-toed predecessor, the eocene Orohippus. The
line of descent appears to have been direct, and the remains now known
supply every important intermediate form. It is, of course, impossible
to say with certainty through which of the three-toed genera of the
pliocene that lived together the succession came. It is not impossible
that the latter species, which appear generically identical, are the
descendants of more distinct pliocene types, as the persistent tendency
in all the earlier forms was in the same direction. Considering the
remarkable development of the group through the tertiary period, and
its existence even later, it seems very strange that none of the species
should have survived, and that we are indebted for our present horse to
the Old World."(7)
PALEONTOLOGY OF EVOLUTION
These and such-like revelations have come to light in our own time--are,
indeed, still being disclosed. Needless to say, no index of any sort now
attempts to conceal them; yet something has been accomplished towards
the same end by the publication of the discoveries in Smithsonian
bulletins and in technical memoirs of government surveys. Fortunately,
however, the results have been rescued from that partial oblivion by
such interpreters as Professors Huxley and Cope, so the unscientific
public has been allowed to gain at least an inkling of the wonderful
progress of paleontology in our generation.
The writings of Huxley in particular epitomize the record. In 1862 he
admitted candidly that the paleontological record as then known, so far
as it bears on the doctrine of progressive development, negatives
that doctrine. In 1870 he was able to "soften somewhat the Brutus-like
severity" of his former verdict, and to assert that the results of
recent researches seem "to leave a clear balance in favor of the
doctrine of the evolution of living forms one from another." Six years
later, when reviewing the work of Marsh in America and of Gaudry
in Pikermi, he declared that, "on the evidence of paleontology, the
evolution of many existing forms of animal life from their predecessors
is no longer an hypothesis, but an historical fact." In 1881 he
asserted that the evidence gathered in the previous decade had been so
unequivocal that, had the transmutation hypothesis not existed, "the
paleontologist would have had to invent it."
Since then the delvers after fossils have piled proof on proof in
bewildering profusion. The fossil-beds in the "bad lands" of western
America seem inexhaustible. And in the Connecticut River Valley near
relatives of the great reptiles which Professor Marsh and others
have found in such profusion in the West left their tracks on the
mud-flats--since turned to sandstone; and a few skeletons also have been
found. The bodies of a race of great reptiles that were the lords of
creation of their day have been dissipated to their elements, while the
chance indentations of their feet as they raced along the shores, mere
footprints on the sands, have been preserved among the most imperishable
of the memory-tablets of the world.
Of the other vertebrate fossils that have been found in the eastern
portions of America, among the most abundant and interesting are the
skeletons of mastodons. Of these one of the largest and most complete is
that which was unearthed in the bed of a drained lake near Newburg, New
York, in 1845. This specimen was larger than the existing elephants,
and had tusks eleven feet in length. It was mounted and described by Dr.
John C. Warren, of Boston, and has been famous for half a century as the
"Warren mastodon."
But to the student of racial development as recorded by the fossils all
these sporadic finds have but incidental interest as compared with the
rich Western fossil-beds to which we have already referred. From records
here unearthed, the racial evolution of many mammals has in the past few
years been made out in greater or less detail. Professor Cope has traced
the ancestry of the camels (which, like the rhinoceroses, hippopotami,
and sundry other forms now spoken of as "Old World," seem to have had
their origin here) with much completeness.
A lemuroid form of mammal, believed to be of the type from which man
has descended, has also been found in these beds. It is thought that the
descendants of this creature, and of the other "Old-World" forms
above referred to, found their way to Asia, probably, as suggested by
Professor Marsh, across a bridge at Bering Strait, to continue their
evolution on the other hemisphere, becoming extinct in the land of their
nativity. The ape-man fossil found in the tertiary strata of the island
of Java in 1891 by the Dutch surgeon Dr. Eugene Dubois, and named
Pithecanthropus erectus, may have been a direct descendant of the
American tribe of primitive lemurs, though this is only a conjecture.
Not all the strange beasts which have left their remains in our "bad
lands" are represented by living descendants. The titanotheres, or
brontotheridae, for example, a gigantic tribe, offshoots of the
same stock which produced the horse and rhinoceros, represented the
culmination of a line of descent. They developed rapidly in a geological
sense, and flourished about the middle of the tertiary period; then,
to use Agassiz's phrase," time fought against them." The story of their
evolution has been worked out by Professors Leidy, Marsh, Cope, and H.
F. Osborne.
A recent bit of paleontological evidence bearing on the question of
the introduction of species is that presented by Dr. J. L. Wortman in
connection with the fossil lineage of the edentates. It was suggested by
Marsh, in 1877, that these creatures, whose modern representatives are
all South American, originated in North America long before the two
continents had any land connection. The stages of degeneration by which
these animals gradually lost the enamel from their teeth, coming finally
to the unique condition of their modern descendants of the sloth tribe,
are illustrated by strikingly graded specimens now preserved in the
American Museum of Natural History, as shown by Dr. Wortman.
All these and a multitude of other recent observations that cannot be
even outlined here tell the same story. With one accord paleontologists
of our time regard the question of the introduction of new species as
solved. As Professor Marsh has said, "to doubt evolution today is to
doubt science; and science is only another name for truth."
Thus the third great battle over the meaning of the fossil records has
come to a conclusion. Again there is a truce to controversy, and it may
seem to the casual observer that the present stand of the science of
fossils is final and impregnable. But does this really mean that a full
synopsis of the story of paleontology has been told? Or do we only await
the coming of the twentieth-century Lamarck or Darwin, who shall
attack the fortified knowledge of to-day with the batteries of a new
generalization?
IV. THE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY
JAMES HUTTON
One might naturally suppose that the science of the earth which lies at
man's feet would at least have kept pace with the science of the distant
stars. But perhaps the very obviousness of the phenomena delayed the
study of the crust of the earth. It is the unattainable that allures and
mystifies and enchants the developing mind. The proverbial child spurns
its toys and cries for the moon.
So in those closing days of the eighteenth century, when astronomers had
gone so far towards explaining the mysteries of the distant portions
of the universe, we find a chaos of opinion regarding the structure
and formation of the earth. Guesses were not wanting to explain the
formation of the world, it is true, but, with one or two exceptions,
these are bizarre indeed. One theory supposed the earth to have been at
first a solid mass of ice, which became animated only after a comet had
dashed against it. Other theories conceived the original globe as a mass
of water, over which floated vapors containing the solid elements, which
in due time were precipitated as a crust upon the waters. In a word, the
various schemes supposed the original mass to have been ice, or water,
or a conglomerate of water and solids, according to the random fancies
of the theorists; and the final separation into land and water was
conceived to have taken place in all the ways which fancy, quite
unchecked by any tenable data, could invent.
Whatever important changes in the general character of the surface of
the globe were conceived to have taken place since its creation were
generally associated with the Mosaic: deluge, and the theories which
attempted to explain this catastrophe were quite on a par with
those which dealt with a remoter period of the earth's history. Some
speculators, holding that the interior of the globe is a great abyss
of waters, conceived that the crust had dropped into this chasm and had
thus been inundated. Others held that the earth had originally revolved
on a vertical axis, and that the sudden change to its present position
bad caused the catastrophic shifting of its oceans. But perhaps the
favorite theory was that which supposed a comet to have wandered near
the earth, and in whirling about it to have carried the waters, through
gravitation, in a vast tide over the continents.
Thus blindly groped the majority of eighteenth-century philosophers in
their attempts to study what we now term geology. Deluded by the old
deductive methods, they founded not a science, but the ghost of a
science, as immaterial and as unlike anything in nature as any other
phantom that could be conjured from the depths of the speculative
imagination. And all the while the beckoning earth lay beneath the feet
of these visionaries; but their eyes were fixed in air.
At last, however, there came a man who had the penetration to see that
the phantom science of geology needed before all else a body corporeal,
and who took to himself the task of supplying it. This was Dr.
James Hutton, of Edinburgh, physician, farmer, and manufacturing
chemist--patient, enthusiastic, level-headed devotee of science.
Inspired by his love of chemistry to study the character of rocks and
soils, Hutton had not gone far before the earth stood revealed to him
in a new light. He saw, what generations of predecessors had blindly
refused to see, that the face of nature everywhere, instead of being
rigid and immutable, is perennially plastic, and year by year is
undergoing metamorphic changes. The solidest rocks are day by day
disintegrated slowly, but none the less surely, by wind and rain and
frost, by mechanical attrition and chemical decomposition, to form the
pulverized earth and clay. This soil is being swept away by perennial
showers, and carried off to the oceans. The oceans themselves beat on
their shores, and eat insidiously into the structure of sands and rocks.
Everywhere, slowly but surely, the surface of the land is being worn
away; its substance is being carried to burial in the seas.
Should this denudation continue long enough, thinks Hutton, the entire
surface of the continents must be worn away. Should it be continued LONG
ENOUGH! And with that thought there flashes on his mind an inspiring
conception--the idea that solar time is long, indefinitely long. That
seems a simple enough thought--almost a truism--to the twentieth-century
mind; but it required genius to conceive it in the eighteenth. Hutton
pondered it, grasped its full import, and made it the basis of his
hypothesis, his "theory of the earth."
MODERN GEOLOGY
The hypothesis is this--that the observed changes of the surface of
the earth, continued through indefinite lapses of time, must result in
conveying all the land at last to the sea; in wearing continents away
till the oceans overflow them. What then? Why, as the continents wear
down, the oceans are filling up. Along their bottoms the detritus of
wasted continents is deposited in strata, together with the bodies of
marine animals and vegetables. Why might not this debris solidify to
form layers of rocks--the basis of new continents? Why not, indeed?
But have we any proof that such formation of rocks in an ocean-bed has,
in fact, occurred? To be sure we have. It is furnished by every bed
of limestone, every outcropping fragment of fossil-bearing rock, every
stratified cliff. How else than through such formation in an ocean-bed
came these rocks to be stratified? How else came they to contain the
shells of once living organisms imbedded in their depths? The ancients,
finding fossil shells imbedded in the rocks, explained them as mere
freaks of "nature and the stars." Less superstitious generations had
repudiated this explanation, but had failed to give a tenable solution
of the mystery. To Hutton it is a mystery no longer. To him it seems
clear that the basis of the present continents was laid in ancient
sea-beds, formed of the detritus of continents yet more ancient.
But two links are still wanting to complete the chain of Hutton's
hypothesis. Through what agency has the ooze of the ocean-bed been
transformed into solid rock? and through what agency has this rock been
lifted above the surface of the water to form new continents? Hutton
looks about him for a clew, and soon he finds it. Everywhere about us
there are outcropping rocks that are not stratified, but which give
evidence to the observant eye of having once been in a molten state.
Different minerals are mixed together; pebbles are scattered through
masses of rock like plums in a pudding; irregular crevices in otherwise
solid masses of rock--so-called veinings--are seen to be filled with
equally solid granite of a different variety, which can have gotten
there in no conceivable way, so Hutton thinks, but by running in while
molten, as liquid metal is run into the moulds of the founder. Even
the stratified rocks, though they seemingly have not been melted, give
evidence in some instances of having been subjected to the action of
heat. Marble, for example, is clearly nothing but calcined limestone.
With such evidence before him, Hutton is at no loss to complete his
hypothesis. The agency which has solidified the ocean-beds, he says,
is subterranean heat. The same agency, acting excessively, has produced
volcanic cataclysms, upheaving ocean-beds to form continents. The rugged
and uneven surfaces of mountains, the tilted and broken character
of stratified rocks everywhere, are the standing witnesses of these
gigantic upheavals.
And with this the imagined cycle is complete. The continents, worn away
and carried to the sea by the action of the elements, have been made
over into rocks again in the ocean-beds, and then raised once more into
continents. And this massive cycle, In Hutton's scheme, is supposed
to have occurred
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