Problems of Genetics
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Title: Problems of Genetics
Author: William Bateson
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*** START OF THE PROJECT GUTENBERG EBOOK PROBLEMS OF GENETICS ***
YALE UNIVERSITY
MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES
PROBLEMS OF GENETICS
SILLIMAN MEMORIAL LECTURES
PUBLISHED BY YALE UNIVERSITY PRESS
ELECTRICITY AND MATTER. _By_ JOSEPH JOHN THOMSON,
D.SC., LL.D., PH.D., F.R.S., _Fellow of Trinity College,
Cambridge, Cavendish Professor of Experimental Physics, Cambridge_.
_Price $1.25 net; postage 10 cents extra._
THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.
_By_ CHARLES S. SHERRINGTON,
D.SC., M.D., HON. LL.D., TOR., F.R.S.,
_Holt Professor of Physiology in the University of Liverpool_.
_Price $3.50 net; postage 25 cents extra._
RADIOACTIVE TRANSFORMATIONS. _By_ ERNEST RUTHERFORD,
D.SC., LL.D., F.R.S., _Macdonald Professor of Physics,
McGill University_.
_Price $3.50 net; postage 22 cents extra._
EXPERIMENTAL AND THEORETICAL APPLICATIONS OF
THERMODYNAMICS TO CHEMISTRY.
_By_ DR. WALTHER NERNST, _Professor and Director of the
Institute of Physical Chemistry in the University of Berlin_.
_Price $1.25 net; postage 10 cents extra._
THE PROBLEMS OF GENETICS. _By_ WILLIAM BATESON, M.A.,
F.R.S., _Director of the John Innes Horticultural Institution,
Merton Park, Surrey, England_.
_Price $4.00 net; postage 25 cents extra._
STELLAR MOTIONS.
WITH SPECIAL REFERENCE TO MOTIONS DETERMINED BY MEANS OF
THE SPECTROGRAPH. _By_ WILLIAM WALLACE CAMPBELL, SC.D., LL.D.,
_Director of the Lick Observatory, University of California_.
_Price $4.00 net; postage 30 cents extra._
THEORIES OF SOLUTIONS. _By_ SVANTE AUGUST ARRHENIUS,
PH.D., SC.D., M.D., _Director of the Physico-Chemical
Department of the Nobel Institute, Stockholm, Sweden_.
_Price $2.25 net; postage 15 cents extra._
IRRITABILITY.
A PHYSIOLOGICAL ANALYSIS OF THE GENERAL EFFECT OF
STIMULI IN LIVING SUBSTANCES.
_By_ MAX VERWORN,
_Professor at Bonn Physiological Institute_.
_Price $3.50 net; postage 20 cents extra._
THE EVOLUTION OF MODERN MEDICINE.
_By_ SIR WILLIAM OSLER, BART., M.D., LL.D., SC.D.,
_Regius Professor of Medicine, Oxford University_.
_Price $3.00 net; postage 40 cents extra._
PROBLEMS OF GENETICS
BY
WILLIAM BATESON, M.A., F.R.S.
DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY
_WITH ILLUSTRATIONS_
[Illustration]
NEW HAVEN: YALE UNIVERSITY PRESS
LONDON: HUMPHREY MILFORD
OXFORD UNIVERSITY PRESS
MCMXIII
Copyright, 1913
By YALE UNIVERSITY
First printed August, 1913, 1000 copies
[** Transcriber's Note:
Underscores "_" before and after a word or phrase indicate ITALICS
in the original text.
Hyphenation was used inconsistently by the author and has been
left as in the original text. ]
THE SILLIMAN FOUNDATION
In the year 1883 a legacy of about eighty-five thousand dollars was left
to the President and Fellows of Yale College in the city of New Haven,
to be held in trust, as a gift from her children, in memory of their
beloved and honored mother, Mrs. Hepsa Ely Silliman.
On this foundation Yale College was requested and directed to establish
an annual course of lectures designed to illustrate the presence and
providence, the wisdom and goodness of God, as manifested in the natural
and moral world. These were to be designated as the Mrs. Hepsa Ely
Silliman Memorial Lectures. It was the belief of the testator that any
orderly presentation of the facts of nature or history contributed
to the end of this foundation more effectively than any attempt to
emphasize the elements of doctrine or of creed; and he therefore
provided that lectures on dogmatic or polemical theology should be
excluded from the scope of this foundation, and that the subjects should
be selected rather from the domains of natural science and history,
giving special prominence to astronomy, chemistry, geology, and anatomy.
It was further directed that each annual course should be made the basis
of a volume to form part of a series constituting a memorial to Mrs.
Silliman. The memorial fund came into the possession of the Corporation
of Yale University in the year 1901; and the present volume constitutes
the fifth of the series of memorial lectures.
PREFACE
This book gives the substance of a series of lectures delivered in Yale
University, where I had the privilege of holding the office of Silliman
Lecturer in 1907.
The delay in publication was brought about by a variety of causes.
Inasmuch as the purpose of the lectures is to discuss some of the wider
problems of biology in the light of knowledge acquired by Mendelian
methods of analysis, it was essential that a fairly full account of
the conclusions established by them should first be undertaken and I
therefore postponed the present work till a book on Mendel's Principles
had been completed.
On attempting a more general discussion of the bearing of the phenomena
on the theory of Evolution, I found myself continually hindered by the
consciousness that such treatment is premature, and by doubt whether
it were not better that the debate should for the present stand
indefinitely adjourned. That species have come into existence by an
evolutionary process no one seriously doubts; but few who are familiar
with the facts that genetic research has revealed are now inclined to
speculate as to the manner by which the process has been accomplished.
Our knowledge of the nature and properties of living things is far too
meagre to justify any such attempts. Suggestions of course can be made:
though, however, these ideas may have a stimulating value in the lecture
room, they look weak and thin when set out in print. The work which may
one day give them a body has yet to be done.
The development of negations is always an ungrateful task apt to be
postponed for the positive business of experiment. Such work is happily
now going forward in most of the centers of scientific life. Of many
of the subjects here treated we already know more than we did in 1907.
The delay in production has made it possible to incorporate these new
contributions.
The book makes no pretence at being a treatise and the number of
illustrative cases has been kept within a moderate compass. A good many
of the examples have been chosen from American natural history, as being
appropriate to a book intended primarily for American readers. The facts
are largely given on the authority of others, and I wish to express my
gratitude for the abundant assistance received from American colleagues,
especially from the staffs of the American Museum in New York, and of
the Boston Museum of Natural History. In connexion with the particular
subjects personal acknowledgments are made.
Dr. F. M. Chapman was so good as to supervise the preparation of the
coloured Plate of _Colaptes_, and to authorize the loan of the Plate
representing the various forms of _Helminthophila_, which is taken from
his _North American Warblers_.
I am under obligation to Messrs. Macmillan & Co., for permission to
reproduce several figures from _Materials for the Study of Variation_,
illustrating subjects which I wished to treat in new associations, and
to M. Leduc for leave to use Fig. 9.
In conclusion I thank my friends in Yale for the high honour they did me
by their invitation to contribute to the series of Silliman Lectures,
and for much kindness received during a delightful sojourn in that
genial home of learning.
TABLE OF CONTENTS.
CHAPTER PAGE
I. INTRODUCTORY. THE PROBLEM OF SPECIES AND VARIETY 1
II. MERISTIC PHENOMENA 31
III. SEGMENTATION, ORGANIC AND MECHANICAL 60
IV. THE CLASSIFICATION OF VARIATION AND THE NATURE
OF SUBSTANTIVE VARIATION 83
NOTE TO CHAPTER IV 94
V. THE MUTATION THEORY 97
NOTE TO CHAPTER V 116
VI. VARIATION AND LOCALITY 118
VII. LOCAL DIFFERENTIATION--_continued_.
OVERLAPPING FORMS 146
VIII. LOCALLY DIFFERENTIATED FORMS--_continued_.
CLIMATIC VARIETIES 164
IX. THE EFFECTS OF CHANGED CONDITIONS 187
X. THE EFFECTS OF CHANGED CONDITIONS--_continued_.
THE CAUSES OF GENETIC VARIATION 212
XI. THE STERILITY OF HYBRIDS. CONCLUDING REMARKS 233
APPENDIX TO CHAPTER X 250
INDEX 251
PROBLEMS OF GENETICS
CHAPTER I
INTRODUCTORY
The purpose of these lectures is to discuss some of the familiar
phenomena of biology in the light of modern discoveries. In the last
decade of the nineteenth century many of us perceived that if any
serious advance was to be made with the group of problems generally
spoken of as the Theory of Evolution, methods of investigation must be
devised and applied of a kind more direct and more penetrating than
those which after the general acceptance of the Darwinian views had been
deemed adequate. Such methods obviously were to be found in a critical
and exhaustive study of the facts of variation and heredity, upon which
all conceptions of evolution are based. To construct a true synthetic
theory of Evolution it was necessary that variation and heredity instead
of being merely postulated as axioms should be minutely examined as
phenomena. Such a study Darwin himself had indeed tentatively begun, but
work of a more thorough and comprehensive quality was required. In the
conventional view which the orthodoxy of the day prescribed, the terms
variation and heredity stood for processes so vague and indefinite that
no analytical investigation of them could be contemplated. So soon,
however, as systematic inquiry into the natural facts was begun it was
at once found that the accepted ideas of variation were unfounded.
Variation was seen very frequently to be a definite and specific
phenomenon, affecting different forms of life in different ways, but
in all its diversity showing manifold and often obvious indications
of regularity. This observation was not in its essence novel. Several
examples of definite variation had been well known to Darwin and
others, but many, especially Darwin himself in his later years, had
nevertheless been disposed to depreciate the significance of such
facts. They consequently then lapsed into general disparagement. Upon
more careful inquiry the abundance of such phenomena proved to be far
greater than was currently supposed, and a discussion of their nature
brought into prominence a consideration of greater weight, namely that
the differences by which these definite or discontinuous variations are
constituted again and again approximate to and are comparable with the
class of differences by which species are distinguished from each other.
The interest of such observations could no longer be denied. The
more they were examined the more apparent it became that by means of
the facts of variation a new light was obtained on the physiological
composition and capabilities of living things. Genetics thus cease to be
merely a method of investigating theories of evolution or of the origin
of species but provide a novel and hitherto untried instrument by which
the nature of the living organism may be explored. Just as in the study
of non-living matter science began by regarding the external properties
of weight, opacity, colour, hardness, mode of occurrence, etc., noting
only such evidences of chemical attributes and powers as chance
spontaneously revealed; and much later proceeded to the discovery that
these casual manifestations of chemical properties, rightly interpreted,
afford a key to the intrinsic nature of the diversity of matter, so in
biology, having examined those features of living things which ordinary
observations can perceive, we come at last to realize that when studied
for their own sake the properties of living organisms in respect of
heredity and variation are indications of their inner nature and provide
evidences of that nature which can be obtained from no other source.
While such ideas were gradually forming in our minds, came the
rediscovery of Mendel's work. Investigations which before had only
been imagined as desirable now became easy to pursue, and questions
as to the genetic inter-relations and compositions of varieties can
now be definitely answered. Without prejudice to what the future may
disclose whether by way of limitation or extension of Mendelian method,
it can be declared with confidence and certainty that we have now the
means of beginning an analysis of living organisms, and distinguishing
many of the units or factors which essentially determine and cause the
development of their several attributes.
Briefly put, the essence of Mendelism lies in the discovery of the
existence of unit characters or factors. For an account of the Mendelian
method, how it is applied and what it has already accomplished,
reference must be made to other works.[1] With this part of the subject
I shall assume a sufficient acquaintance. In these lectures I have
rather set myself the task of considering how certain problems appear
when viewed from the standpoint to which the application of these
methods has led us. It is indeed somewhat premature to discuss such
questions. The work of Mendelian analysis is progressing with great
rapidity and anything I can say may very soon be superseded as out
of date. Nevertheless a discussion of this kind may be of at least
temporary service in directing inquiry to the points of special interest.
THE PROBLEM OF SPECIES AND VARIETY
Nowhere does our new knowledge of heredity and variation apply more
directly than to the problem what is a species and what is a variety? I
cannot assert that we are already in a position to answer this important
question, but as will presently appear, our mode of attack and the
answers we expect to receive are not those that were contemplated by our
predecessors. If we glance at the history of the scientific conception
of Species we find many signs that it was not till comparatively recent
times that the definiteness of species became a strict canon of the
scientific faith and that attempts were made to give precise limits
to that conception. When the diversity of living things began to be
accurately studied in the sixteenth and seventeenth centuries names
were applied in the loosest fashion, and in giving a name to an animal
or a plant the naturalists of those times had no ulterior intention.
Names were bestowed on those creatures about which the writer proposed
to speak. When Gesner or Aldrovandi refer to all the kinds of horses,
unicorns, dogs, mermaids, etc., which they had seen or read of, giving
to each a descriptive name, they do not mean to "elevate" each named
kind to "specific rank"; and if anyone had asked them what they meant by
a species, it is practically certain that they would have had not the
slightest idea what the question might imply, or any suspicion that it
raised a fundamental problem of nature.
Spontaneous generation being a matter of daily observation, then
unquestioned, and supernatural events of all kinds being commonly
reported by many witnesses, transmutation of species had no inherent
improbability. Matthioli,[2] for instance, did not expect to be charged
with heresy when he declared _Stirpium mutatio_ to be of ordinary
occurrence. After giving instances of induced modifications he wrote,
"Tantum enim in plantis naturae germanitas potest, ut non solum saepe
praedictos praestet effectus, sed etiam ut alteram in alteram stirpem
facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum
in lolium, hordeum in avenam, et ocymum in serpyllum."
I do not know who first emphasized the need for a clear understanding
of the sense in which the term species is to be applied. In the second
half of the seventeenth century Ray shows some degree of concern on
this matter. In the introduction to the _Historia Plantarum_, 1686, he
discusses some of the difficulties and lays down the principle that
varieties which can be produced from the seed of the same plant are to
be regarded as belonging to one species, being, I believe, the first
to suggest this definition. That new species can come into existence
he denies as inconsistent with Genesis 2, in which it is declared that
God finished the work of Creation in six days. Nevertheless he does not
wholly discredit the possibility of a "transmutation" of species, such
that one species may as an exceptional occurrence give rise by seed
to another and nearly allied species. Of such a phenomenon he gives
illustrations the authenticity of which he says he is, against his will,
compelled to admit. He adds that some might doubt whether in the cases
quoted the two forms concerned are really distinct species, but the
passage is none the less of value for it shews that the conception of
species as being distinct unchangeable entities was not to Ray the dogma
sacrosanct and unquestionable which it afterwards became.[3]
In the beginning of the eighteenth century Marchant,[4] having observed
the sudden appearance of a lacinated variety of _Mercurialis_, makes the
suggestion that species in general may have arisen by similar mutations.
Indeed from various passages it is manifest that to the authors of the
seventeenth and early eighteenth centuries species appeared simply as
groups more or less definite, the boundaries of which it was unnecessary
to determine with great exactitude. Such views were in accord with the
general scientific conception of the time. The mutability of species is
for example sometimes likened (see for instance Sharrock, loc. cit.) to
the metamorphoses of insects, and it is to be remembered that the search
for the Philosopher's Stone by which the transmutation of metals was to
be effected had only recently fallen into discredit as a pursuit.
The notion indeed of a peculiar, fixed meaning to be attached to species
as distinct from variety is I think but rarely to be found categorically
expressed in prae-Linnaean writings.
But with the appearance of the _Systema Naturae_ a great change
supervened. Linnaeus was before all a man of order. Foreseeing the
immense practical gain to science that must come from a codification of
nomenclature, he invented such a system.
It is not in question that Linnaeus did great things for us and made
Natural History a manageable and accessible collection of facts instead
of a disorderly heap; but orderliness of mind has another side, and
inventors and interpreters of systems soon attribute to them a force and
a precision which in fact they have not.
The systematist is primarily a giver of names, as Ray with his broader
views perceived. Linnaeus too in the exordium to the _Systema Naturae_
naively remarks, that he is setting out to continue the work which
Adam began in the Golden Age, to give names to the living creatures.
Naming however involves very delicate processes of mind and of logic.
Carried out by the light of meagre and imperfect knowledge it entails
all the mischievous consequences of premature definition, and promotes
facile illusions of finality. So was it with the Linnaean system. An
interesting piece of biological history might be written respecting the
growth and gradual hardening of the conception of Species. To readers
of Linnaeus's own writings it is well known that his views cannot be
summarized in a few words. Expressed as they were at various times
during a long life and in various connexions, they present those divers
inconsistencies which commonly reflect a mind retaining the power of
development. Nothing certainly could be clearer than the often quoted
declaration of the _Philosophia Botanica_, "Species tot numeramus quot
diversae formae in principio sunt creatae," with the associated passage
"Varietates sunt plantae ejusdem speciei mutatae a caussa quacunque
occasionali." Those sayings however do not stand alone. In several
places, notably in the famous dissertation on the peloric _Linaria_
he explicitly contemplates the possibility that new species may arise
by crossing, declaring nevertheless that he thinks such an event to
be improbable. In that essay he refers to Marchant's observation on a
laciniate _Mercurialis_, but though he states clearly that that plant
should only be regarded as a variety of the normal, he does not express
any opinion that the contemporary genesis of new species must be an
impossibility. In the later dissertation on Hybrid Plants he returns to
the same topic. Again though he states the belief that species cannot
be generated by cross-breedings, he treats the subject not as heretical
absurdity but as one deserving respectful consideration.
The significance of the aphorisms that precede the lectures on the
Natural Orders is not easy to apprehend. These are expressed with the
utmost formality, and we cannot doubt that in them we have Linnaeus's
own words, though for the record we are dependent on the transcripts of
his pupils.
The text of the first five is as follows:
1. Creator T. O. in primordio vestiit Vegetabile _Medullare_
principiis constitutivis diversi _Corticalis_ unde tot difformia
individua, quot _Ordines_ Naturales prognata.
2. _Classicas_ has (1) plantas Omnipotens miscuit inter se,
unde tot _Genera_ ordinum, quot inde plantae.
3. _Genericas_ has (2) miscuit Natura, unde tot _Species_
congeneres quot hodie existunt.
4. _Species_ has miscuit Casus, unde totidem quot passim
occurrunt, _Varietates_.
5. Suadent haec (1-4) Creatoris leges a simplicibus ad
Composita.
Naturae leges generationis in hybridis.
Hominis leges ex observatis a posteriori.
I am not clear as to the parts assigned in the first sentence
respectively to the "_Medulla_" and the "_Cortex_," beyond that Linnaeus
conceived that multiformity was first brought about by diversity in
the "_Cortex_." The passage is rendered still more obscure if read in
connection with the essay on "_Generatio Ambigena_," where he expresses
the conviction that the _Medulla_ is contributed by the mother, and the
_Cortex_ by the father, both in plants and animals.[5]
But however that may be, he regards this original diversity as resulting
in the constitution of the Natural Orders, each represented by one
individual.
In the second aphorism the Omnipotent is represented as creating the
genera by intermixing the individual _plantae classicae_, or prototypes
of the Natural Orders.
The third statement is the most remarkable, for in it he declares that
Species were formed by the act of Nature, who by inter-mixing the genera
produced _Species congeneres_, namely species inside each genus, to the
number which now exist. Lastly, Chance or Accident, intermixing the
species, produced as many varieties as there are about us.
Linnaeus thus evidently regarded the intermixing of an originally
limited number of types as the sufficient cause of all subsequent
diversity, and it is clear that he draws an antithesis between
_Creator_, _Natura_, and _Casus_, assigning to each a special part
in the operations. The acts resulting in the formation of genera are
obviously regarded as completed within the days of the Creation, but the
words do not definitely show that the parts played by Nature and Chance
were so limited.
Recently also E. L. Greene[6] has called attention to some curious
utterances buried in the _Species Plantarum_, in which Linnaeus refers
to intermediate and transitional species, using language that even
suggests evolutionary proclivities of a modern kind, and it is not easy
to interpret them otherwise.
Whatever Linnaeus himself believed to be the truth, the effect of his
writings was to induce a conviction that the species of animals and
plants were immutably fixed. Linnaeus had reduced the whole mass of
names to order and the old fantastical transformations with the growth
of knowledge had lapsed into discredit; the fixity of species was
taken for granted, but not till the overt proclamation of evolutionary
doctrine by Lamarck do we find the strenuous and passionate assertions
of immutability characteristic of the first half of the nineteenth
century.
It is not to be supposed that the champions of fixity were unacquainted
with varietal differences and with the problem thus created, but in
their view these difficulties were apparent merely, and by sufficiently
careful observation they supposed that the critical and permanent
distinctions of the true species could be discovered, and the
impermanent variations detected and set aside.
This at all events was the opinion formed by the great body of
naturalists at the end of the eighteenth and beginning of the nineteenth
centuries, and to all intents and purposes in spite of the growth of
evolutionary ideas, it remains the guiding principle of systematists
to the present day. There are 'good species' and 'bad species' and the
systematists of Europe and America spend most of their time in making
and debating them.
In some of its aspects the problem of course confronted earlier
naturalists. Parkinson for instance (1640) in introducing his treatment
of _Hieracium_ wrote, "To set forth the whole family of the Hawkeweedes
in due forme and order is such a world of worke that I am in much doubt
of mine own abilitie, it having lyen heavie on his shoudiers that
hath already waded through them ... for such a multitude of varieties
in forme pertaining to one herbe is not to be found againe in _rerum
natura_ as I thinke," and the same idea, that the difficulty lay rather
in man's imperfect powers of discrimination than in the nature of the
materials to be discriminated, is reflected in many treatises early and
late.
It was however with the great ouburst of scientific activity which
followed Linnaeus that the difficulty became acute. Simultaneously
vast masses of new material were being collected from all parts of the
world into the museums, and the products of the older countries were
re-examined with a fresh zeal and on a scale of quantity previously
unattempted. But the problem how to name the forms and where to draw
lines, how much should be included under one name and where a new name
was required, all this was felt, rather as a cataloguer's difficulty
than as a physiological problem. And so we still hear on the one hand
of the confusion caused by excessive "splitting" and subdivisions, and
on the other of the uncritical "lumpers" who associate together under
one name forms which another collector or observer would like to see
distinguished.
In spite of Darwin's hopes, the acceptance of his views has led to no
real improvement--scarcely indeed to any change at all in either the
practice or aims of systematists. In a famous passage in the _Origin_ he
confidently declares that when his interpretation is generally adopted
"Systematists will be able to pursue their labours as at present; but
they will not be incessantly haunted by the shadowy doubt whether this
or that form be a true species. This, I feel sure, and I speak after
experience, will be no slight relief. The endless disputes whether or
not some fifty species of British brambles are good species will cease."
Those disputes nevertheless proceed almost exactly as before. It is
true that biologists in general do not, as formerly, participate in
these discussions because they have abandoned systematics altogether;
but those who are engaged in the actual work of naming and cataloguing
animals and plants usually debate the old questions in the old way.
There is still the same divergence of opinion and of practice, some
inclining to make much of small differences, others to neglect them.
Not only does the work of the systematists as a whole proceed as if
Darwin had never written but their attitude towards these problems is
but little changed. In support of this statement I may refer to several
British Museum Catalogues, much of the _Biologia Centrali-Americana_,
Ridgway's _Birds of North America_, the _Fauna Hawaiensis_, indeed to
almost any of the most important systematic publications of England,
America, or any other country. These works are compiled by the most
proficient systematists of all countries in the several groups, but
with rare exceptions they show little misgiving as to the fundamental
reality of specific differences. That the systematists consider the
species-unit as of primary importance is shown by the fact that the
whole business of collection and distribution of specimens is arranged
with regard to it.
Almost always the collections are arranged in such a way that the
phenomena of variation are masked. Forms intermediate between two
species are, if possible, sorted into separate boxes under a third
specific name. If a species is liable to be constantly associated
with a mutational form, the mutants are picked out, regardless of the
circumstances of their origin, from the samples among which they were
captured, and put apart under a special name. Only by a minute study
of the original labels of the specimens and by redistributing them
according to locality and dates, can their natural relations be traced.
The published accounts of these collections often take no notice of
variations, others make them the subject of casual reference. Very few
indeed treat them as of much importance. From such indications it is
surely evident that the systematists attach to the conception of species
a significance altogether different from that which Darwin contemplated.
I am well aware that some very eminent systematists regard the whole
problem as solved. They hold as Darwin did that specific diversity
has no physiological foundation or causation apart from fitness, and
that species are impermanent groups, the delimitations of which are
ultimately determined by environmental exigency or "fitness." The
specific diversity of living things is thus regarded as being something
quite different in nature from the specific diversity of inorganic
substances. In practice those who share these opinions are, as might
be anticipated, to be found among the 'lumpers' rather than among the
'splitters.' In their work, certainly, the Darwinian theory is actually
followed as a guiding principle; unanalysed inter-gradations of all
kinds are accepted as impugning the integrity of species; the underlying
physiological problem is forgotten, and while the product is almost
valueless as a contribution to biological research, I can scarcely
suppose that it aids greatly in the advances of other branches of our
science.
But why is it that, with these exceptions, the consequences of the
admittedly general acceptance of a theory of evolution are so little
reflected in the systematic treatment of living things? Surely the
reason is that though the systematist may be convinced of the general
truth of the evolution theory at large, he is still of opinion that
species are really distinct things. For him there are still 'good'
species and 'bad' species and his experience tells him that the
distinction between the two is not simply a question of degree or a
matter of opinion.
To some it may seem that this is mere perversity, a refusal to see
obvious truth, a manifestation of the spirit of the collector rather
than of the naturalist. But while recognising that from a magnification
of the conception of species the systematists are occasionally led
into absurdity I do not think the grounds for their belief have in
recent times been examined with the consideration they deserve. The
phenomenon of specific diversity is manifested to a similar degree by
living things belonging to all the great groups, from the highest to the
lowest, Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
and Bacteria, all present diversities of such a kind that among them the
existence of specific differences can on the whole be recognised with
a similar degree of success and with very similar limitations. In all
these groups there are many species quite definite and unmistakable, and
others practically indefinite. The universal presence of specificity,
as we may call it, similarly limited and characterised, is one of its
most remarkable features. Not only is this specificity thus universally
present among the different forms of life, but it manifests itself
in respect of the most diverse characteristics which living things
display. Species may thus be distinguished by peculiarities of form,
of number, of geometrical arrangement, of chemical constitution and
properties, of sexual differentiation, of development, and of many
other properties. In any one or in several of these features together,
species may be found distinguished from other species. It is also to be
observed that the definiteness of these distinctions has no essential
dependence on the nature of the characteristic which manifests them.
It is for example sometimes said that colour-distinctions are of small
systematic importance, but every systematist is familiar with examples
(like that of the wild species of _Gallus_) in which colours though
complex, show very little variation. On the other hand features of
structure, sexual differentiation, and other attributes which by our
standards are estimated as essential, may be declared to show much
variation or little, not according to any principle which can be
detected, but simply as the attention happens to be applied to one
species or group of species, or to another. In many groups of animals
and plants observers have hit upon characters which were for a time
thought to be finally diagnostic of species. The Lepidoptera and Diptera
for instance, have been re-classified according to their neuration.
Through a considerable range of forms determinations may be easily made
on these characters, but as is now well known, neuration is no more
immune from variation than any other feature of organisation, and in
some species great variability is the rule. Again it was once believed
by some that the genitalia of the Lepidoptera provided a basis of final
determination--with a similar sequel. In some groups, for example
the Lycaenidae, or the Hesperidae, there are forms almost or quite
indistinguishable on external examination, but a glance at the genitalia
suffices to distinguish numerous species, while on the contrary among
Pieridae a great range of species show scarcely any difference in these
respects: and again in occasional species the genitalia show very
considerable variations.
The proposition that animals and plants are on the whole divisible into
definite and recognisable species is an approximation to the truth.
Such a statement is readily defensible, whereas to assert the contrary
would be palpably absurd. For example, a very competent authority
lately wrote: "In the whole Lepidopterous fauna of England there is no
species of really uncertain limits."[7] Others may be disposed to make
certain reservations, but such exceptions would be so few as scarcely to
impair the validity of the general statement. The declaration might be
extended to other orders and other lands.
We know, of course, that the phenomenon of specific diversity is
complicated by local differentiation: that, in general, forms which
cannot disperse themselves freely exhibit a multitude of local races,
and that of these some are obviously adaptative, and that a few even owe
their peculiarity to direct environmental effects. Every systematist
also is perfectly aware that in dealing with collections from little
explored countries the occurrence of polymorphism or even of sporadic
variation may make the practical business of distinguishing the species
difficult and perhaps for the time impossible; still, conceding that a
great part of the diversity is due to geographical differentiation, and
that some is sporadic variation, our experience of our own floras and
faunas encourages the belief that if we were thoroughly familiar with
these exotic productions it would usually be possible to assign their
specific limitations with an approach to certainty.
For apart from any question of the justice of these wider inferences,
if we examine the phenomenon of specificity as it appears in those
examples which are nearest to hand, surely we find signs in plenty that
specific distinction is no mere consequence of Natural Selection. The
strength of this proposition has lain mainly in the appeal to ignorance.
Steadily with the growth of knowledge has its cogency diminished, and
such a belief could only have been formulated at a time when the facts
of variation were unknown.
In Darwin's time no serious attempt had been made to examine the
manifestations of variability. A vast assemblage of miscellaneous facts
could formerly be adduced as seemingly comparable illustrations of the
phenomenon "Variation." Time has shown this mass of evidence to be
capable of analysis. When first promulgated it produced the impression
that variability was a phenomenon generally distributed amongst living
things in such a way that the specific divisions must be arbitrary.
When this variability is sorted out, and is seen to be in part a result
of hybridisation, in part a consequence of the persistence of hybrids
by parthenogenetic reproduction, a polymorphism due to the continued
presence of individuals representing various combinations of Mendelian
allelomorphs, partly also the transient effect of alteration in external
circumstances, we see how cautious we must be in drawing inferences as
to the indefiniteness of specific limits from a bare knowledge that
intermediates exist. Conversely, from the accident of collocation or
from a misleading resemblance in features we deem essential, forms
genetically distinct are often confounded together, and thus the
divergence of such forms in their other features, which we declare
to be non-essential, passes as an example of variation. Lastly, and
this is perhaps the most fertile of all the sources of confusion, the
impression of the indefiniteness of species is created by the existence
of numerous local forms, isolated geographically from each other, forms
whose differences may be referable to any one of the categories I have
enumerated.
The advance has been from many sides. Something has come from the work
of systematists, something from cultural experiments, something from
the direct study of variation as it appears in nature, but progress
is especially due to experimental investigation of heredity. From all
these lines of inquiry we get the same answer; that what the naturalists
of fifty years ago regarded as variation is not one phenomenon but
many, and that what they would have adduced as evidence against the
definiteness of species may not in fact be capable of this construction
at all.
If we may once more introduce a physical analogy, the distinctions with
which the systematic naturalist is concerned in the study of living
things are as multifarious as those by which chemists were confronted in
the early days of their science. Diversities due to mechanical mixtures,
to allotropy, to differences of temperature and pressure, or to degree
of hydration, had all to be severally distinguished before the essential
diversity due to variety of chemical constitution stood out clearly,
and I surmise that not till a stricter analysis of the diversities of
animals and plants has been made on a comprehensive scale, shall we be
in a position to declare with any confidence whether there is or is not
a natural and physiological distinction between species and variety.
As I have said above, it is in the cases nearest to hand that the
problem may be most effectively studied. Comparison between forms from
dissimilar situations contributes something; but it is by a close
examination of the behaviour, especially the genetic behaviour, of
familiar species when living in the presence of their nearest allies
that the most direct light on the problem is to be obtained. I cannot
understand the attitude of those who, contemplating such facts as this
examination elicits, can complacently declare that specific difference
is a mere question of degree. With the spread of evolutionary ideas to
speak much of the fixity of species has become unfashionable, and yet
how striking and inscrutable are the manifestations of that fixity!
Consider the group of species composing the _agrestis_ section of the
genus _Veronica_, namely _Tournefortii_, _agrestis_, and _polita_.
These three grow side by side in my garden, as they do in suitable
situations over a vast area of the temperate regions. I have for years
noticed them with some care and become familiar with their distinctions
and resemblances. Never is there any real doubt as to the identity of
any plant. The species show some variability, but I have never seen
one which assumed any of the distinguishing features of the others. A
glance at the fruits decides at once to which species a plant belongs. I
find it impossible to believe that the fixity of these distinctions is
directly dependent on their value as aids in the struggle for existence.
The mode of existence of the three forms in so far as we can tell is
closely similar. By whatever standard we reckon systematic affinity I
suppose we shall agree that these species come very near indeed to each
other. Bentham even takes the view that _polita_ is a mere variety of
_agrestis_.
Now in such cases as this it has been argued that the specific
features of the several types have been separately developed in as
many distinct localities, and that their present association is due
to subsequent redistribution. Of these Veronicas indeed we know that
one, _Tournefortii_ (= _Buxbaumii_) is as a matter of fact a recent
introduction from the east.[8] But this course of argument leads to
still further difficulties. For if it is true that the peculiarities
of the several species have been perfected and preserved on account
of their survival-value to their possessors, it follows that there
must be many ways of attaining the same result. But since sufficient
adaptation may be ensured in so many ways, the disappearance of the
common parent of these forms is difficult to understand. Obviously it
must have been a plant very similar in general construction to its
modern representatives. Like them it must have been an annual weed, with
an organisation conformable to that mode of life. Why then, after having
been duly perfected for that existence should it have been entirely
superseded in favour of a number of other distinct contrivances for
doing the same thing, and--if a gradual transition be predicated--not
only by them, but by each intermediate stage between them and the
original progenitor? Surely the obvious inference from such facts is
that the burden cast upon the theory of gradual selection is far greater
than it can bear; that adaptation is not in practice a very close fit,
and that the distinctions between these several species of Veronica have
not arisen on account of their survival-value but rather because none
of their diversities was so damaging as to lead to the extermination
of its possessor. When we see these various Veronicas each rigidly
reproducing its parental type, all comfortably surviving in competition
with each other, are we not forced to the conclusion that _tolerance_
has as much to do with the diversity of species as the stringency of
Selection? Certainly these species owe their continued existence to the
fact that they are each good enough to live, but how shall we refer the
distinctions between them directly or indirectly to the determination of
Natural Selection?
The control of Selection is loose while the conformity to specific
distinction is often very strict and precise, and no less so even when
several closely related species co-exist in the same area and in the
same circumstances.
The theory of Selection fails at exactly the point where it was devised
to help: _Specific_ distinction.
Let us examine a somewhat different set of facts in the case of another
pair of nearly allied species _Lychnis diurna_ and _vespertina_. The
two plants have much in common. Both are dioecious perennials, with
somewhat similar flowers, the one crimson, the other white. Each however
has its peculiarities which are discernible in almost any part of its
structure, whether flower, leaf, fruit or seed, distinctions which
would enable a person thoroughly familiar with the plants to determine
at once from which species even a small piece had been taken. There is
so much resemblance however as readily to support the surmise that the
two were mere varieties of one species. Bentham, following Linnaeus,
in fact actually makes this suggestion, with what propriety we will
afterwards consider. Now this case is typical of many. The two forms
have a wide distribution, occurring sometimes separately, sometimes
in juxtaposition. _L. diurna_ is a plant of hedgerows and sheltered
situations. _L. vespertina_ is common in fields and open spaces, where
_diurna_ is hardly ever found; but not rarely _vespertina_ occurs in
association with _diurna_ in the places which that plant frequents. In
this case I do not doubt that we have to do with organisms of somewhat
different aptitudes. That _L. vespertina_ has powers which _diurna_
has not is shown very clearly by the fact that _diurna_ is sometimes
entirely absent from areas where _vespertina_ can abound.[9] But in
order to understand the true genetic relations of the two plants to
each other it is necessary to observe their behaviour when they meet
as they not unfrequently do. If the _Lychnis_ population of such a
locality be examined it will be found to consist of many undoubted
and unmodified _diurna_, a number--sometimes few, sometimes many--of
similarly unmodified _vespertina_, and an uncertain but usually rather
small proportion of plants obviously hybrids between the two. How
is it possible to reconcile these facts with the view that specific
distinction has no natural basis apart from environmental exigency?
Darwinian orthodoxy suggests that by a gradual process of Natural
Selection either one of these two types was evolved from the other,
or both from a third type. I cannot imagine that anyone familiar with
the facts would propose the first hypothesis in the case of _Lychnis_,
nor can I conceive of any process, whether gradual or sudden, by
which _diurna_ could have come out of _vespertina_, or _vespertina_
out of _diurna_. Both however may no doubt have been derived from
some original third type. It is conceivable that _Lychnis macrocarpa_
of Boissier, a native of Southern Spain and Morocco, may be this
original form. This species is said to combine a white flower (like
that of _L. vespertina_), with capsule-teeth rolled back (like those
of _diurna_).[10] But whatever the common progenitor may have been, if
we are to believe that these two species have been evolved from it by
a gradual process of Natural Selection based on adaptation, enormous
assumptions must be made regarding the special fitness of these two
forms and the special unfitness of the common parent, and these
assumptions must be specially invoked and repeated for each several
feature of structure or habits distinguishing the three forms.
Why, if the common parent was strong enough to live to give rise to
these two species, is it either altogether lost now, or at least absent
from the whole of Northern Europe? Its two putative descendants, though
so distinct from each other, are, as we have seen, able often to
occupy the same ground. If they were gradually derived from a common
progenitor--necessarily very like themselves--can we believe that
this original form should always, in all the diversities of soil and
situation which they inhabit, be unable to exist? Some one may fancy
that the hybrids which are found in the situations occupied by both
forms are this original parental species. But nothing can be more
certain than that these plants are simply heterozygous combinations
made by the union of gametes bearing the characters of _diurna_ and
_vespertina_.[11] For they may be reproduced exactly in F_{1} or in
later generations of that cross when it is artificially made; when bred
from their families exhibit palpable phenomena of segregation more or
less complex; and usually, if perhaps not always, they are partially
sterile.[12] In a locality on the Norfolk coast that I know well,
there is a strip of rough ground chiefly sand-bank, which runs along
the shore. This ground is full of _vespertina_. Not a hundred yards
inland is a lane containing _diurna_, and among the _vespertina_ on
the sand-bank are always some of the hybrid form, doubtless the result
of fertilisation from the neighbouring _diurna_ population. Seed saved
from these hybrids gave _vespertina_ and hybrids again, having obviously
been fertilised by other _vespertina_ or by other hybrids, and I have no
doubt that such hybrid plants if fertilised by _diurna_ would have shown
some _diurna_ offspring. The absence of _diurna_ in such localities may
fairly be construed as an indication that _diurna_ is there at a real
disadvantage in the competition for life.
But if, admitting this, we proceed to consider how the special aptitude
of _vespertina_ is constituted, or what it is that puts _diurna_ at
a disadvantage, we find ourselves quite unable to show the slightest
connexion between the success of one or the failure of the other on the
one hand, and _the specific characteristics_ which distinguish the two
forms on the other. The orthodox Selectionist would, as usual, appeal to
ignorance. We ask what can _vespertina_ gain by its white flowers, its
more lanceolate leaves, its grey seeds, its almost erect capsule-teeth,
its longer fruits, which _diurna_ loses by reason of its red flowers,
more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter
fruits? We are told that each of these things _may_ affect the viability
of their possessors. We cannot assert that this is untrue, but we should
like to have evidence that it is true. The same problem confronts us
in thousands upon thousands of examples, and as time goes on we begin
to feel that speculative appeals to ignorance, though dialectically
admissible, provide an insufficient basis for a proposition which,
if granted, is to become the foundation of a vast scheme of positive
construction.
One thing must be abundantly clear to all, that to treat two forms so
profoundly different as one, because intermediates of unknown nature can
be shown to exist between them, is a mere shirking of the difficulties,
and this course indeed creates artificial obstacles in the way of those
who are seeking to discover the origin of organic diversity.
In the enthusiasm with which evolutionary ideas were received the
specificity of living things was almost forgotten. The exactitude with
which the members of a species so often conform in the diagnostic,
specific features passed out of account; and the scientific world by
dwelling with a constant emphasis on the fact of variability, persuaded
itself readily that species had after all been a mere figment of
the human mind. Without presuming to declare what future research
only can reveal, I anticipate that, when variation has been properly
examined and the several kinds of variability have been successfully
distinguished according to their respective natures, the result will
render the natural definiteness of species increasingly apparent.
Formerly in such a case as that of the two _Lychnis_ species, the series
of "intermediates" was taken to be a palpable proof that _vespertina_
"graded" to _diurna_. It is this fact, doubtless, upon which Bentham
would have relied in suggesting that both may be one species.[13]
Genetic tests, though as yet imperfectly applied, make it almost certain
that these inter-grading forms are not in any true sense variations
from either species in the direction of the other, but combinations of
elements derived from both.
The points in which very closely allied species are distinguished
from each other may be found in the most diverse features of their
organisation. Sometimes specific difference is to be seen in a
character which we can believe to be important in the struggle, but at
least as often it is some little detail that we cannot but regard as
trivial which suffices to differentiate the two species. Even when the
diagnostic point is of such a nature that we can imagine it to make a
serious difference in the economy we are absolutely at a loss to suggest
why this feature should be a necessity to species A and unnecessary to
species B its nearest ally. The house sparrow (_Passer domesticus_) is
in general structure very like the tree sparrow (_P. montanus_). They
differ in small points of colour. For instance _montanus_ has a black
patch on the cheek which is absent in _domesticus_. The presence in the
one species and the absence in the other are equally definite, and in
both cases we are equally unable to suggest any consideration of utility
in relation to these features. The two species are distinguished also by
a characteristic that may well be supposed to be of great significance.
In _domesticus_ the two sexes are strongly differentiated, the cock
being more ornate than the hen. On the other hand the two sexes in
_montanus_ are alike, and, if we take a standard from _domesticus_,
we may fairly say that in _montanus_ the hen has the colouration of
the male. It is not unreasonable to suppose that such a distinction
may betoken some great difference in physiological economy, but the
economical significance of this perhaps important distinction is just as
unaccountable as that of the seemingly trivial but equally diagnostic
colour-point.
I have spoken of the fixed characteristics of the two species. If
we turn to a very different feature, their respective liability to
albinistic variation, we find ourselves in precisely similar difficulty.
_Passer domesticus_ is a species in which individuals more or less pied
occur with especial frequency, but in _P. montanus_ such variation is
extremely rare if it occurs at all. The writer of the section on Birds
in the _Royal Natural History_ (III., 1894-5, p. 393) calls attention to
this fact and remarks that in that species he knows no such instance.
The two species therefore, apart from any differences that we can
suppose to be related to their respective habits, are characterised by
small fixed distinctions in colour-markings, by a striking difference
in secondary sexual characters, and by a difference in variability. In
all these respects we can form no surmise as to any economic reason why
the one species should be differentiated in the one way and the other in
the other way, and I believe it is mere self-deception which suggests
the hope that with fuller knowledge reasons of this nature would be
discovered.
The two common British wasps, _Vespa vulgaris_ and _Vespa germanica_,
are another pair of species closely allied although sharply
distinguished, which suggest similar reflexions. Both usually make
subterranean nests but of somewhat different materials. _V. vulgaris_
uses rotten wood from which the nest derives a characteristic yellow
colour, while _V. germanica_ scrapes off the weathered surfaces of
palings and other exposed timber, material which is converted into
the grey walls of the nest. The stalk by which the nest is suspended
(usually to a root) in the case of _germanica_ passes freely through
a hole in the external envelope, but _vulgaris_ unites this external
wall solidly to the stalk. In bodily appearance and structure the
two species are so much alike that they have often been confounded
even by naturalists, and to the untrained observer they are quite
indistinguishable. There are nevertheless small points of difference
which almost though not quite always suffice to distinguish the
two forms. For example the yellow part of the sinus of the eyes is
emarginate in _vulgaris_ but not emarginate in _germanica_. _V.
vulgaris_ often has black spots on the tibiae while in _germanica_ the
tibiae are usually plain yellow. In both species there is a horizontal
yellow stripe on the thorax, but whereas in _vulgaris_ this is a plain
narrow stripe, it is in _germanica_ enlarged downwards in the middle.
These and other apparently trivial details of colouration, though not
absolutely constant, are yet so nearly constant that irregularities
in these respects are quite exceptional. Lastly the genitalia of the
males, though not very different, present small structural points
of distinction which are enough to distinguish the two species at a
glance.[14]
In considering the meaning of the distinctions between these two wasps
we meet the old problem illustrated by the Sparrows. The two species
have somewhat different habits of life and we should readily expect
to find differences of bodily organisation corresponding with the
differences of habits. But is that what we do find? Surely not. To
suppose that there is a correspondence between the little points of
colour and structure which we see and the respective modes of life of
the two species is perfectly gratuitous. We have no inkling of the
nature of such a correspondence, how it can be constituted, or in what
it may consist.
Is it not time to abandon these fanciful expectations which are never
realised? Everywhere both among animals and plants does the problem of
specific difference reiterate itself in the same form. In view of such
facts as I have related and might indefinitely multiply, the fixity of
specific characters cannot readily be held to be a measure of their
economic importance to their possessors. The incidence of specific
fixity is arbitrary and capricious, sometimes lighting on a feature or
a property which can be supposed to matter much, but as often is it
attached to the most trifling of superficial peculiarities.
The incidence of _variability_ is no less paradoxical, and without
investigation of the particular case no one can say what will be found
to show much or little variability. The very characteristic which in
one species may exhibit extreme variability may in an allied species
show extreme constancy. Illustrations will occur to any naturalist, but
nowhere is this truth more strikingly presented than in the British
Noctuid Moths. Many are so variable that, in the common phrase,
"scarcely two can be found alike," while others show comparatively
slight variation. It need scarcely be remarked that, in the instances
I have in mind, the evidence of great variability is in no way due to
the abundance with which the particular species occurs, for common
species may show constancy, and less abundant species may show great
variability. The polymorphism seems to be now at least a general
property of the variable species, as the fixity is a property of the
fixed species. In illustration I may refer to the following examples.
_Dianthoecia capsincola_ is a common and widely distributed moth which
feeds on _Lychnis_. It shows little variation. _Dianthoecia carpophaga_
is another species which feeds chiefly on _Silene_. Its habits are
very similar to those of _capsincola_. Like that species it has a wide
geographical range and is abundant in its localities, but in contrast
to the fixity of _capsincola_, _carpophaga_ exhibits a complex series
of varieties. _Agrotis suffusa_ (= _ypsilon_) is a moth widely spread
through the southern half of England. It is very constant in colour and
markings. _Agrotis segetum_ and _tritici_ are excessively variable both
in ground colour and markings, being found in an immense profusion of
dissimilar forms throughout their distribution. Of these and several
other species of _Agrotis_ there are many named varieties, some of
which have by various writers been regarded as specifically distinct.
Of the genus _Noctua_ many species (e. g. _festiva_) show a similar
polymorphism, but _N. triangulum_, though showing some variation in
certain respects, is usually very constant to its type, and the same is
true of _N. umbrosa_.
In several species of _Taeniocampa_, especially _instabilis_, the
multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is
a comparatively constant species. The genus _Plusia_ contains a number
of constant species, but in _Plusia interrogationis_ we meet the fact
that the central silvery mark undergoes endless variation. "Truly no
two are alike," says Mr. Tutt, "and to look down a long series of
_interrogationis_ is something like looking at a series of Chinese
characters." In contrast to this we have the fact that in _Plusia gamma_
the very similar silvery mark is by no means variable.
I have taken this series of cases from the Noctuid moths, but it would
be as easy to illustrate the same proposition from the Geometridae or
the Micro-Lepidoptera.[15] I have a long series of _Peronea cristana_,
for example, which was given to me by Mr. W. H. B. Fletcher, of Bognor.
All were beaten out of the same hedge, and their polymorphism is such
that no one unaccustomed to such examples could suppose that they
belonged to a single species. Another common form, _P. schalleriana_,
which lives in similar circumstances, exhibits comparatively slight
variability.
It should be expressly noted that the variation of which I am speaking
is a genuine polymorphism. Several of the species enumerated exhibit
also geographical variation, possessing definite and often strikingly
distinct races peculiar to certain localities; but apart from the
existence of such local differentiation, stands out the fact upon which
I would lay stress, that some species are excessively variable while
others are by comparison constant, in circumstances that we may fairly
regard as comparable.
This fact is difficult to reconcile with the conventional view that
specific type is directly determined by Natural Selection and that
the precision with which a species conforms to its pattern is an
indication of the closeness of that control. Anyone familiar with the
characteristics of Moths will agree that the Noctuids, Geometrids and
Tortricids are creatures whose existence depends in some degree on the
success with which they can escape detection by their enemies in the
imaginal state. We are therefore not surprised to find that some species
of these orders exhibit definite geographical variation in conformity
with the character of the ground, which may reasonably be supposed to
aid in their protection. If this were all, there would be nothing to
cause surprise. We might even be disposed to allow that variability
might contribute to the perpetuation of animals so situated, on the
principle that among a variety of surroundings some would probably be
in harmony with the objects on which they rest. But we cannot admit
the plausibility of an argument which demands on the one hand that
the extreme precision with which species A adheres in the minutest
details of its colour and pattern to a certain type shall be ascribed
to the protective fitness of those details, and on the other hand that
the abundant variability of species B shall be ascribed to the same
determination. If it is absolutely necessary for A to conform to one
type how comes it that B may range through some twenty distinct forms,
any two of which differ more from each other than the regular species
of many other genera? The only reply I can conceive is a suggestion
that there _may_ be some circumstance which differentiates the various
classes of cases, that the exigencies of the fixed species _may_ be
different from those of the variable. Those who make such appeals to
ignorance do not always perhaps realise whither this course of reasoning
may lead. If admissible here the same argument would lead us to suggest
that because albino moles have for an indefinite period occurred on a
certain land near Bath there may be something in the soil or in the
conditions of life near Bath which requires a proportion of albinos in
its mole population. Or again, because the butterfly _Thais rumina_
in one locality, Digne in the south of France, has a percentage of
individuals of the variety _Honoratii_ (with certain normally yellow
spots on the hind wing coloured bright red) and nowhere else throughout
its distribution, that therefore we may suggest that there is some
difference in the condition of life at Digne which makes the continuance
of _Honoratii_ there possible and beneficial.
A polymorphism offering a parallel to that of the variable moths is
afforded by the breeding plumage of the Ruff, the male of _Machetes
pugnax_. The variety of plumage which these cocks exhibit is such
that the statement that no two can be found alike is only a venial
exaggeration. Newton remarks[16] "that all this wonderful 'show' is
the consequence of the polygamous habit of the Ruff can scarcely
be doubtful"; but even if it be conceded that the great external
differentiation of the cocks may be a result of sexual selection, the
problem of their _polymorphism_ remains unsolved, for, as we are well
aware, polygamy is not usually associated with polymorphism of the male.
The Black Cock (_Tetrao tetrix_), for example, is as polygamous as the
Ruff, but in that and countless other cases, both sexes are constant to
one type of plumage.
When we thus compare the polymorphism of one species with the fixity
of another, and attempt to determine the causes which have led to
these extraordinary contrasts, two distinct lines of argument are
open to us. We may ascribe the difference either to causes external
to the organisms, primarily, that is to say, to a difference in the
exigencies of Adaptation under Natural Selection; or on the other hand
we may conceive the difference as due to innate distinctions in the
chemical and physiological constitutions of the fixed and the variable
respectively. There is truth undoubtedly in both conceptions. If the
mole were physiologically incapable of producing an albino that variety
would not have come into being, and if the albino were totally incapable
of getting its living it would not be able to hold its own. Were
_Plotheia frontalis_ constructed on a chemical plan which admitted of no
variation, the countless varieties would not have been produced; and if
one of its varieties had an overwhelming success out of all proportion
to that of the rest, then the species would soon become monomorphic
again. We cannot declare that Natural Selection has no part in the
determination of fixity or variability; nevertheless looking at the
whole mass of fact which a study of the incidence of variation provides,
I incline to the view that the variability of polymorphic forms should
be regarded rather as a thing tolerated than as an element contributing
directly to their chances of life; and on the other hand that the fixity
of the monomorphic forms should be looked upon not so much as a proof
that Natural Selection controls them with a greater stringency, but
rather as evidence of a natural and intrinsic stability of chemical
constitution.
Compare the condition of a variable form like the male Ruff (or in
a less degree the Red Grouse in both its sexes) with that of the
common Pheasant which is comparatively constant. In the Pheasant no
doubt variations do occur as in other wild birds, but apart from the
effects of mongrelisation the species is unquestionably uniform. Could
it seriously be proposed that we should regard the constancy of the
pheasant's plumage in this country as depending on the special fitness
of that type of colouration? Even if the pheasant be not an alien in
Western Europe, it has certainly been protected for centuries, and for a
considerable period has existed in a state of semi-domestication. Such
conditions should give good opportunity for polymorphism to be produced.
In some coverts various aberrations do of course occur and persist,
yet there is nothing indicative of a general relaxation of the fixity
of the specific type, and the pheasant remains substantially a fixed
species.[17] The common pheasant (_Phasianus colchicus_) even shows
little of that disposition to form local races which appears in the
species of Further India. Are we not then on safer ground in regarding
the fixity of our species as a property inherent in its own nature
and constitution? Just as in ages of domestication no rose has ever
given off a blue variety so has the pheasant never broken out into the
polymorphism of the Ruff.
As soon as it is realised how largely the phenomena of variation and
stability must be an index of the internal constitution of organisms,
and not mere consequences of their relations to the outer world, such
phenomena acquire a new and more profound significance.
FOOTNOTES:
[1] In _Mendel's Principles of Heredity_ (Cambridge University Press,
1909) I have dealt with this subject, giving an account of the principal
facts discovered up to the beginning of 1909.
[2] Matthioli Opera, Ed. 1598, p. 8, originally published 1565.
[3] Ray's instances relate to Kales, and in most of these examples
we can see that there was no question of mutation or transmutation
at all, but that the occurrence was due either to mistake or to
cross-fertilisation. Sharrock, to whom Ray refers, was inclined to
discredit stories of transmutation, but he has also this passage
(_History of the Propagation and Improvement of Vegetables by the
Concurrence of Art and Nature_, Oxford, 1660, p. 29):
"It is indeed growen to be a great question, whether the transmutation
of a species be possible either in the vegetable, Animal, or Minerall
Kingdome. For the possibility of it in the vegetable; I have heard _Mr.
Bobart_ and his _Son_ often report it, and proffer to make oath that the
Crocus and Gladiolus, as likewise the Leucoium, and Hyacinths by a long
standing without replanting have in his garden changed from one kind to
the other: and for satisfaction about the curiosity in the presence of
_Mr. Boyle_ I tooke up some bulbs of the very numericall roots whereof
the relation was made, though the alteration was perfected before, where
we saw the diverse bulbs growing as it were on the same stoole, close
together, but no bulb half of the one kind, and the other half of the
other: But the changetime being past it was reason we should believe the
report of good artists in matters of their own faculty."
Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts
were professional botanists, the father was author of a Catalogue of
the plants in the Hortus Medicus at Oxford, and the son was afterwards
Curator of the Oxford Garden.
[4] _Mém. Ac. roy. des Sci._ for 1719 (1721), p. 59.
[5] _Amoen. Acad._, 1789, vol. 6. I do not know whether attention has
been called to the curious mistake which Linnaeus makes in the course of
this argument. He cites the differences between the Mule and the Hinny
in illustration of his thesis, pointing out that the Mule is externally
more like a horse and the Hinny more like an ass. This, he says, is
because the Mule has the horse for a father, and the Hinny the ass, thus
inverting the actual facts!
[6] _Proc. Washington Ac. Sci._, 1909, XI, pp. 17-26.
[7] J. W. Tutt, in _Ent. Rec._, 1909, XXI, p. 185.
[8] E. Lehmann (_Bull. l'Herb. Boissier_, Ser. 2, VIII, 1908, p. 229)
has published an admirable paper on the interrelationships of these
species and has instituted cultural experiments which will probably
much elucidate the nature of their specific distinctness. As regards
the existence of intermediate forms he comes to the conclusion that
two only can be so regarded. The first was described by Kuntze from
specimens found on a flower-pot on board a Caspian steamer, from which
Lehmann proposes the new specific name _Siaretensis_. This comes between
_polita_ and _filiformis_, a close ally of _Tournefortii_. The other,
which combines some of the features of both _polita_ and _Tournefortii_,
was found in the province of Asterabad.
[9] In Cambridgeshire for example _vespertina_ is common but _diurna_ is
absent. Whether this absence is connected with the general presence of
chalk I cannot say. When introduced artificially _diurna_ establishes
itself, for a time at least, without any apparent difficulty and
occasionally escapes from the garden on to the neighbouring roadside.
[10] Conceivably however it may be a segregated combination. For an
account of this plant see Boissier, _Voy. Bot. Midi de l'Espagne_, 1839,
II, 722.
[11] A discussion of this subject with references to literature is
given by Rolfe, in an excellent paper on "Hybridisation viewed from the
standpoint of Systematic Botany" (_Jour. R. Hort. Soc._, XXIV, 1900, p.
197). He concludes: "The simple fact is that the two plants (_L. diurna_
and _vespertina_) are thoroughly distinct in numerous particulars, and
affect such different habitats that in some localities one or the other
of them is completely wanting. But when their stations are adjacent they
hybridise together very readily, and it is here that these intermediate
forms occur which have puzzled botanists so much." The same paper
contains valuable information concerning several cognate illustrations.
[12] In only two cases have I seen such plants (both females) completely
sterile.
[13] As is well known, in an even more notorious example, he proposed
to unite _Primula vulgaris_, _P. elatior_, and _P. acaulis_, similarly
relying on the existence of "intermediates," which we now well know to
be mongrels between the species.
[14] For an account of the distinctions between _Vespa vulgaris_ and
_germanica_ see Ch. Janet, _Études sur les Fourmis, les Guêpes et les
Abeilles_, 11^e, Note. Sur _Vespa germanica_ et _V. vulgaris_. Limoges
(Ducourtieux), 1895; and R. du Buysson, Monographie des Guêpes, _Ann.
Soc. Ent. France_, 1903, Vol. LXXII, p. 603, Pl. VIII.
[15] The statements made above are for the most part taken from Barrett,
C. G., _Lepidoptera of the British Islands_, and from Tutt, J. W., _The
British Noctuae and their Varieties_. The reader who is unfamiliar
with the amazing polymorphism exhibited by some of these moths should
if possible take an opportunity of looking over a long series in a
collection, or, if that be impossible, refer to the admirable coloured
plates published by Barrett. It may not be superfluous to observe that
plenty of similar examples are known in other countries. For instance
_Plotheia frontalis_, a Noctuid which often abounds in Ceylon, shows
an equally bewildering wealth of forms. If a dozen specimens of such a
species were to be brought home from some little known country, each
individual would almost certainly be described as the type of a distinct
species. (See the coloured plate published by Sir G. Hampson, Cat. Brit.
Mus., Heterocera, Vol. IX.)
[16] _Dict. of Birds_, p. 800. It would be interesting and profitable
to attempt in a long series of Ruffs to determine the Mendelian factors
which by their combinations give rise to this complex assemblage
of varietal forms. A few such factors both of colour and pattern
can be at once distinguished, and it is noticeable that some of the
resulting types of barring, spangling and penciling show a perceptible
correspondence with some of the types of colouration found in the breeds
of domestic fowls.
[17] Howard Saunders (_Illust. Manual of British Birds_, 1899, p. 499)
states that there is evidence that the pheasant had become naturalized
in the south of England before the Norman invasion. He adds, "little, if
any, deviation from the typical _P. colchicus_ took place up to the end
of last century, when the introduction of the Chinese Ring-necked _P.
torquatus_ commenced, which has left almost indelible marks, especially
with regard to the characteristic white collar."
CHAPTER II
MERISTIC PHENOMENA
Twenty years ago in describing the facts of Variation, argument was
necessary to show that these phenomena had a special value in the
sciences of Zoology and Botany. This value is now universally understood
and appreciated. In spite however of the general attention devoted to
the study of Variation, and the accumulation of material bearing on the
problem, no satisfactory or searching classification of the phenomena is
possible. The reason for this failure is that a real classification must
presuppose knowledge of the chemistry and physics of living things which
at present is quite beyond our reach.
It is however becoming probable that if more knowledge of the chemical
and physical structure of organisms is to be attained, the clue will
be found through Genetics, and thus that even in the uncoordinated
accumulation of facts of Variation we are providing the means of
analysis applicable not only to them, but to the problems of normality
also.
The only classification that we can yet institute with any confidence
among the phenomena of Variation is that which distinguishes on the one
hand variations in the processes of division from variations in the
nature of the substances divided.
Variations in the processes of division are most often made apparent by
a change in the number of the parts, and are therefore called _Meristic_
Variations, while the changes in actual composition of material are
spoken of as _Substantive_ Variations. The Meristic Variations form on
the whole a natural and fairly well defined group, but the Substantive
Variations are obviously a heterogeneous assemblage.
Though this distinction does not go very far, it is useful, and in
all probability fundamental. It is of value inasmuch as it brings
into prominence the distinct and peculiar part which the process
of division, or, more generally, repetition of parts, plays in the
constitution of the forms of living things.
That there may be a real independence between the Meristic and the
Substantive phenomena is evident from the fact both that Meristic
changes may occur without Substantive Variation, and that the substances
composing an organism may change without any perceptible alteration
in its meristic structure. When the distinction between these two
classes of phenomena is perceived it will be realised that the study
of genetics has on the one hand a physical, or perhaps more strictly
a mechanical aspect, which relates to the manner in which material is
divided and distributed; and also a chemical aspect, which relates
to the constitution of the materials themselves. Somewhat as the
philosophers of the seventeenth and eighteenth centuries were awaiting
both a chemical and a mechanical discovery which should serve as a
key to the problems of unorganised matter, so have biologists been
awaiting two several clues. In Mendelian analysis we have now, it is
true, something comparable with the clue of chemistry, but there is
still little prospect of penetrating the obscurity which envelops the
mechanical aspect of our phenomena. To make clear the application of
the terms chemical and mechanical to the problem of Genetics the nature
of that problem must be more fully described. In its most concrete form
this problem is expressed in the question, how does a cell divide? If
the organism is unicellular, and the single cell is the whole body,
then the process of heredity is accomplished in the single operation of
cell-division. Similarly in animals and plants whose bodies are made
up of many cells, the whole process of heredity is accomplished in the
cell-divisions by which the germ-cells are formed. When therefore we see
a cell dividing, we are witnessing the process by which the form and the
properties of the daughter-cells are determined.
Now this process has the two aspects which I have called mechanical
and chemical. The term "_Entwicklungsmechanik_" has familiarised us
with the application of the word mechanics to these processes, but on
reflexion it will be seen that this comprehensive term includes two
sorts of events which are sometimes readily distinguishable. There
is the event by which the cell _divides_, and the event by which the
two halves or their descendants are or may be _differentiated_. It
is common knowledge that in some cell-divisions two similar halves,
indistinguishable in appearance, properties, and subsequent fate, may
be produced, while in other divisions daughter-cells with distinct
properties and powers are formed. We cannot imagine but that in the
first case, when the resulting cells are identical, the division is a
mechanical process by which the mother-cell is simply cut in two; while
in order that two differentiated halves may be produced, some event must
have taken place by which a chemical distinction between the two halves
is effected.[1] In any ordinary Mendelian case we have a clear proof
that such a chemical difference may be established between germ-cells.
The facts of colour-inheritance for instance prove that germ-cells,
otherwise identical, may be formed _possessing_ the chromogen-factor
which is necessary to the formation of colour in the flowers, or
_destitute_ of that factor. Similarly the germ-cells may possess the
ferment which, by its action on the chromogenic substance, produces the
colour, or they may be without that ferment. The same line of argument
applied to a great range of cases. Nevertheless, though differences
in chemical properties are often thus constituted by cell-divisions,
and though we are thus able to make a quasi-chemical analysis of the
individual by determining and enumerating these properties, yet it is
evident that the distribution of these factors is not itself a chemical
process. This is proved by the fact that similar divisions may be
effected between halves which are exactly alike, and also by the fact
that the numbers in which the various types of germ-cells are formed
negative any suggestion of valency between them. The recognition of the
unit-factors may lead--indeed must lead--to great advances in chemical
physiology which without that clue would have been impossible, but
in causation the chemical phenomena of heredity must be regarded as
secondary to the physical or mechanical phenomena by which the cells
and their constituents are divided and separated. When therefore we
speak of the _essential_ phenomena of heredity we mean the mechanics
of division, especially, though not, as we shall see, exclusively, of
_cell_-division; and in the relation between the two halves of the
dividing cell we have the problem presented in what seems to be its
simplest form.
In attempting to form some conception of the processes by which bodily
characteristics are transmitted, or--to avoid that confusing metaphor
of "transmission"--how it comes about that the offspring can grow to
resemble its parent, continuity of the germ-substance which in some
animals is a visible phenomenon,[2] gives at least apparent help. An egg
for example on becoming adult develops in certain parts a particular
pigment. The eggs of that adult when they reach the appropriate age
develop the same pigment. We have no clear picture of the mechanism by
which this process is effected, but when we realise that the pigment
results from the interaction of certain substances, and that since
all the eggs are in reality pieces of the same material, it seems,
unless we inquire closely, not unnatural that the several pieces of the
material should exhibit the same colours at the same periods of their
development. The continuity of the material of the germs suggests that
there is a continuity of the materials from which the pigment is formed,
and that thus an actual bit of those substances passes into each egg
ready at the appropriate moment to generate the pigment. The argument
thus outlined applies to all _substantive_ characteristics. In each case
we can imagine, if we will, the appearance of that characteristic as due
to the contribution of its rudiment from the germ tissues.
When we consider more critically it becomes evident that the aid given
by this mental picture is of very doubtful reality, for even if it were
true that any predestined particle actually corresponding with the
pigment-forming materials is definitely passed on from germ to germ,
yet the power of increase which must be attributed to it remains so
incomprehensible that the mystery is hardly at all illuminated.
When however we pass from the substantive to the meristic characters,
the conception that the character depends on the possession by the germ
of a particle of a specific material becomes even less plausible. Hardly
by any effort of imagination can we see any way by which the division
of the vertebral column into _x_ segments or into _y_ segments, or of a
Medusa into 4 segments or into 6, can be determined by the possession
or by the want of a material particle. The distinction must surely be
of a different order. If we are to look for a physical analogy at all
we should rather be led to suppose that these differences in segmental
numbers corresponded with changes in the amplitude or number of dividing
waves than with any change in the substance or material divided.
PHENOMENA OF DIVISION
I have said that in the division of a cell we seem to see the problem in
its simplest form, but it is important to observe that the problem of
division may be presented by the bodies of animals and plants in forms
which are independent of the divisions between cells. The existence of
pattern implies a repetition of parts, and repetition of parts when
developed in a material originally homogeneous can only be created by
division. Cell-division is probably only a special case of a process
similar to that by which the pattern of the skeleton is laid down in
a unicellular body such as that of a Radiolarian or Foraminiferan.
Attempts have lately been made to apply mathematical treatment to
problems of biology. It has sometimes seemed to me that it is in the
geometrical phenomena of life that the most hopeful field for the
introduction of mathematics will be found. If anyone will compare one
of our animal patterns, say that of a zebra's hide, with patterns known
to be of purely mechanical production, he will need no argument to
convince him that there must be an essential similarity between the
processes by which the two kinds of patterns were made and that parts
at least of the analysis applicable to the mechanical patterns are
applicable to the zebra stripes also. Patterns mechanically produced are
of many and very diverse kinds. One of the most familiar examples, and
one presenting some especially striking analogies to organic patterns,
is that provided by the ripples of a mackerel sky, or those made in a
flat sandy beach by the wind or the ebbing tide. With a little search
we can find among the ripple-marks, and in other patterns produced by
simple physical means, the closest parallels to all the phenomena of
striping as we see them in our animals. The forking of the stripes, the
differentiation of two "faces," the deflections round the limbs and
so forth, which in the body we know to be phenomena of division, are
common both to the mechanical and the animal patterns. We cannot tell
what in the zebra corresponds to the wind or the flow of the current,
but we can perceive that in the distribution of the pigments, that
is to say, of the chromogen-substances or of the ferments which act
upon them, a rhythmical disturbance has been set up which has produced
the pattern we see; and I think we are entitled to the inference that
in the formation of patterns in animals and plants mechanical forces
are operating which ought to be, and will prove to be, capable of
mathematical analysis. The comparison between the striping of a living
organism and the sand-ripples will serve us yet a little farther,
for a pattern may either be formed by actual cell-divisions, and
the distribution of differentiation coincidently determined, or--as
visibly in the pigmentation of many animal and plant tissues--the
pattern may be laid down and the pigment (for example) distributed
through a tissue across or independently of the cell-divisions of the
tissue. Our tissues therefore are like a beach composed of sands of
different kinds, and different kinds of sands may show distinct and
interpenetrating ripples. When the essential analogy between these
various classes of phenomena is perceived, no one will be astonished at,
or reluctant to admit, the reality of discontinuity in Variation, and if
we are as far as ever from knowing the actual causation of pattern we
ought not to feel surprised that it may arise suddenly or be suddenly
modified in descent. Biologists have felt it easier to conceive the
evolution of a striped animal like a zebra from a self-coloured type
like a horse (or of the self-coloured from the striped) as a process
involving many intergradational steps; but so far as the _pattern_ is
concerned, the change may have been decided by a single event, just as
the multitudinous and ordered rippling of a beach may be created or
obliterated at one tide.
[Illustration: FIG. 1. Tusk of Indian elephant, showing an abnormal
segmentation.]
This point is well illustrated by the tusk of an Indian elephant which
I lately found in a London sale-room. This tusk is by some unknown
cause, presumably a chronic inflammation, thrown up into thirteen
well-marked ridges which closely simulate a series of segments (Fig. 1).
Whatever the cause the condition shows how easily a normally unsegmented
structure may be converted into a series of repeated parts.
The spread of segmentation through tissues normally unsegmented is very
clearly exemplified in the skates' jaws shown in Fig. 2. The right
side of the upper figure shows the normal arrangement in the species
_Rhinoptera jussieui_, but the structure on the left side is very
different. The probable relations of the several rows of teeth to the
normal rows is indicated by the lettering, but it is evident that by
the appearance of new planes of division constituting separate centers
of growth, the series has been recast. The pattern of the left side is
so definite that had the variation affected the right side also, no
systematist would have hesitated to give the specimen a new specific
name. The other two drawings show similar variations of a less extensive
kind, the nature of which is explained by the lettering of the rows of
teeth.
[Illustration: FIG. 2. Jaws of Skates (_Rhinoptera_) showing meristic
variation. (For a detailed discussion see _Materials for the Study of
Variation_, p. 259.)]
This power to divide is a fundamental attribute of life, and of that
power cell-division is a special example. In regard to almost all the
chief vital phenomena we can say with truth that science has made some
progress. If I mention respiration, metabolism, digestion, each of these
words calls to mind something more than a bare statement that such
acts are performed by an animal or a plant. Each stands for volumes of
successful experiment and research, But the expression cell-division,
the fundamental act which typifies the rest, and on which they all
depend, remains a bare name. We can see with the microscope the outward
symptoms of division, but we have no surmise as to the nature of the
process by which the division is begun or accomplished. I know nothing
which to a man well trained in scientific knowledge and method brings
so vivid a realisation of our ignorance of the nature of life as the
mystery of cell-division. What is a living thing? The best answer in
few words that I know is one which my old teacher, Michael Foster, used
to give in his lectures introductory to biology. "A living thing is a
vortex of chemical and molecular change." This description gives much,
if not all, that is of the essence of life. The living thing is unlike
ordinary matter in the fact that, through it, matter is always passing.
Matter is essential to it; but, provided that the flow in and out is
unimpeded, the life-process can go on so far as we know indefinitely.
Yet the living "vortex" differs from all others in the fact that it
can divide and throw off other "vortices," through which again matter
continually swirls.
We may perhaps take the parallel a stage further. A simple vortex, like
a smoke-ring, if projected in a suitable way will twist and form two
rings. If each loop as it is formed could grow and then twist again to
form more loops, we should have a model representing several of the
essential features of living things.
It is this power of spontaneous division which most sharply
distinguishes the living from the non-living. In the excellent book
dealing with the problems of development, lately published by Mr.
Jenkinson a special emphasis is very properly laid on the distinction
between the processes of division, and those of differentiation. Too
often in discussions of the developmental processes the distinction
is obscured. He regards differentiation as the "central difficulty."
"Growth and division of the nucleus and the cells," he tells us,
are side-issues. This view is quite defensible, but I suspect that
the division _is_ the central difficulty, and that if we could get
a rationale of what is happening in cell-division we should not be
long before we had a clue to the nature of differentiation. It may be
self-deception, but I do not feel it impossible to form some hypothesis
as to the mode of differentiation, but in no mood of freest speculation
are we ever able to form a guess as to the nature of the division. We
see differentiations occurring in the course of chemical action, in
some phenomena of vibration and so forth: but where do we see anything
like the spontaneous division of the living cell? Excite a gold-leaf
electroscope, and the leaves separate, but we know that is because they
were double before. In electrolysis various substances separate out at
the positive and negative poles respectively. Now if in cell-division
the two daughter-cells were always dissimilar--that is to say, if
differentiation always occurred--we could conceive some rough comparison
with such dissociations. But we know the dissimilarity between
daughter-cells is not essential. In the reproduction of unicellular
organisms and many other cases, the products formed at the two poles
are, so far as we can tell, identical. Any assumption to the contrary,
if we were disposed to make it, would involve us in difficulties still
more serious. At any rate, therefore, if differentiation be really the
central difficulty in development, it is division which is the essential
problem of heredity.
Sir George Darwin and Professor Jeans tell us that "gravitational
instability" consequent on the condensation of gases is "the primary
agent at work in the actual evolution of the universe," which has led to
the division of the heavenly bodies. The greatest advance I can conceive
in biology would be the discovery of the nature of the instability which
leads to the continual division of the cell. When I look at a dividing
cell I feel as an astronomer might do if he beheld the formation of a
double star: that an original act of creation is taking place before me.
Enigmatical as the phenomenon seems, I am not without hope that, if it
were studied for its own sake, dissociated from the complications which
obscure it when regarded as a mere incident in development, some hint as
to the nature of division could be found. It is I fear a problem rather
for the physicist than for the biologist. The sentiment may not be a
popular one to utter before an assembly of biologists, but looking at
the truth impersonally I suspect that when at length minds of first rate
analytical power are attracted to biological problems, some advance will
be made of the kind which we are awaiting.
The study of the phenomena of bodily symmetry offers perhaps the
most hopeful point of attack. The essential fact in reproduction is
cell-division, and the essential basis of hereditary resemblance is
the symmetry of cell-division. The phenomena of twinning provide a
convincing demonstration that this is so. By twinning we mean the
production of equivalent structures by division. The process is one
which may affect the whole body of an animal or plant, or certain of
its parts. The term twin as ordinarily used refers to the simultaneous
birth of two individuals. Those who are naturalists know that such twins
are of two kinds, (1) twins that are not more alike than any other
two members of the same family, and (2) twins that are so much alike
that even intimate friends mistake them. These latter twins, except in
imaginative literature, are always of the same sex.
It is scarcely necessary for me to repeat the evidence from which it has
been concluded that without doubt such twins arise by division of the
same fertilised ovum. There is a perfect series of gradations connecting
them with the various forms of double monsters united by homologous
parts. They have been shown several times to be enclosed in the same
chorion, and the proofs of experimental embryology show that in several
animals by the separation of the two first hemispheres of a dividing egg
twins can be produced. Lastly we have recently had the extraordinarily
interesting demonstration of Loeb, to which I may specially refer.
Herbst some years ago found that in sea water, from which all lime salts
had been removed, the segments of the living egg fall apart as they are
formed. Using this method Loeb has shown that a temporary immersion in
lime-free sea water may result in the production of 90 per cent. of
twins. We are therefore safe in regarding the homologous or "identical"
twins as resulting from the divisions of one fertilised egg, while the
non-identical or "fraternal" twins, as they are called, arise by the
fertilisation of two separate ova.[3]
In the resemblance of identical twins we have an extreme case of
hereditary likeness[4] and a proof, if any were needed, that the
cause of individual variation is to be sought in the differentiation
of germ-cells. The resemblance of identical twins depends on two
circumstances, First, since only two germ-cells take part in their
production, difference between the germ cells of the same individual
cannot affect them. Secondly the division of the fertilised ovum,
the process by which they became two instead of one, must have been
a symmetrical division. The structure of twins raises however one
extremely significant difficulty, which as yet we cannot in any way
explain. The resemblance between twins is a phenomenon of symmetry,
like the resemblance between the two sides of a bilaterally symmetrical
body. Not only is the general resemblance readily so interpreted, but
we know also that in double monsters, namely unseparated twins, various
anatomical abnormalities shown by the one half-body are frequently shown
by the other half-also.[5] The two belong to one system of symmetry.
How then does it happen that the body of one of a pair of twins does
not show a transposition of viscera? We know that the relation of right
and left implies that the one should be the mirror-image of the other.
Such a relation of images may be maintained even in minute details.
For example if the same pattern of finger-print is given by the fingers
of the two hands, one is the reverse of the other. In double monsters,
namely unseparated twins, there is evidence that an inversion of viscera
does occur with some frequency. Evidence from such cases is not so clear
and simple as might be expected, because as a matter of fact, the heart
and stomach, upon which the asymmetry of the viscera chiefly depend, are
usually common to the two bodies. Duplicity generally affects either
the anterior end alone, or the posterior end alone. The division is
generally _from the heart forwards_, giving two heads and two pairs of
anterior limbs on a common trunk, or _from the heart backwards_, giving
two pairs of posterior limbs with the anterior body common. In either
case, though the bodies may be grouped in a common system of symmetry,
neither can be proved to show definite reversal of the parts. To see
that reversal recourse must be had to more extreme duplications, such as
the famous Siamese Twins. They, as a matter of fact, were an excellent
instance of the proposition that twins are related as mirror-images,
for both of them had eleven pairs of ribs instead of the normal twelve,
and one of them had a partial reversal of viscera.[6] (Küchenmeister,
_Verlagerung_, etc., p. 204.)
If anyone could show how it is that neither of a pair of twins has
transposition of viscera the whole mystery of division would, I expect,
be greatly illuminated.[7] At present we have simply to accept the fact
that twins, by virtue of their detachment from each other, have the
power of resuming the polarity which is proper to any normal individual.
It was nevertheless with great interest that I read Wilder's recent
observation[8] that occasionally in identical twins the finger-print of
one or both the index-fingers may be reversed, showing that there is
after all some truth in the notion that reversal should occur in them.
There is another phenomenon by twinning which, if we could understand
it, might help. I refer to the free-martin, the subject of one of John
Hunter's masterpieces of anatomical description. In horned cattle twin
births are rare, and when twins of opposite sexes are born, the male
is perfect and normal, but the reproductive organs of the female are
deformed and sterile, being known as a free-martin. The same thing
occasionally happens in sheep, suggesting that in sheep also twins may
be formed by the division of one ovum; for it is impossible to suppose
that mere development in juxtaposition can produce a change of this
character. I mention the free-martin because it raises a question of
absorbing interest. It is conceivable that we should interpret it by
reference to the phenomenon of gynandromorphism, seen occasionally in
insects, and also in birds as a great rarity. In the gynandromorph one
side of the body is male, the other female. A bullfinch for instance has
been described with a sharp line of division down the breast between the
red feathers of the cock on one side and the brown feathers of the hen
on the other. (Poll, H., _SB. Ges. Nat. Fr._, Berlin, 1909, p. 338.) In
such cases neither side is sexually perfect. If the halves of such a
gynandromorph came apart, perhaps one would be a free-martin.
The behaviour of homologous twinning in heredity has been little
studied. It does not exist as a normal feature in any animal which
is amenable to experiment, and we cannot positively assert that a
comparable phenomenon exists in plants; for in them--the Orange, for
example--polyembryony may evidently be produced by a parthenogenetic
development of nucellar tissue. It is possible that in Man twinning
is due to a peculiarity of the mother, not of the father. It may and
not rarely does descend from mother to daughter, but whether it can be
passed on through a male generation to a daughter again, there is not
sufficient evidence to show. The facts as far as they go are consistent
with the inference which may be drawn from Loeb's experiment, that the
twinning of a fertilized ovum may be determined not by the germ-cells
which united to form it, but by the environment in which it begins to
develop. The opinion that twinning may descend through the male directly
has been lately expressed by Dr. J. Oliver in the _Eugenics Review_
(1912), on the evidence of cases in which twins had occurred among the
relations of fathers of twins, but I do not know of any comprehensive
collection of evidence bearing on the subject.
Besides twinning of the whole body a comparable duplicity of various
parts of the same body may occur. Such divisions affect especially those
organs which have an axis of bilateral symmetry, such as the thumb, a
cotyledon, a median petal, the frond of a fern or the anal fin of a
fish. From the little yet known it is clear that the genetic analysis
of these conditions must be very difficult, but evidence of any kind
regarding them will be valuable. We want especially to know whether
these divisions are due to the _addition_ of some factor or power which
enables the part to divide, or whether the division results from the
_absence_ of something which in the normal body prevents the part from
dividing. Breeding experiments, so far as they go, suggest that the less
divided state is usually dominant to the more divided.[9] The two-celled
Tomato fruit is dominant to the many-celled type. The Manx Cat's tail,
with its suppression of caudal segmentation is a partial dominant over
the normal tail. The tail of the Fowl in what is called the "Rumpless"
condition is at least superficially comparable with that of the Manx
Cat, and though the evidence is not wholly consistent, Davenport
obtained facts indicating that this suppressed condition of the caudal
vertebrae is an imperfect dominant.[10]
Some evidence may also be derived from other examples of differences
which at first sight appear to be substantive though they are more
probably meristic in ultimate nature. The distinction between the
normal and the "Angora" hair of the Rabbit is a case in point. We can
scarcely doubt that one of the essential differences between these two
types is that in the Angora coat the hair-follicles are more finely
divided than they are in the normal coat, and we know that the normal,
or less-divided condition, is dominant to the Angora, or more finely
divided.
[Illustration: FIG. 3. _I_, _II_, _III_, various degrees of syndactyly
affecting the medius and annularis in the hand; _IV_, syndactyly
affecting the index and medius in the foot. (After Annandale.)]
In the case of the solid-hoofed or "mule-footed" swine, the evidence
shows, as Spillman has lately pointed out,[11] that the condition
behaves as a dominant. The essential feature of this abnormality is
that the digits III and IV are partially united. The union is greatest
peripherally. Sometimes the third phalanges only are joined to form one
bone, but the second and even the first phalanges may also be compounded
together. Here the variation is obviously meristic and consists in a
failure to divide, the normal separation of the median digits of the
foot being suppressed.
[Illustration: FIG. 4. Case of complete syndactyly in the foot. _II_ and
_III_, digit apparently representing the index and medius. _c_^{2} +
_c_^{3}, bone apparently representing the middle and external cuneiform;
_cb_, cuboid; _c_^{1}, internal cuneiform. (After Gruber.)]
Webbing between the digits, in at least some of its manifestations, is
a variation of similar nature. The family recorded by Newsholme[12]
very clearly shows the dominance of this condition. The case is
morphologically of great interest and must undoubtedly have a bearing on
the problems of the mechanics of Division. In discussing the phenomena
of syndactylism I pointed out some years ago that the digits most
frequently united in the human hand are III and IV, while in the foot,
union most frequently takes place between II and III.[13] In Newsholme's
family the union was always between II and III of the foot, except in
the case of one male who had the digits III and IV of the right _hand_
alone webbed together. There can be little doubt that the geometrical
system on which the foot is planned has an axis of symmetry passing
between the digits II and III, while the corresponding axis in the hand
passes between III and IV. Union between such digits may therefore be
regarded as comparable with any non-division or "coalescence" of lateral
structures in a middle line, and when as in these examples such a
condition is shown to be a dominant we cannot avoid the inference that
some concrete factor has the power of suppressing or inhibiting this
division. Figs. 3 and 4 illustrate degrees of union between digits in
the human hand and foot.
It is not in question that various other forms of irregular webbing
and coalescence of digits exist, and respecting the genetic behaviour
of these practically nothing is as yet known. Such a case is described
by Walker,[14] in which the first and second metacarpals of both
feet were fused in mother and daughter, and several more are found
in literature. Contrasted with these phenomena we have the curious
fact that in the Pigeon, Staples-Browne found webbing of the toes a
_recessive_ character. The question thus arises whether this webbing is
of the same nature as that shown to be a dominant in Man, and indeed
whether the phenomenon in pigeons is really meristic at all. There is
some difference perceptible between the two conditions; for in Man
there is not so much a development of a special web-like skin uniting
the digits as a want of proper division between the digits themselves,
and in extreme cases two digits may be represented by a single one. In
the Pigeon I am not aware that a real union of this kind has ever been
observed, and though the web-like skin may extend the whole length of
the digits and be so narrow as to prevent the spread of the toes, it
may, I think, be maintained that the unity of the digits is unimpaired.
For the present the nature of this variation in the pigeon's feet must
be regarded as doubtful, and we should note that if it is actually an
example of a more perfect division being dominant to a less perfect
division, the case is a marked exception to the general rule that
non-division is dominant to division.
Reference must also be made to the phenomenon of fasciation in the stems
of plants. As Mendel showed in the case of _Pisum_ this condition is
often a recessive. The appearances suggest that the difference between a
normal and a fasciated plant consists in the inability of the fasciated
plant to separate its lateral branches. The nature of the condition is
however very obscure and it is equally likely that some multiplication
of the growing point is the essential phenomenon.[15]
Stockard's interesting experiments[16] illustrate this question. He
showed that by treating the embryos of a fish (_Fundulus heteroclitus_)
with a dilute solution of magnesium salts, various cyclopian
monstrosities were frequently produced. These have been called cases
of _fusion_ of the optic vesicles. I would prefer to regard them as
cases of a division suppressed or restricted by the control of the
environment. Conversely, the splendid discovery of Loeb, that an
unfertilised egg will divide and develop parthenogenetically without
fertilisation, as a consequence of exposure to various media, may be
interpreted as suggesting that the action of those media releases
the strains already present in the ovum, though I admit that an
interpretation based on the converse hypothesis, that the medium acts as
a stimulus, is as yet by no means excluded.
In these cases we come nearest to the direct causation or the direct
inhibition of a division, but the meaning of the evidence is still
ambiguous. I incline to compare Loeb's parthenogenesis with the
development (and of course accompanying cell-division) of dormant buds
on stems which have been cut back.
It is interesting to note that sometimes as an abnormality, the
faculty of division gets out of hand and runs a course apparently
uncontrolled. A remarkable instance of this condition is seen in
_Begonia_ "_phyllomaniaca_", which breaks out into buds at any point on
the stem, petioles, or leaves, each bud having, like other buds, the
power of becoming a new plant if removed. We would give much to know the
genetic properties of _B. phyllomaniaca_, and in conjunction with Mr. W.
O. Backhouse I have for some time been experimenting with this plant.
It proved totally sterile. Its own anthers produce no pollen, and all
attempts to fertilise it with other species failed though the pollen of
a great number of forms was tried.
Recently however we have succeeded in making plants which are in every
respect _Begonia phyllomaniaca_, so far as the characters of stems and
leaves are concerned. These plants, of which we have sixteen, were made
by fertilising _B. heracleifolia_ with _B. polyantha_. They are all
beginning to break out in "phyllomania." As yet they have not flowered,
but as they agree in all details with _phyllomaniaca_ there can be
little doubt that the original plant bearing that name was a hybrid
similarly produced. The production of "phyllomania" on a hybrid Begonia
has also been previously recorded by Duchartre.[17] In this case the
cross was made between _B. incarnata_ and _lucida_. The synonymy of
the last species is unfortunately obscure, and I have not succeeded in
repeating the experiment.
[Illustration: FIG. 5. Piece of petiole of _Begonia phyllomaniaca_. The
proximal end is to the right of the figure.]
From these facts it seems practically certain that the condition is one
which is due to the meeting of complementary factors. At first sight
we may incline to think that the phyllomania is in some way due to the
sterility. This however cannot be seriously maintained; for not only is
sterility in plants not usually associated with such manifestations, but
we know a Begonia called "Wilhelma" which is exactly _phyllomaniaca_ and
equally sterile, though it has no trace of phyllomania. This plant arose
in the nurseries of MM. P. Bruant of Poitiers, and has generally been
described as a seedling of _phyllomaniaca_, but from the total sterility
of that form this account of its origin must be set aside.
[Illustration: FIG. 6. Two right hind feet of polydactyle cats. _II_
shows the lowest development of the condition yet recorded. The digit,
_d_^{1}, which stands as hallux is fully formed and has three phalanges.
Both it and the digit marked _d_^{2} are formed as _left_ digits. In the
normal hind foot of the cat the hallux is represented by a rudiment only.
_I_ shows a further development of the condition. In this foot there are
_six_ digits. _d_^{1} has two phalanges, but both it and _d_^{2} and
_d_^{3} are shaped as left digits. Thus _d_^{3}, which in the normal
foot would be shaped as a right digit, is transformed so as to look like
a _left_ digit.]
The phenomenon in this case can hardly be regarded as due to the
excitation of dormant buds, for it is apparent on examination that
the new growths are not placed in any fixed geometrical relation to
the original plant. They arise on the petiole, for example, as small
green outgrowths each of which gradually becomes a tiny leaf. The
attitude of these leaves is quite indeterminate, and they may point
in any direction, some having their apices turned peripherally, some
centrally, and others in various oblique or transverse positions (Fig.
5). These little leaves are thus comparable with seedlings, in that
their polarity is not related to, or consequent upon that of the parent
plant. They have in fact that "individuality," which we associate with
germinal reproduction.
There are many curious phenomena seen in the behaviour of parts normally
repeated in bilateral symmetry which may some day guide us towards
an understanding of the mechanics of division. A part like a hand,
which needs the other hand to complete its symmetry, cannot twin by
mere division, yet by proliferation and special modifications on the
radial side of the same limb, even a hand may be twinned. In the well
known polydactyle cats a change of this kind is very common and indeed
almost the rule. When extra digits appear at the inner (tibial) side
of the limb, they are shaped as digits of the other side, and even the
normal digit II (index) is usually converted into the mirror-image
of its normal self. The limb then develops a new symmetry in itself.
Nevertheless it is not easy to interpret these facts as meaning that
there has been some interruption in the control which one side of the
body exercises over the other. The heredity of polydactylism is complex
but there is little doubt that the condition familiar in the Cat is a
dominant. In some human cases also the descent is that of a dominant,
but irregularities are so frequent that no general rule can yet be
perceived. The dominance of such a condition is an exception to the
principle that the less-divided is usually dominant to the more-divided,
a fact which probably should be interpreted as meaning that divisions
are of more than one kind.
Among ordinary somatic divisions, whether of organs, cells, or patterns
of differentiation, the control of symmetry is usually manifested. There
is however one class of somatic differentiations which are exceptionally
interesting from the fact that they may show a complete independence
of such geometrical control. The most familiar examples of these
geometrically uncontrolled Variations are to be seen in bud-sports.
The normal differentiation of the organs of a plant is arranged on
a definite geometrical system, which to those who have never given
special attention to such things before, will often seem surprisingly
precise. The arrangement of the leaves on uninjured, free-growing shoots
can generally be seen to follow a very definite order, just as do the
flowers or the parts of the flowers. If however bud sports occur,
then though the parts included in the sports show all the geometrical
peculiarities proper to the sport-variety, yet the sporting-buds
themselves are not related to each other according to any geometrical
plan.
A very familiar illustration is provided by the distribution of colour
in those Carnations that are not self-coloured. The pigment may, as in
Picotees, be distributed peripherally with great regularity to the edges
of the petals; or, as in Bizarres and Flakes, it may be scattered in
radial sectors which show no geometrical regularity. Now in this case
the pigments are the same in both types of flower, and the chemical
factors concerned in their production must surely be the same. The
difference must lie in the mechanical processes of distribution of
the pigment. In the Picotee we see the orderly differentiation which
we associate with normality; in the Bizarre we see the disorderly
differentiation characteristic of bud-sports. The distribution of colour
in this case lies outside the scheme of symmetry of the plant.
Such a distribution is characteristic of bud-sports, and of certain
other differentiations in both plants and animals, which I cannot on
this occasion discuss. Now reflexion will show that these facts have
an intimate bearing on the mechanical problems of heredity. For first
in the bud-sports we are witnessing the distribution of factors which
distinguish genetic varieties. We do not know the physical nature of
those factors, but if we must give them a name, I suppose we should call
them "ferments" exactly as Boyle did in 1666. He is discussing how it
comes about that a bud, budded on a stock, becomes a branch bearing the
fruit of its special kind. He notes that though the bud inserted be "not
so big oftentimes as a Pea," yet "whether by the help of some peculiar
kind of Strainer or by the Operation of some powerful Ferment lodged in
it, or by both these, or some other cause," the sap is "so far changed
as to constitute a Fruit quite otherwise qualify'd."[18] We can add
nothing to his speculation, and we believe still that by a differential
distribution of "ferments" the sports are produced. All the factors are
together present in the normal parts; some are left out in the sport. In
an analogous case however, that of a variegated _Pelargonium_ which has
green and also albino shoots, Baur proved that the shoots pure in colour
are also pure in their posterity. There can be no doubt that the sports
of Carnations, Azaleas, Chrysanthemums, etc., would behave in the same
way.
The well-known Azaleas Perle de Ledeburg, President Kerchove,
and _Vervaeana_ are familiar illustrations. Perle de Ledeburg is
predominantly white, but it has red streaks in some of its flowers. It
not very rarely gives off a self-red sport. This is evidently due to
the development of a bud in a red-bearing area of the stem. The red in
this plant is not under "geometrical control." Many plants have white
flowers with no markings, but if the red markings are geometrically
ordered differentiations, no self-coloured sports are formed. The case
of _Vervaeana_ is a good illustration of this proposition. It has white
flowers with red markings arranged in an orderly manner on the lower
parts of the petals, especially on the dorsal petals. This is one of the
Azaleas most liable to have red sports, and at first sight it might seem
that the sport represented the red of the central marks. Examination
however of a good many flowers shows that irregular red streaks like
those of Perle de Ledeburg occur, about as commonly as in that variety.
_Vervaeana_ in fact is Perle de Ledeburg with _definite_ red markings
added, and its red sports obviously are those branches the germs of
which came in a patch of the stem bearing these red elements. That this
is the true account is rendered quite obvious by the fact that the red
of the sport is a colour somewhat different from that of the definite
marks, and that these marks are still present on the red ground of the
sporting flowers.
It will be understood that these remarks apply to those cases in which
the production of sports is habitual or frequent, and I imagine in
all such examples it will be found that there are indications of
irregularity in the distribution of the differentiations such as to
justify the view that they are not under that geometrical control which
governs the normal differentiation of the parts. The question next
arises whether these considerations apply also to the production of a
bud-sport as a rare exception, but by the nature of the case it is not
possible to say positively whether the appearance of an exceptional
sport is due to the unsuspected presence of a pre-existing fragment of
material having a special constitution, or to the origin, _de novo_, of
such a material. For instance one of the garden forms of _Pelargonium_
known as _altum_ is liable perhaps once in some hundreds of flowers to
have one or two magenta petals. The normal colour is a brilliant red;
and as we may be fairly sure that this red is recessive to magenta the
interpretation would be quite different according as the appearance of
the magenta is regarded as due to the presence of small areas endowed
with magentaness, or to the spontaneous generation of the factor for
that pigment. Either interpretation is possible on the facts, but the
view that the whole plant has in it scarce mosaic particles of magenta
seems on the whole more consistent with present knowledge.
In _Pelargonium altum_ the enzyme causing the magenta colours must
be distributed in very small areas, but a case in which the magenta
is similarly arranged in a much coarser patchwork may be seen in the
_Pelargonium_ "Don Juan," which often bears whole trusses or branches of
red flowers upon plants having the normal dominant magenta trusses. In
most cases there is little doubt that though the magenta flowered parts
can "sport" to red, the red parts could not produce the magenta flowers.
The asymmetrical, or to speak more precisely, the disorderly, mingling
of the colours in the somatic parts is thus an indication of a similarly
disorderly mixing of the factors for those colours in the germ-tissues,
so that some of the gametes bear enough of the colour-factors to make a
self-coloured plant, while others bear so little that the plant to which
they give rise is a patchwork. If this view is correct we may extend it
so far as to consider whether the fineness or coarseness of the mixture
visible in the flowers or leaves may not give an indication of the
degree to which the factors are subdivided among the germ-cells. We know
very little about the genetic properties of striped varieties. In both
_Antirrhinum_ and _Mirabilis_ it has been found that the striped may
occasionally and irregularly throw self-coloured plants, and therefore
the striping cannot be regarded simply as a recessive character. On the
other hand in _Primula Sinensis_ there are well-known flaked varieties
which ordinarily at least breed true. Whether these ever throw selfs I
do not know, but if they do it must be quite exceptionally. The power
of these flaked plants to breed true is, I suspect, connected with the
fact that in their flowers the coloured and white parts are _intimately_
mixed, this intimate mixture thus being an indication of a similarly
intimate mixture in the germ-cells. It would be important to ascertain
whether self-fertilised seed from the occasional flowers in which the
colour has run together to join a large patch gives more self-coloured
plants than the intimately flaked flowers do.
The next fact may eventually prove of great importance. We have seen
that in bud-sports the differentiation is of the same nature as
that between pure types, and also that in the sporting plant this
differentiation is distributed without any reference to the plant's
axis, or any other consideration of symmetry. Now among the germ-cells
of a Mendelian hybrid exactly such characters are being distributed
allelomorphically, and there again we have strong evidence for believing
that the distribution obeys no pattern. For example, we can in the case
of seeds still _in situ_ perceive how the characters were distributed
among the germ-cells, and there is certainly no obvious pattern
connecting them, nor can we suppose that there is an actual pattern
obscured.
Of this one illustration is especially curious. Individual plants of
the same species are, as regards the decussations of their leaves and
in other respects, _either rights or lefts_. The fact is not emphasized
in modern botany and is in some danger of being forgotten. When, as
in the flowers of Arum, some _Gladioli_, _Exacum_, _St. Paulia_, or
the fruits of _Loasa_, rights and lefts occur on the same stem,
they come off alternately. But if, as in the seedlings of Barley the
twist of the first leaf be examined, it will be seen to be either a
right-or left-handed screw. An ear of barley, say a two-row barley,
is a definitely symmetrical structure. The seeds stand in their
envelopes back to back in definite positions. Each has its organs
placed in perfectly definite places. _If these seeds were buds_ their
differentiations would be grouped into a common plan. One might expect
that the differentiations of these embryos would still fall into the
pattern; but they do not, and so far as I have tested them, any one
may be a right or a left, just as each may carry any of the Mendelian
allelomorphs possessed by the parent plant, without reference to the
differentiation of any other seed. The fertilisation may be responsible,
but our experience of the allelomorphic characters suggest that the
irregularity is in the egg-cells themselves.[19]
_Germ cells thus differ from somatic cells in the fact that their
differentiations are outside the geometrical order which governs
the differentiation of the somatic cells._ I can think of possible
exceptions, but I have confidence that the rule is true and I regard it
as of great significance.
The old riddle, what is an individual, finds at least a partial solution
in the reply that an individual is a group of parts differentiated in a
geometrically interdependent order. With the germ-cell a new geometrical
order, with independent polarity is almost if not quite always, begun,
and with this geometrical independence the power of rejuvenescence may
possibly be associated.
The problems thus raised are unsolved, but they do not look insoluble.
The solution may be nearer than we have thought. In a study of the
geometry of differentiation, germinal and somatic, there is a way
of watching and perhaps analyzing what may be distinguished as the
mechanical phenomena of heredity. If any one could in the cases of
the Picotee and the Bizarre Carnation, respectively, detect the real
distinction between the two types of distribution, he would make a
most notable advance. Any one acquainted with mechanical devices can
construct a model which will reproduce some of these distinctions more
or less faithfully. The point I would not lose sight of is that the
analogy with such models must for a long way be a true and valuable
guide. I trust that some one with the right intellectual equipment will
endeavor to follow this guide; and I am sanguine enough to think that a
comprehensive study of the geometrical phenomena of differentiation will
suggest to a penetrative mind that critical experiment which may one day
reveal the meaning of spontaneous division, the mystery through which
lies the road, perhaps the most hopeful, to a knowledge of the nature of
life.
FOOTNOTES:
[1] In saying this we make no assumption as to the particular
cell-division at which differentiation occurs. This may be one of the
maturation-divisions, or it may perhaps be much earlier.
[2] From the recent discoveries of Erwin Baur we are led to surmise
that in the flowering plants the sub-epidermal layer, or some of its
elements, may legitimately be regarded as a similar germ-substance,
continuous in Weismann's sense.
[3] These fraternal twins, which show no special resemblance to each
other, are like the multiple births of other animals, and there is no
disposition for them to be of the same sex. In the sheep, for example,
statistics show that the frequency of pairs of twins, male and female,
is approximately double that of the frequency of pairs, both male or
both female, as it should be if the sex-distribution were fortuitous.
For instance Bernadin (_La Bergerie de Rambouillet_, 1890, p. 100)
gives the following figures for twin-lambs in Merinos: both male, 87;
both female, 83; sexes mixed, 187. The 9-banded Armadillo (_Dasypus
novemcinctus_), in which the young born in one litter are said to be
always of one sex, is the only known exception in Vertebrates, and
is presumably a genuine case of normal polyembryony (see especially,
Rosner, _Bull. Ac. Soc. Cracovie_, 1901, p. 443, and Newman and
Patterson, _Biol. Bull._, XVII, 1909, p. 181), and an important paper
lately published by H. H. Newman and J. T. Patterson, _Jour. Morph._,
1911, XXII, p. 855.
[4] A good collection of evidence as to disease in homologous twins was
lately published by E. A. Cockayne, _Brit. Jour. Child. Diseases_, Nov.,
1911.
[5] Cp. Windle, B. C. A., _Jour. Anal. Phys._, XXVI, p. 295.
[6] Mr. E. Nettleship tells me that in the course of collecting
pedigrees of families containing colour-blind members he has discovered
two cases (shortly to be published) of pairs of twins, which on account
of their very close resemblances must be deemed homologous, one of each
pair being colour-blind and the other normal. Such a distinction between
closely similar twins is most curious and unexpected.
[7] Another paradoxical phenomenon of the same nature occurs in the
Narwhal The males normally have the _left_ tusk alone developed, the
corresponding right tusk remaining as an undeveloped rudiment in its
socket. The left tusk is a left-handed screw. Occasionally the right
tusk is also developed and grows to the same length as that of the
left side, but in such specimens the right tusk is also a left-hand
screw like the tusk of the other side, instead of being reversed as we
should certainly have expected. It need scarcely be remarked that in the
case of the horns of antelopes, and in other examples of spiral organs
arranged in pairs, that of one side of the body is the mirror image of
that on the other side. The Narwhal's tusks in being both twisted in the
same direction are thus highly anomalous, and are comparable with pairs
of twins.
[8] Wilder, H. H., _Amer. Jour. Anat._, 1904, III, p. 452.
[9] Polydactylism which is often a dominant and the web-foot of Pigeons
which is recessive should be remembered as possible exceptions (see p.
49).
[10] Davenport inclined at first to regard rumplessness as a recessive,
but in his latest publication on the subject he definitely concludes
that it is an imperfect dominant. This conclusion accords well with
evidence quoted by Darwin (_An. and Plts._, II, ed. 2, p. 4) that
rumpless fowls may throw tailed offspring. (_Amer. Nat._, 1910, XLIV, p.
134.)
[11] Spillman, W. J., _Amer. Breeders Mag._, 1910, I, p. 178.
[12] Newsholme, _Lancet_, December 10, 1910, p. 1690.
[13] _Materials for the Study of Variation_, 1894, p. 358.
[14] Walker, G., _Johns Hopkins Hospital Bulletin_, XII, 1901, p. 129.
[15] Cp. R. H. Compton, _New Phytologist_, 1911, p. 249.
[16] _Arch. f. Entwickelungsmech._, 1907, XXIII, p. 249.
[17] Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.
[18] R. Boyle, _The Origine of Formes and Qualities_, Oxford, 1666.
[19] Remarkable experiments on this question have lately been carried
out by R. H. Compton (_Camb. Phil. Soc._, XV, 1910, p. 495), showing
that in a certain Barley, "Plumage Corn," the average ratio of left to
right is about 1.5. A fuller paper has since been published by Compton,
_Jour. Genetics_, 1912, II, I, p. 53.
CHAPTER III
SEGMENTATION, ORGANIC AND MECHANICAL
Models may be and often have been devised imitating some of the
phenomena of division, but none of them have reproduced the peculiarity
which characterises divisions of living tissues, that _the position
of chemical differentiation_ is _determined by those divisions_. For
example, models of segmentation, whether radial or linear, may be made
by the vibration of plates as in the familiar Chladni figures of the
physical laboratory, or by the bowing of a tube dusted on the inside
with lycopodium powder, and in various other ways. The sand or the
powder will be heaped up in the nodes or regions of least movement, and
the patterns thus formed reproduce many of the geometrical features
of segmentation. But in the segmentations of living things the nodes
and internodes, once determined by the dividing forces, would each
become the seat of appropriate and distinct chemical processes leading
to the differentiation of the parts, and the deposition of the bones,
petals, spines, hairs, and other organs in relation to the meristic
ground-plan. The "ripples" of meristic division not merely divide but
differentiate, and when a "ripple" forks the result is not merely a
division but a reduplication of the organ through which the fork runs.
An example illustrating such a consequence is that of the half-vertebrae
of the Python. On the left side the vertebra is single (Fig. 7) and
bears a single rib, but on the right side a division has occurred with
the result that two half-vertebrae, each bearing a rib, are formed, one
standing in succession to the other. We cannot, indeed, imagine any
operation of physiological division carried out in such an organ as a
vertebra, passing through a plane at right angles to the long axis of
the body, which does not necessarily involve the further process of
reduplication.
As the meristic system of distribution spreads through the body,
chemical differentiations follow in its track, with segmentation and
pattern as the visible result. Could we analyse these simultaneous
phenomena and show how it is that the places of chemical differentiation
are determined by the system of division, progress would then be rapid.
It is here that all speculation fails.
[Illustration: FIGS. 7 and 8. Two examples of imperfect division in the
vertebræ of a python. _I_, the vertebræ 147-150 from the right side,
showing imperfect division between the 148th and 149th. The condition on
the left side of this vertebra was the same. _II_, the dorsal surface of
vertebræ 165-167. On the right side the 166th is double and bears two
ribs, but on the left side it is normal and has one rib only.]
Many attempts have been made to interpret the processes of division
and repetition, in terms of mechanics, or at least to refer them to
their nearest mechanical analogies, so far with little success. The
problem is beset with difficulties as yet insurmountable and of these
one must be especially noticed. In the living thing the process by which
repetition and patterns come into being consists partly in division but
partly also in growth. We have no means of studying the phenomena of
pattern-formation except in association with that of growth. Growth soon
ceases unless division takes place, and if growth is impossible division
soon ceases also. In consequence of this fact that the final pattern
is partly a product of growth, it can never be used as unimpeachable
evidence of the primary geometrical relations of the members as laid
down in the divisions.
In the last chapter in referring to the problem of repetition I
introduced an analogy, comparing the patterns of the organic world
with those produced in unorganised materials by wave-motion. In the
preliminary stage of ignorance, having no more trustworthy clue, I do
not think it wholly unprofitable to consider the applicability of this
analogy somewhat more fully. It possesses, as I hope to show, at least
so much validity as to encourage the belief that morphology may safely
discard one source of long-standing error and confusion.
Those who have studied the structure of parts repeated in series will
have encountered the old morphological problem of "Serial Homology,"
which has absorbed so much of the attention of naturalists and
especially of zoologists at various periods. This problem includes
two separate questions. The first of these is the origin in evolution
of the resemblance between two organs occurring in a repeated series,
of which the fore and hind limbs of Vertebrates are the prerogative
instance. From the fact that these resemblances can be traced very far,
often into minute details of structure, many anatomists have inclined to
the opinion that the resemblance must originally have been still more
complete, and that the two limbs, for instance, must have acquired their
present forms by the differentiation of two identical groups of parts.
Similar questions arise whenever parts are repeated in series, whether
the series be linear or radial, and, though less obviously, even when
the repetition is bilateral only. In each such example the question
arises, is the resemblance between the parts the remains of a still
closer resemblance, or is differentiation original? Sometimes the
view that these parts have arisen by the differentiation of a series
of identical parts is plausible enough, as for example when the
peculiarities of various appendages of a Decapod Crustacean are referred
to modifications of the Phyllopod series. In application to other cases
however we soon meet with difficulty, and the suggestion that the
segments of a vertebrate were originally all alike is seen at once to be
absurd, for the reason that a creature so constituted could not exist,
and that, differentiation of at least one anterior and one posterior
segment, is an essential condition of a viable organism consisting of
parts repeated in a linear series. Between these two terminal segments
it is possible to imagine the addition of one segment, or of a series
of approximately similar segments; but when once it is realised that
the terminals must have been differentiated from the beginning, it
will be seen that the problem of the origin of the resemblance between
segments is not rendered more comprehensible by the suggestion that
even the intervening members were originally alike. Seeing indeed that
some differentiation must have existed primordially it is as easy to
imagine that the original body was composed of a series grading from the
condition of the anterior segment to that of the posterior, as any other
arrangement. The existence of a linear or successive series in fact
postulates a polarity of the whole, and in such a system the conception
of an ideal segment containing all the parts represented in the others
has manifestly no place. The introduction of that conception though
sanctioned by the great masters of comparative anatomy, has, as I think,
really delayed the progress of a rational study of the phenomena of
division. The same notion has been applied to every class of repetition
both in animals and plants, generally with the same unhappy results. In
the cruder forms in which this doctrine was taught thirty years ago it
is now seldom expressed, but modified presentations of it still survive
and confuse our judgments.
The process of repetition of parts in the bodies of organisms is
however a periodic phenomenon. This much, provided we remain free from
prejudice as to the nature and causation of the period or rhythm, we
may safely declare, and a comparison may thus be instituted between the
consequences of meristic repetition in the bodies of living things and
those repetitions which in the inorganic world are due to rhythmical
processes. Of such processes there is a practically unlimited diversity
and we have nothing to indicate with which of them our repetitions
should rather be compared.
[Illustration: FIG. 9. Osmotic growths simulating segmentation. (After
Leduc.)]
In some respects perhaps the best models of living organisms yet made
are the "osmotic growths" produced by Leduc.[1] These curious structures
were formed by placing a fragment of a salt, for instance calcium
chloride, in a solution of some colloidal substance. As the solid takes
up water from the solution a permeable pellicle or membrane is formed
around it. The vesicle thus enclosed grows by further absorption of
water, often extending in a linear direction, and in many examples this
growth occurs by a series of rhythmically interrupted extensions. Some
of the growths thus formed are remarkably like organic structures,
and might pass for a series of antennary segments or many other
organs consisting of a linear series of repeated parts. In admitting
the essential resemblance between these "osmotic growths" and living
bodies or their organs I lay less stress on the general conformation
of the growths, which often as Leduc points out, recall the forms
of fungi or hydroids, but rather on the fact that the interruptions
in the development of these systems are so closely analogous to the
segmentations or repetitions of parts characteristic of living things
(Fig. 9). In the same way I am less impressed by Leduc's models of
Karyokinesis, wonderful as they nevertheless are, for the division is
here imitated by putting separate drops on the gelatine film. What we
most want to know is how in the living creature one drop becomes two.
The models of linear segmentation have the remarkable merit that they do
in some measure imitate the process of actual division or repetition.
So in a somewhat modified method Leduc, by causing the diffusion
of a solution in a gelatine film, produced rhythmical or periodic
precipitations strikingly reminiscent of various organic tissues, for
here also the process of periodic repetition is imitated with success.
It is a feature common to these and to all other rhythmical repetitions
produced by purely mechanical forces that there is resemblance between
the members of the series, and that this similarity of conformation may
be maintained in most complex detail. When however in the mechanical
series some of the members differ from the rest we have no difficulty
in recognising that these differences--which correspond with the
differentiations of the organic series--are due to special heterogeneity
in the conditions or in the materials, and it never occurs to us to
suppose that all the members must have been primordially alike. For
example, in the case of ripple-marks on the sand, which I choose as one
of the most familiar and obvious illustrations of a repeated series
due to mechanical agencies, if we notice one ripple different in form
from those adjacent to it, we do not suppose that this variation must
have been brought about by deformation of a ripple which was at first
formed like the others, but we ascribe it to a difference in the sand at
that point, or to a difference in the way in which the wind or the tide
dealt with it. We may press the analogy further by observing that in as
much as such a series of waves has a beginning and an end, it possesses
polarity like that of the various linear series of parts in organisms,
and even the formation of each member must influence the shape of its
successor. Since in an organism the beginning and end of the series
are always included, some differentiation among the repetitions must
be inevitable. If therefore it be conceded, as I think it must, that
segmentation and pattern are the consequence of a periodic process we
realize that it is at least as easy to imagine the formation of such a
series of parts having family likeness combined with differentiation
as it would be to conceive of their arising primordially as a series
of identical repetitions. The suggestion that the likenesses which we
now perceive are the remains of a still more complete resemblance is a
substitution of a more complex conception for a simpler one.
The other question raised by the problem of Serial Homology is how far
there is a correspondence between individual members of series when
the series differ from each other either in the number of parts, or in
the mode of distribution of differentiation among them. Students, for
example, of vertebrate morphology debate whether the _n_th vertebra
which carries the pelvic girdle in Lizard A is individually homologous
with the _n_ + _x_th vertebra which fulfils this function in Lizard B,
or whether it is not more truly homologous with the vertebra standing in
the _n_th ordinal position, though that vertebra in Lizard B is free.
In various and more complex aspects the same question is debated in
regard to the cranial and spinal nerves, the branches of the aorta,
the appendages of Arthropoda, and indeed in regard to all such series
of differentiated parts in linear or successive repetition. Persons
exercised with these problems should before making up their minds
consider how similar questions would be answered in the case of any
series of rhythmical repetitions formed by mechanical agencies. In the
case of our illustration of the ripples in the sand, given the same
forces acting on the same materials in the same area, the number of
ripples produced will be the same, and the _n_th ripple counting from
the end of the series will stand in the same place whenever the series
is evoked. If any of the conditions be changed, the number and shapes
can be changed too, and a fresh "distribution of differentiation"
created. Stated in this form it is evident that the considerations
which would guide the judgment in the case of the sand ripples are not
essentially different from those which govern the problem of individual
homology in its application to vertebrae, nerves, or digits.
The fact that the unit of repetition is also the unit of growth is the
source of the obscurity which veils the process. When we compare the
skeleton of a long-tailed monkey with that of a short-tailed or tailless
ape we see at once how readily the additional series of caudal segments
may be described as a consequence of the propagation of the "waves" of
segmentation beyond the point where they die out in the shorter column,
and we see that with an extension of the series of repetitions there is
growth and extension of material.
The considerations which apply to this example will be found operating
in many cases of the variation of terminal members of linear series.
Some of these series, like the teeth of the dog, end in a terminal
member of a size greatly reduced below that of the next to it. Even when
there is thus a definite specialisation of the last member of the series
it not infrequently happens that the addition, by variation, of a member
beyond the normal terminal, is accompanied by a very palpable increase
in size of the member which stands numerically in the place of the
normal terminal.[2] So also with variation in the number of ribs, when
a lumbar vertebra varies homoeotically into the likeness of the last
dorsal and bears a rib, the rib placed next in front of this, which in
the normal trunk is the last, shows a definite increase in development.
The consequences of such homoeoses are sometimes very extensive,
involving readjustments of differentiation affecting a long series of
members, as may easily be seen by comparing the vertebral columns of
several individual Sloths[3] (whether _Bradypus_ or _Choloepus_) to take
a specially striking example.
It may be urged that no feature as yet enables us to perceive wherein
lies the primary distinction which determines such variation, whether it
is due to a difference in the dividing forces or in the material to be
divided. If for instance we were to imitate such a series of segments
by pressing hanging drops of a viscous fluid out of a paint-tube by
successive squeezes, the number of times the tube is contracted before
it is empty will give the number of the segments, but their size may
depend either on the force of the contractions or on the capacity of
the tube, or on various other factors. Nevertheless in the case of the
variation of terminal members, whatever be the nature of the rhythmical
impulse which produces the series of organs, the elevation of the
normally terminal member in correspondence with the addition of another
is what we should expect.
If the organism acquired its full size first and the delimitation of
the parts took place afterwards, there might be some hope that the
resemblance between living patterns and those mechanically caused by
wave-motion might be shown to be a consequence of some real similarity
of causation, but in view of the part played by growth, appeal to these
mechanical phenomena cannot be declared to have more than illustrative
value. Similarly in as much as living patterns appear, and almost
certainly do in reality come into existence by a rhythmical process,
comparisons of these patterns with those developed in crystalline
structures, and in the various fields of force are, as it seems to me,
inadmissible, or at least inappropriate.
However their intermittence be determined, the rhythms of division must
be looked upon as the immediate source of those geometrically ordered
repetitions universally characteristic of organic life. In the same
category we may thus group the segmentation of the Vertebrates and of
the Arthropods, the concentric growth of the Lamellibranch shells or of
Fishes' scales, the ripples on the horns of a goat, or the skeletons of
the Foraminifera or of the Heliozoa. In the case of plant-structures
Church[4] has admirably shown, with an abundance of detail, how on
analysis the definiteness of phyllotaxis is an expression of such
rhythm in the division of the apical tissues, and how the spirals
and "orthostichies" displayed in the grown plant are its ultimate
consequences. The problem thus narrows itself down to the question of
the mode whereby these rhythms are determined.
It is natural that we should incline to refer them to a chemical source.
If we think of the illustration just given, of the segmentation of a
viscous fluid into drops by successive contractions of a soft-walled
tube we can, I think, conceive of such rhythmic contractions as due to
summations of chemical stimuli, somewhat as are the beats of the heart.
But when we recognize the vast diversity of materials the distribution
of which is determined by an ostensibly similar rhythmic process it
seems hopeless to look forward to a directly chemical solution. That the
chemical degradation of protoplasm or of materials which it contains is
the source of the energy used in the divisions cannot be in dispute, but
that these divisions can be themselves the manifestations of chemical
action seems in the highest degree improbable.
We may therefore insist with some confidence on the distinction between
the Meristic and the substantive constitution of organisms, between,
that is to say, the system according to which the materials are divided
and the essential composition of the materials, conscious of the fact
that the energy of division is supplied from the materials, and that
in the ontogeny the manner in which the divisions are effected must
depend secondarily on the nature of the substances to be divided.
The mechanical processes of division remain a distinguishable group
of phenomena, and variations in the substances to be distributed in
division may be independent of variations in the system by which the
distribution is effected.
Modern genetic analysis supplies many remarkable examples of this
distinction. When formerly we compared the leaves of a normal palmatifid
Chinese Primula with the pinnatifid leaves[5] of its fern-leaved variety
we were quite unable to say whether the difference between the two types
of leaf was due to a difference in the material cut up in the process
of division or to a difference in that process itself. Knowledge that
the distinction is determined by a single segregable factor tends to
prove that the critical difference is one of substance. So also in the
Silky fowl we know that the condition of its feathers is due to the
absence of some one factor present in the normal form. We may conceive
such differences as due to change of form in the successive "waves" of
division, but we cannot yet imagine segregation otherwise than as acting
by the removal or retention of a material element. Future observation
by some novel method may suggest some other possibility, but such cases
bring before us very clearly the difficulties by which the problem is
beset.
[Illustration: FIG. 10. The palm-and fern type of leaf in _Primula
Sinensis_. The palm is dominant and the fern is recessive.]
In another region of observation phenomena occur which as it seems to
me put it beyond question that the meristic forces are essentially
independent of the materials upon which they act, save, in the remoter
sense, in so far as these materials are the sources of energy. The
physiology of those regenerations and repetitions which follow upon
mutilation supplies a group of facts which both stimulate and limit
speculation. No satisfactory interpretations of these extraordinary
occurrences has ever been found, but we already know enough to feel
sure that in them we are witnessing indications which should lead
to the discovery of the true mechanics of repetition and pattern.
The consequences of mutilation in causing new growth or perhaps more
strictly in enabling new growth to take place, are such that they cannot
be interpreted as responses to chemical stimuli in any sense which
the word chemical at present connotes. Powers are released by mutilation
of which in the normal conditions of life no sign can be detected. All
who have tried to analyse the phenomena of regeneration are compelled
to have recourse to the metaphor of equilibrium, speaking of the normal
body as in a state of strain or tension (Morgan) which when disturbed by
mutilation results in new division and growth. The forces of division
are inacessible to ordinary means of stimulation. Applications, for
example, of heat or of electricity excite no responses of a positive
kind unless the stimuli are so violent as to bring about actual
destruction.[6] These agents do not, to use a loose expression, come
into touch with the meristic forces. Changes in the chemical environment
of cells may, as in the experiments of Loeb and of Stockard produce
definite effects, but the facts suggest that these effects are due
rather to alterations in the living material than to influence exerted
directly on the forces of division themselves.
By destruction of tissue however the forces both of growth and of
division also may often be called into action with a resulting
regeneration. Interruption of the solid connexion between the parts may
produce the same effects, as for example when the new heads or tails
grow on the divided edges of Planarians (Morgan), or when from each half
embryo partially separated from its normally corresponding half, a new
half is formed with a twin monster as the result.
Often classed with regenerations but in reality quite distinct from
them are those special and most interesting examples where the growth
of a _paired_ structure is excited by a simple wound. Some of the best
known of these instances are presented by the paired extra appendages
of Insects and Crustacea. Some years ago I made an examination of all
the examples of such monstrosities to which access was to be obtained,
and it was with no ordinary feeling of excitement that I found that
these supernumerary structures were commonly disposed on a recognizable
geometrical plan, having definite spatial relations both to each other
and to the normal limb from which they grew. The more recent researches
of Tornier[7] and especially his experiments on the Frog have shown
that a cut into the posterior limb-bud induces the outgrowth of such a
_pair_ of limbs at the wounded place. Few observations can compare with
this in novelty or significance; and though we cannot yet interpret
these phenomena or place them in their proper relations with normal
occurrences, we feel convinced that here is an observation which is no
mere isolated curiosity but a discovery destined to throw a new light on
biological mechanics. The supernumerary legs of the Frog are evidently
grouped in a system of symmetry similar to that which those of the
Arthropods exhibit, and though in Arthropods paired repetitions have not
been actually produced by injury under experimental conditions we need
now have no hesitation in referring them to these causes as Przibram has
done.
At this point some of the special features of the supernumerary
appendages become important. First they may arise at any point on the
normal limb, being found in all situations from the base to the apex.
Nor are they limited as to the surface from which they spring, arising
sometimes from the dorsal, anterior, ventral, or posterior surfaces, or
at points intermediate between these principal surfaces.
With rare and dubious exceptions, the parts which are contained in these
extra appendages are only those which lie _peripheral to their point of
origin_. Thus when the point of origin is in the apical joint of the
tarsus, the extra growth if completely developed consists of a double
tarsal apex bearing two pairs of claws. If they arise from the tibia,
two complete tarsi are added. If they spring from the actual base of
the appendage then two complete appendages may be developed in addition
to the normal one. We must therefore conclude that in any point on a
normal appendage the power exists which, if released, may produce a bud
containing in it a paired set of the parts peripheral to this point.
[Illustration: FIG. 11. Diagrams of the geometrical relations which are
generally exhibited by extra pairs of appendages in Arthropoda. The
sections are supposed to be those of the apex of a tibia in a beetle.
_A_, anterior, _P_, posterior, _D_, dorsal, _V_, ventral. _M_^{1},
_M_^{2} are the imaginary planes of reflexion. The shaded figure is in
each case a limb formed like that of the other side of the body, and the
outer unshaded figures are shaped like the normal for the side on which
the appendages are. On the several radii are shown the extra pairs in
their several possible relations to the normal from which they arise.
The normal is drawn in thick lines in the center.]
Next the geometrical relations of the halves of the supernumerary pair
are determined by the position in which they stand in regard to the
original appendage. These relations are best explained by the diagram
(Fig. 11), from which it will be seen that the two supernumerary
appendages stand as images of each other; and, of them, that which is
adjacent to the normal appendage forms an image of it. Thus if the
supernumerary pair arise from a point on the dorsal surface of the
normal appendage, the two _ventral_ surfaces of the extra pair will
face each other. If they arise on the anterior surface of the normal
appendage, their morphologically posterior surfaces will be adjacent,
and so on.
These facts give us a view of the relations of the two halves of a
dividing bud very different from that which is to be derived from the
exclusive study of normal structures. Ordinary morphological conceptions
no longer apply. The distribution of the parts shows that the bud or
rudiment which becomes the supernumerary pair may break or open out in
various ways according to its relations to the normal limb. Its planes
of division are decided by its geometrical relations to the normal body.
Especially curious are some of the cases in which the extra pair are
imperfectly formed. The appearance produced is then that of two limbs
in various stages of coalescence, though in reality of course they are
stages of imperfect separation. The plane of "coalescence" may fall
anywhere, and the two appendages may thus be compounded with each other
much as an object partially immersed in mercury "compounds" with its
optical image reflected from the surface.
Supernumerary paired structures are not usually, if ever, formed when
an appendage is simply amputated. Cases occasionally are seen which
nevertheless seem to be of this nature. Borradaile,[8] for example,
described a crab (_Cancer pagurus_) having in place of the right chela
three _small_ chelae arising from a common base, where the appearances
suggested that the three reduced limbs replaced a single normal limb.
From the details reported however it seems still possible that one of
the chelae (that lettered F. I in Borradaile's figure) may be the normal
one, and the other two an extra pair. The chela which I suspect to be
the normal is in several respects deformed as well as being reduced in
size, and this deformity may perhaps have ensued as a consequence of the
same wound which excited the growth of the extra pair. Its reduced size
may be due to the same injury, which may quite well have checked its
growth to full proportions.
Admitting doubt in these ambiguous cases it seems to be a general rule
that for the production of the extra pair the normal limb should persist
in connexion with the body. Moreover it is practically certain that
in no case can a _single_, viz. an unpaired, duplicate of the normal
appendage grow from it. Many examples have been described as of this
nature, but all of them may be with confidence regarded as instances of
a supernumerary pair in which only the two morphologically anterior or
the two morphologically posterior surfaces are developed. We have thus
the paradox that a limb of one side of the body, say the right, has in
it the power to form a pair of limbs, right and left, as an outgrowth of
itself, but cannot form a second left limb alone.
A very interesting question arises whether it is strictly correct to
describe the extra pair as a right and a left, or whether they are not
rather two lefts or two rights of which one is reversed. This question
did not occur to me when in former years I studied these subjects. It
was suggested to me by Dr. Przibram. The answer might have an important
bearing on biological mechanics, but I know no evidence from which the
point can be determined with certainty. In order to decide this question
it would be necessary to have cases in which the paired repetition
affected a limb markedly differentiated on the two sides of the body,
and of course the development of the extra parts in order to be decisive
must be fairly complete. One example only is known to me which at all
satisfies these requirements, that of the lobster's chela figured (after
Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig.
184, III.
Here the drawing distinctly suggests that one of the extra
dactylopodites, namely that lettered R, is differentiated as a left
and not merely a reversed right. For the teeth on this dactylopodite
are those of a cutting claw, not of a crushing claw, whereas the
dactylopodites R' and L' bear crushing teeth. The figure makes it fairly
certain also that the limb affected was a crushing claw. Accepting this
interpretation, we reach the remarkable conclusion that the bud of new
growth consisted of halves differentiated into cutter and crusher as the
normal claws are, and that the extra crusher is geometrically a left
but physiologically a right. Though shaped as a left in respect of the
direction in which it points, the extra crusher is really an optically
reversed right, while the dactylopodite R, which is placed pointing like
a right, is really a reversed left (Fig. 12).
[Illustration: FIG. 12. Right claw of lobster bearing a pair of extra
dactylopodites (after van Beneden). The fine toothing on R suggests that
this is part of a cutting claw, though the limb bearing it is a crusher.]
If these indications are reliable[9] and are established by further
observation we shall be led to the conclusion that the bud which
becomes an extra pair of limbs does not merely contain the parts proper
to the side on which it grows, but is comparable with the original
zygotic cell, and consists not simply of two halves, but of two halves
differentiated as a right and a left like the two halves of the normal
body.
Phenomena of this kind, evoked by mutilation or injury, together with
the cognate observations on regeneration throw very curious lights
on the nature of living things. To an understanding of the nature of
the mechanics of living matter and its relation to matter at large
they offer the most hopeful line of approach. I allude especially to
the examples in which it has been established that the part which is
produced after mutilation is a structure different from that which was
removed. The term "regeneration" was introduced before such phenomena
were discovered, and though every one recognizes its inapplicability
to these remarkable cases, the word still misleads us by presenting
a wrong picture to the mind. The expression "heteromorphosis" (Loeb)
has been appropriately applied to various phenomena of this kind, and
Morgan has given the name "morphallaxis" to another group of cases in
which the renewal occurs by the transformation of a previously existing
part.[10] But we must continually remember that all these occurrences
which we know only as abnormalities and curiosities must in reality be
exemplifications of the normal mechanics of division and growth. The
conditions needed to call them forth are abnormal, but the responses
which the system makes are evidences of its normal constitution. When
therefore, for example, the posterior end of a worm produces a reversed
tail from its cut end we have a proof that there must be in the normal
body forces ready to cause this outgrowth. The new structure is not
an ill-shaped head-end, for, as Morgan shows, the nephridial ducts
have their funnels perforating the segments in a reversed direction.
The "tension" of growth is actually reversed.[11] So also when in a
Planarian amputation of the body immediately behind the head leads to
the formation of a new reversed head at the back of the normal head,
while amputation further back leads to the regeneration of a new tail,
these responses give indications of forces normally present in the
body of the Planarian. Such facts open up a great field of speculation
and research. Especially important it would be to determine where the
critical region may be at which the one response is replaced by the
other. I suppose it is even possible that there is some neutral zone in
which neither kind of response is made.
Physical parallels to the phenomena of regeneration are not easy
to find and we still cannot penetrate beyond the empirical facts.
Przibram has laid stress on the general resemblance between the new
growth of an amputated part in an animal and the way in which a broken
crystal repairs itself when placed in the mother-solution. That the
two processes have interesting points of likeness cannot be denied.
It must however never be forgotten that there is one feature strongly
distinguishing the two; for I believe it is universally recognized
by physicists that all the phenomena of geometrical regularity which
crystals display are ultimately dependent on the forms of the particles
of the crystalline body. This cannot in any sense be supposed to hold
in regard to protoplasm or its constituents. The definiteness of
crystals is also an unlikely guide for the reason that it is absolute
and perfect, or in other words because this kind of regularity cannot
be disturbed at all without a change so great that the substance itself
is altered; whereas we know that the forms of living things are capable
of such changes, great and small, that we must regard perfection of
form, whether manifested in symmetry or in number, as an ideal which
will only be produced in the absence of disturbance. The symmetry of the
living things is like the symmetry of the concentric waves in a pool
caused by a splash. Perfect circles are made only in the imaginary case
of mathematical uniformity, but the system maintains an approximate
symmetry though liable to manifold deformation.
Since the geometrical order of the living body cannot be a direct
function of the materials it must be referred to some more proximate
control. In renewing a part the body must possess the power of seizing
particles of many dissimilar kinds, and whirl them into their several
and proper places. The action in renewal, like that of original growth,
may be compared--very crudely--with the action of a separator which
simultaneously distributes a variety of heterogeneous materials in an
orderly fashion; but in the living body the thing distributed must
rather be the _appetency_ for special materials, not the materials
themselves.
If the analogy of crystals be set aside and we seek for other parallels
to regeneration there are none very obvious. I have sometimes wondered
whether it might not be possible to institute a fruitful comparison
between the renewal of parts and the reformation of waves of certain
classes after obliteration. In several respects, as I have already said,
some curious resemblances with the repetitions formed by wave-motion
are to be traced in our organic phenomena, and though admitting that
I cannot develop these comparisons, I think nevertheless they may be
worth bearing in mind. When, after obliteration, an eddy in a stream,
or a ripple-mark (a more complex case of eddy-formation) in blown sand
is re-formed, we have an example in which pattern is reconstituted and
growth takes place not by virtue of the composition of the materials--in
this case the water or the sand--but by the way in which they are acted
upon by extraneous forces.
A feature in the actual mode by which ripple-marks are reconstituted may
not be without interest in connexion with our phenomena of regeneration.
When, for example, the wind is blowing steadily over a surface of fine,
dry sand, the familiar ripple-marks are formed by a heaping of the sand
in lines transverse to the direction of the wind. The heaping is due to
the formation of eddies corresponding with positions of instability.
When the wind is steady and the sand homogeneous, the distances between
the ripples, or wave-lengths, are sensibly equal. If while the wind
continues to blow, the ripples are obliterated with a soft brush they
will quickly be re-formed over the whole area, but I have noticed that
at first their wave-length is approximately half that of the ripples
in the undisturbed parts of the system.[12] The normal wave-length is
restored by the gradual accentuation of alternate ripples. Of course
the sand-ripples are in reality slowly travelling forward in the
direction towards which the wind is blowing, and for this our living
segmentations afford no obvious parallel, but the appearances in the
area of reformation, and especially the forking of the old ridges where
they join the new ones, are curiously reminiscent of the irregularities
of segmentation seen in regenerated structures. The value of the
considerations adduced in the chapter is, I admit, very small. The
utmost that can be claimed for them is that mechanical segmentations,
like those seen in ripple-mark, or in Leduc's osmotic growths, show
how by the action of a continuous force in one direction, repeated
and serially homologous divisions can be produced having features of
similarity common to those repetitions by which organic forms and
patterns are characterised. The analogy supplies a vicarious picture of
the phenomena which in default of one more true may in a slight degree
assist our thoughts. It suggests that the rhythms of segmentation may be
the consequence of a single force definite in direction and continuously
acting during the time of growth. The polarity of the organism would
thus be the expression of the fact that this meristic force is
definitely directed after it has once been excited, and the reversal
seen in some products of regeneration suggest further that it is capable
of being reflected. This polarity cannot be a property of the material,
as such, but is determined by a force acting on that material, just as
the polarity of a magnet is not determined by the arrangement of its
particles, but by the direction in which the current flows.
To some it may appear that even to embark on such discussions as this is
to enter into a perilous flirtation with vitalistic theories. How, they
may ask, can any force competent to produce chemical and geometrical
differentiation in the body be distinguished from the "Entelechy" of
Driesch? Let me admit that in this reflexion there is one element of
truth. If those who proclaim a vitalistic faith intend thereby to
affirm that in the processes by which growth and division are effected
in the body, a part is played by an orderly force which we cannot
_now_ translate into terms of any known mechanics, what observant man
is not a vitalist? Driesch's first volume, putting as it does into
intelligible language that positive deduction from the facts--especially
of regeneration--should carry a vivid realisation of this truth to any
mind. If after their existence is realised, it is desired that these
unknown forces of order should have a name, and the word entelechy is
proposed, the only objection I have to make is that the adoption of a
term from Aristotelian philosophy carries a plain hint that we propose
to relegate the future study of the problem to metaphysic.
From this implication the vitalist does not shrink. But I cannot find in
the facts yet known to us any justification of so hopeless a course. It
was but yesterday that the study of _Entwicklungsmechanik_ was begun,
and if in our slight survey we have not yet seen how the living machine
is to be expressed in terms of natural knowledge that is poor cause for
despair. Driesch sums up his argument thus:[13]
"It seems to me that there is only one conclusion possible. If we are
going to explain what happens in our harmonious-equipotential systems
by the aid of causality based upon the constellation of single chemical
factors and events, there _must_ be some such thing as a machine. Now
the assumption of the existence of a machine proves to be absolutely
absurd in the light of the experimental facts. _Therefore there can
be neither any sort of a machine nor any sort of causality based upon
constellation underlying the differentiation of harmonious-equipotential
systems._"
"For a machine, typical with regard to the three chief dimensions
of space, cannot remain itself if you remove parts of it or if you
rearrange its parts at will."
To the last clause a note is added as follows:
"The pressure experiments and the dislocation experiments come into
account here; for the sake of simplicity they have not been alluded to
in the main line of our argument."
I doubt whether any man has sufficient knowledge of all possible
machines to give reality to this statement. In spite also of the
astonishing results of experiments in dislocation, doubt may further be
expressed as to whether they have been tried in such variety or on such
a scale as to justify the suggestion that the living organism remains
itself if its parts are rearranged at will. All we know is that it can
"remain itself" when much is removed, and when much rearrangement has
been affected, which is a different thing altogether.
I scarcely like to venture into a region of which my ignorance is so
profound, but remembering the powers of eddies to re-form after partial
obliteration or disturbance, I almost wonder whether they are not
essentially machines which remain themselves when parts of them are
removed.
Real progress in this most obscure province is not likely to be made
till it attracts the attention of physicists; and though they for long
may have to forego the application of exact quantitative methods, I
confidently anticipate that careful comparison between the phenomena
of repetition formed in living organisms and the various kinds of
segmentation produced by mechanical agencies would be productive of
illuminating discoveries.
FOOTNOTES:
[1] Stéphane Leduc, _Théorie Physico-Chymique de la Vie_, Paris, 1910.
[2] _Materials for the Study of Variation_, No. 249, p. 217; and p. 272.
[3] _Materials_, p. 118.
[4] Church, A. H., _On the Relation of Phyllotaxis to Mechanical Laws_,
London, 1904.
[5] It is a question whether the dominance of the palmatifid leaf over
the pinnatifid is not really an example of the dominance of a lower
number of segmentations over a higher. From the uncertainty whether two
given leaves of two separate plants are actually comparable one cannot
institute quite satisfactory numerical comparisons, but I think the view
that the "Fern" leaf has more lobes than an otherwise similar "Palm"
leaf may be fairly maintained. If this be admitted, the "Palm" leaf
represents the dominant low number and its round shape is a consequence
of the greater powers of growth which are so often possessed by the
members of a shorter series.
[6] It is perhaps of importance to remember that in certain species
of bacteria (e. g. _Bacillus Anthracis_) division may cease where the
organism is cultivated under certain artificial conditions though growth
continues. In this way very long unsegmented threads are produced.
[7] _Arch. f. Entwm._, XX, 1905, p. 76; _Sitzungsb. d. Ges. Naturf._,
Berlin, 1907, p. 41, etc.
[8] Borradaile, L. A., _Jour. Marine Zool._, 1897, No. 8.
[9] Dr. Przibram, I should mention, concludes that on the whole the
facts are against this interpretation, but as more evidence is certainly
required, I call attention to the possibility.
[10] Morgan, T. H., _Regeneration_, 1901.
[11] It would be interesting to know whether growth continues at the
original posterior end after the new "posterior" end has been formed in
front.
[12] In the actual case observed, the ripples unsmoothed had a
wave-length of about 2-1/2 inches; and when the new ones were first
formed, there were about 30 ridges in the length originally traversed by
15 or 16.
[13] _The Science and Philosophy of the Organism_; Gifford Lectures,
1907. London, 1908, p. 141.
CHAPTER IV
THE CLASSIFICATION OF VARIATION AND THE NATURE OF
SUBSTANTIVE FACTORS
We have now seen that among the normal physiological processes the
phenomena of division form a recognisable, and in all likelihood a
naturally distinct group. Variations in these respects may thus be
regarded as constituting a special class among variations in general.
The substantive variations have only one property in common--the
negative one that they are not Meristic. The work of classifying them
and distinguishing them according to their several types demands a
knowledge of the chemistry of life far higher than that to which science
has yet attained. In reference to some of the simplest variations Garrod
has introduced the appropriate term "Chemical sports." The condition in
man known as Alkaptonuria in which the urine is red is due especially
to the absence of the enzyme which decomposes the excretory substance,
alkapton. The "chemical sport" here consists in the inability to break
up the benzene ring. The chemical feature which distinguishes and is the
proximate cause of several colour-varieties can now in a few cases be
declared. The work of Miss Wheldale has shown that colour-varieties may
be produced by the absence of the chromogen compound the oxidation of
which gives rise to sap-colours, by differences in the completeness of
this process of oxidation, and by a process of reduction supervening on
or perhaps suppressing the oxidation. Some of these processes moreover
may be brought about by the combined action of two bodies, the one an
enzyme, for example an oxygenase, and the other a substance regarded as
a peroxide, contributing the oxygen necessary for the oxidation to take
place. Variation in colour may thus be brought about by the addition or
omission of any one of the bodies concerned in the action.
Similar variations, or rather similar series of variations will
undoubtedly hereafter be identified in reference to all the various
kinds of chemical processes upon which the structure and functions of
living things depend. The identification of these processes and of
the bodies concerned in them will lead to a real classification of
Substantive Variations.
To forecast the lines on which such classification will proceed is to
look too far ahead. We may nevertheless anticipate with some confidence
that future analysis will recognise among the contributing elements,
some which are intrinsic and inalienable, and others which are extrinsic
and superadded.
We already know that there may be such interdependence among the
substantive characters that to disentangle them will be a work of
extreme difficulty. The mere fact that in our estimation characters
belong to distinct physiological systems is no proof of their actual
independence. In illustration may be mentioned the sap-colour in Stocks
and the development of hoariness on the leaves and stems, which Miss
Saunders's experiments have shown to be intimately connected, so that
in certain varieties no hoariness is produced unless the elements for
sap-colour are already present in the individual plant.
The first step in the classification of substantive variations is
therefore to determine which are due to the addition of new elements
or factors, and which are produced by the omission of old ones. _A
priori_ there is no valid criterion by which this can be known, and
actual experiments in analytical breeding can alone provide the
knowledge required. Some very curious results have by this method been
obtained, which throw an altogether unexpected light on these problems.
For example, in order that the remarkable development of mesoblastic
black pigment characteristic of the Silky Fowl should be developed, it
is practically certain that two distinct variations from such a type
as _Gallus bankiva_ must have occurred. I assume, as is reasonable,
that _G. bankiva_ has genetic properties similar to those of the Brown
Leghorn breed which has been used in the experiments which Mr. Punnett
and I have conducted. _Gallus bankiva_ was not available but the Brown
Leghorn agrees with it very closely in colouration, and probably in
the general physiology of its pigmentation. Setting aside the various
structural differences between the two breeds, the Silky is immediately
distinguished from the Leghorn by the fact that the skin of the whole
body including that of the face and comb appears to be of a deep
purplish colour. The face and comb of the Leghorn are red and the skin
of the body is whitish yellow. On examination it is found that the
purple colour of the Silky is in reality due to the distribution of a
deep black pigment in the mesoblastic membranes throughout the body. The
somatopleura, the pleura, _pia mater_, the dermis, and in most organs
the connective tissue and the sheaths of the blood-vessels, are thus
impregnated with black. No such pigmentation exists in the Leghorn.
As the result of an elaborate series of experimental matings we have
proved that the distinction between the Leghorn and the Silky consists
primarily in the fact that the Silky possesses a pigment-producing
factor, _P_, which is not present in the Leghorn.
This variation must undoubtedly have been one of _addition_. But besides
this there is another difference of an altogether dissimilar nature; for
the Brown Leghorn possesses a factor which has the power of partially or
completely restricting the operation of the pigment-producing factor,
_P_. Moreover in respect of this pigment-restricting factor which we
may call _D_, the sexes of the Brown Leghorn differ, for the male is
homozygous or _DD_, but the female is heterozygous, _Dd_. Thus in order
that the black-skinned breed could be evolved from such a type as a
Brown Leghorn it must be necessary _both_ that _P_ should be added and
that _D_ should drop out. We have not the faintest conception of the
process by which either of these events have come to pass, but there is
no reasonable doubt that in the evolution of the Silky fowl they did
actually happen.
We may anticipate that numerous interdependences of this kind will be
discovered.
Before any indisputable progress can be made with the problem of
evolution it is necessary that we should acquire some real knowledge
of the genesis of that class of phenomena which formed the subject of
the last chapter. So long as the process of division remains entirely
mysterious we can form no conception even of the haziest sort as
to the nature of living organisms, or of the proximate causes which
determine their forms, still less can we attempt any answer to those
remoter questions of origin and destiny which form the subject of the
philosopher's contemplation. It is in no spirit of dogmatism that I
have ventured to indicate the direction in which I look for a solution,
though I have none to offer. It may well be that before any solution is
attained, our knowledge of the nature of unorganised matter must first
be increased. For a long time yet we may have to halt, but we none
the less do well to prepare ourselves to utilise any means of advance
that may be offered, by carefully reconnoitering the ground we have to
traverse. The real difficulty which blocks our progress is ignorance of
the nature of division, or to use the more general term, of repetition.
Let us turn to the more familiar problem of the causes of variation. Now
since variation consists as much in meristic change as in alteration in
substance or material, there is one great range of problems of causation
from which we are as yet entirely cut off. We know nothing of the
causation of division, and we have scarcely an observation, experiment
or surmise touching the causes by which the meristic processes may be
altered.
Of the way in which variations in the substantive composition of
organisms are caused we have almost as little real evidence, but we are
beginning to know in what such variations must consist. These changes
must occur either by the addition or loss of factors.
We must not lose sight of the fact that though the factors operate by
the production of enzymes, of bodies on which these enzymes can act, and
of intermediary substances necessary to complete the enzyme-action, yet
these bodies themselves can scarcely be themselves genetic factors, but
consequences of their existence. What then are the factors themselves?
Whence do they come? How do they become integral parts of the organism?
Whence, for example, came the power which is present in a White Leghorn
of destroying--probably reducing--the pigment in its feathers? That
power is now a definite possession of the breed, present in all its
germ-cells, male and female, taking part in their symmetrical divisions,
and passed on equally to all as much as is the protoplasm or any other
attribute of the breed. From the body of the bird the critical and
efficient substance could in all likelihood be isolated by suitable
means, just as the glycogen of the liver can be. But even when this
extraction has been accomplished and the reducing body isolated, we
shall know no more than we did before respecting the mode by which
the power to produce it was conferred on the fowl, any more than we
know how the walls of its blood-vessels acquired the power to form a
fibrin-ferment.
It is when the scope of such considerations as this are fully grasped
that we realise the fatuousness of the conventional treatment which the
problem of the causes of variation commonly receives. Environmental
change, chemical injury, differences in food supply, in temperature,
in moisture, or the like have been proposed as "causes." Admitting
as we must do, that changes may be produced--usually inhibitions of
development--by subjecting living things to changes in these respects,
how can we suppose it in the smallest degree likely that very precise,
new, and adaptative powers can be conferred on the germs by such
treatment? Reports of positive genetic consequences observed comparable
with those I have mentioned, become from time to time current. We
should I think regard them with the gravest doubt. Few, so far as I am
aware, have ever been confirmed, though clear and repeated confirmation
should be demanded before we suffer ourselves at all to build upon such
evidence. In a subsequent chapter some of these cases will be considered
in detail.
In no class of cases would the transmission of an acquired character
superficially appear so probable as in those where power of resisting
the attack of a pathogenic organism is acquired in the lifetime of
the zygote. The possession of such a power is moreover a distinction
comparable with those which differentiate varieties and species. It
is due to the development in the blood of specific substances which
pervade the whole fluid. This development is exactly one of those
"appropriate responses to stimuli" which naturalists who incline to
regard adaptation as a direct consequence of an environmental influence
might most readily invoke as an illustration of their views. And yet all
evidence is definitely unfavourable to the suggestion of an inheritance
of the acquired power of resistance. Such change as can be perceived
in the virulence of the attacks on successive generations may be most
easily regarded as due to the extermination of the more susceptible
strains, and perhaps in some measure to variation in the invading
organisms themselves, an "acquired character" of quite different import.
The specific "anti-body" may have been produced in response to the
stimulus of disease, but the power to produce it without this special
stimulus is not included in the germ-cells any more than a pigment.
All that they bear is the _power to produce_ the anti-bodies when the
stimulus is applied.
If we could conceive of an organism like one of those to which disease
may be due becoming actually incorporated with the system of its host,
so as to form a constituent of its germ-cells and to take part in the
symmetry of their divisions, we should have something analogous to the
case of a species which acquires a new factor and emits a dominant
variety. When we see the phenomenon in this light we realise the
obscurity of the problem. The appearance of recessive varieties is
comparatively easy to understand. All that is implied is the omission of
a constituent. How precisely the omission is effected we cannot suggest,
but it is not very difficult to suppose that by some mechanical fault
of cell-division a power may be lost. Such variation by unpacking,
or analysis of a previously existing complex, though unaccountable,
is not inconceivable. But whence come the new dominants? Whether we
imagine that they are created by some rearrangement or other change
internal to the organism, or whether we try to conceive them as due to
the assumption of something from without we are confronted by equally
hopeless difficulty.
The mystery of the origin of a dominant increases when it is realised
that there is scarcely any recent and authentic account of such an event
occurring under critical observation, which can be taken as a basis for
discussion. The literature of horticulture for example abounds in cases
alleged, but I do not think anyone can produce an illustration quite
free from doubt. Such evidence is usually open to the suspicion that the
plant was either introduced by some accident, or that it arose from a
cross with a pre-existing dominant, or that it owed its origin to the
meeting of complementary factors. In medical literature almost alone
however, there are numerous records of the spontaneous origin of various
abnormal conditions in man which habitually behave as dominants, and of
the authenticity of some of these there can be no doubt.
When we know that such conditions as hereditary cataract or various
deformities of the fingers behave as dominants, we recognize that
those conditions must be due to the addition of some element to the
constitution of the normal man. In the collections of pedigrees relating
to such pathological dominants there are usually to be found alleged
instances of the origin of the condition _de novo_. Not only do these
records occur with such frequency that they cannot be readily set aside
as errors, but from general considerations it must be obvious that as
these malformations are not common to normal humanity they must at
some moment of time have been introduced. The lay reader may not be
so much impressed with the difficulty as we are. He is accustomed to
regard the origin of _any_ new character as equally mysterious, but when
once dominants are distinguished from recessives the problem wears a
new aspect. Thus the appearance of high artistic gifts, whether as an
attribute of a race or as a sporadic event among the children of parents
destitute of such faculties, is not very surprising, for we feel fairly
sure that the faculty is a recessive, due to the loss of a controlling
or inhibiting factor; but the _de novo_ origin of brachydactylous
fingers in a child of normal parents is of quite a different nature, and
must indicate the action of some new specific cause.
Whether such evidence is applicable to the general problem of evolution
may with some plausibility be questioned; but there is an obvious
significance in the fact that it is among these pathological occurrences
that we meet with phenomena most nearly resembling the spontaneous
origin of dominant factors, and I cannot see such pedigrees as these
without recalling Virchow's aphorism that every variation owes its
origin to some pathological accident. In the evolution of domestic
poultry, if _Gallus bankiva_ be indeed the parent form of all our
breeds, at least some half dozen new factors must have been added during
the process. In _bankiva_ there is, for example, no factor for rose
comb, pea comb, barring on the feathers, or for the various dominant
types of dark plumage. Whence came all these? It is, I think, by no
means impossible that some other wild species now extinct did take
part in the constitution of domestic poultry. It seems indeed to me
improbable that the heavy breeds descend from _bankiva_. Both in regard
to domestic races of fowls, pigeons, and some other forms, the belief in
origin within the period of human civilization from one simple primitive
wild type seems on a balance of probabilities insecurely founded, but
allowing something for multiplicity of origin we still fall far short
of the requisite total of factors. Elements exist in our domesticated
breeds which we may feel with confidence have come in since their
captivity began. Such elements in fowls are dominant whiteness, extra
toe, feathered leg, frizzling, etc., so that even hypothetical extension
of the range of origin is only a slight alleviation of the difficulty.
Somehow or other, therefore, we must recognize that dominant factors
do arise. Whether they are created by internal change, or whether, as
seems to me not wholly beyond possibility, they obtain entrance from
without, there is no evidence to show. If they were proved to enter from
without, like pathogenic organisms, we should have to account for the
extraordinary fact that they are distributed with fair constancy to half
the gametes of the heterozygote.
In proportion as the nature of dominants grows more clear so does it
become increasingly difficult to make any plausible suggestion as to
their possible derivation. On the other hand the origin of a recessive
variety by the loss of a factor is a process so readily imagined that
our wonder is rather that the phenomenon is not observed far more
often. Some slip in the accurate working of the mechanical process of
division, and a factor gets left out, the loss being attested by the
appearance of a recessive variety in some subsequent generation.
Consistently with this presentation of the facts we find that, as in our
domesticated animals and plants, a diversity of recessives may appear
within a moderately short period, and that when variations come they
often do not come alone. Witness the cultural history of the Sweet Pea,
_Primula Sinensis_, _Primula obconica_, _Nemesia strumosa_ and many such
examples in which variation when it did come was abundant. The fact
cannot be too often emphasized that in the vast proportion of these
examples of substantive variation under domestication, as well as of
substantive variation in the natural state, the change has come about by
omission, not by addition. To take, for example, the case of the Potato,
in which so many spontaneous bud-variations have been recorded, East
after a careful study of the evidence has lately declared his belief
that all are of this nature, and the opinion might be extended to many
other groups of cases whether of bud or seminal variation. Morgan draws
the same conclusion in reference to the many varieties he has studied in
_Drosophila_.
In the Sweet Pea, a form
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