An Examination of Weismannism
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Title: An Examination of Weismannism
Author: George John Romanes
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AN EXAMINATION
OF
WEISMANNISM
Oxford
HORACE HART, PRINTER TO THE UNIVERSITY
[Illustration: AUGUST WEISMANN]
AN EXAMINATION
OF
WEISMANNISM
BY
GEORGE JOHN ROMANES
M.A., LL.D., F.R.S.
HONORARY FELLOW OF GONVILLE AND CAIUS COLLEGE, CAMBRIDGE
London
LONGMANS, GREEN, AND CO.
1893
PREFACE
AS already stated in the Preface to the second edition of _Darwin and
after Darwin_, Part I, severe and protracted illness has hitherto
prevented me from proceeding to the publication of Part II. It is
now more than a year since I had to suspend work of every kind, and
therefore, although at that time Part II was almost ready for press,
I have not yet been able to write its concluding chapters. Shortly
before and during this interval Professor Weismann has produced his
essays on _Amphimixis_ and _The Germ-plasm_. These works present
extensive additions to, and considerable modifications of, his previous
theories as collected together in the English translation, under
the title _Essays on Heredity, Vol. I_. Consequently, it has become
necessary for me either to re-write the examination of his system which
I had prepared for Part II of my own treatise, or else to leave that
examination as it stood, and to add a further chapter dealing with
those later developments of his system to which I have just alluded.
After due reflection I have decided upon the latter course, because
in this way we are most likely to obtain a clear view of the growth of
Weismann’s elaborate structure of theories—a view which it is almost
necessary, for the purposes of criticism, that we should obtain.
Having decided upon this point, it occurred to me that certain
advantages would be gained by removing the whole criticism from the
position which it was originally intended to occupy as a section of my
forthcoming volume on the Post-Darwinian period. For, in consequence
of the criticism having been written at successive intervals during
the last six or eight years as Professor Weismann’s works successively
appeared, it has now swelled to a bulk which would unduly encumber
the volume just mentioned. Again, the growth of Professor Weismann’s
system has of late become so rapid, that if the criticism is to keep
pace with it in future, the best plan will doubtless be the one which
it is now my intention to adopt—viz., to publish the criticism in a
separate form, and in comparatively small editions, so that further
chapters may be added with as much celerity as Professor Weismann
may hereafter produce his successive works. Lastly, where so much
elaborate speculation and so many changes of doctrine are concerned, it
is inevitable that some misunderstandings on the part of a critic are
likely to have arisen; and therefore, should Professor Weismann deem it
worth his while to correct any such failings on my part, the plan of
publication just alluded to will furnish me with the best opportunity
of dealing with whatever he may have to say.
It must be understood, however, that under the term “Weismannism” I
do not include any reference to the important question with which the
name of Weismann has been mainly associated—i.e., the inheritance or
non-inheritance of acquired characters. This is a question of fact,
which stands to be answered by the inductive methods of observation
and experiment: not by the deductive methods of general reasoning.
Of course Professor Weismann is fully entitled to assume a negative
answer as a basis whereon to construct his theory of the continuity of
germ-plasm; but no amount of speculation as to what the mechanism of
heredity is likely to be if once this assumption is granted, can even
so much as tend to prove that the assumption itself is true. Therefore,
in this “examination of Weismannism” I intend to restrict our attention
to the elaborate system of theories which Weismann has reared upon his
fundamental postulate of the non-inheritance of acquired characters,
reserving for my next volume our consideration of this postulate itself.
Lest, however, it should be felt that “an examination of Weismannism”
in which the question of the transmission of acquired characters is
omitted must indeed prove a case of Hamlet without the Prince of
Denmark, I may be allowed to make two observations. In the first
place, this great question of fact is clearly quite distinct from
that of any theories which may be framed upon either side of it.
And, in the second place, the question was not raised by Weismann. It
appears, indeed, from what he says, that he never caught a glimpse of
it till about ten years ago, and that he then did so as a result of
his own independent thought. Moreover, it is perfectly true that to
him belongs the great merit of having been the first to call general
attention to the subject, and so to arouse a world-wide interest with
reference to it. But to suppose that the question was first propounded
by Weismann is merely to display a want of acquaintance with the
course of Darwinian thought in this country. As far back as 1874 I had
long conversations with Darwin himself upon the matter, and under his
guidance performed what I suppose are the only systematic experiments
which have ever been undertaken with regard to it. These occupied
more than five years of almost exclusive devotion; but, as they all
proved failures, they were never published. Therefore I here mention
them merely for the purpose of showing that the idea of what is now
called a “continuity of germ-plasm” was present to Darwin’s mind as
a logically possible alternative to the one which he adopted in his
theory of pangenesis—an alternative, therefore, which he was anxious
to exclude by way of experimental disproof. If it be said that no one
could have been aware of this in the absence of publication, I reply
that I think it may be perceived by any one who reads attentively his
chapter on Pangenesis. Moreover, early in the seventies his cousin,
Mr. Francis Galton, published a “Theory of Heredity,” which, as we
shall see in the course of the following pages, presented as distinctly
as could possibly be presented the question of the transmission of
acquired characters, and answered it in almost exactly the same manner
as Weismann did about ten years later. Lastly, as Weismann has himself
been careful to point out, he was likewise anticipated in this matter
by Jäger (1878), and Nussbaum and Rauber (1880).
For these reasons, then, I exclude this question from the following
examination of what I think we ought to understand as distinctively
“Weismannism.”
G. J. R.
CHRIST CHURCH, OXFORD,
_July, 1893_.
CONTENTS
CHAP. PAGE
I. STATEMENT OF WEISMANN’S SYSTEM UP TO THE YEAR 1886 1
II. LATER ADDITIONS TO WEISMANN’S SYSTEM UP TO THE YEAR 1892 28
III. WEISMANN’S THEORY OF HEREDITY (1891) 48
IV. EXAMINATION OF WEISMANN’S THEORY OF EVOLUTION (1891) 86
V. WEISMANNISM UP TO DATE (1893) 117
APPENDIX I.—ON GERM-PLASM 173
” II.—ON TELEGONY 191
AN
EXAMINATION OF WEISMANNISM.
CHAPTER I.
STATEMENT OF WEISMANN’S SYSTEM UP TO THE YEAR 1886[1].
SEEING that Professor Weismann’s theory of heredity, besides being
somewhat elaborate in itself, is presented in a series of disconnected
essays, originally published at different times, it is a matter of
no small difficulty to gather from the present collection of them a
complete view of the system as a whole. Therefore I propose to give
a brief sketch of his several cognate theories, arranged in a manner
calculated to show their logical connexion one with another. And,
in order also to show the relation in which his resulting theory
of heredity stands to what has hitherto been the more usual way of
regarding the facts, I will begin by furnishing a similarly condensed
account of Mr. Darwin’s theory upon the subject. It will be observed
that these two theories constitute the logical extremes of explanatory
thought; and therefore it may be said, in a general way, that all
other modern theories of heredity—such as those of Spencer, Häckel,
Elsberg, Galton, Nägeli, His, Brooks, Hertwig, and De Vries—occupy
positions more or less intermediate between these two extremes.
Therefore, also, we need not wait to consider these intermediate
theories[2].
“When closely analyzed, Mr. Darwin’s theory—or the “provisional
hypothesis of Pangenesis”—will be found to embody altogether seven
assumptions, namely:—
1. That all the component cells of a multicellular organism throw off
inconceivably minute germs, or “gemmules,” which are then dispersed
throughout the whole system.
2. That these gemmules, when so dispersed and supplied with proper
nutriment, multiply by self-division, and, under suitable conditions,
are capable of developing into physiological cells like those from
which they were originally and severally derived.
3. That, while still in this gemmular condition, these cell-seeds
have for one another a mutual affinity, which leads to their being
collected from all parts of the system by the reproductive glands of
the organism; and that, when so collected, they go to constitute the
essential material of the sexual elements—ova and spermatozoa being
thus aggregated packets of gemmules, which have emanated from all the
cells of all the tissues of the organism.
4. That the development of a new organism, out of the fusion of two
such packets of gemmules, is due to a summation of all the developments
of some of the gemmules which these two packets contain.
5. That a large proportional number of the gemmules in each packet,
however, fail to develop, and are then transmitted in a dormant
state to future generations, in any of which they may be developed
subsequently—thus giving rise to the phenomena of reversion or atavism.
6. That in all cases the development of gemmules into the form of their
parent cells depends on their suitable union with other partially
developed gemmules, which precede them in the regular course of growth.
7. That gemmules are thrown off by all physiological cells, not only
during the adult state of the organism, but during all stages of
its development. Or, in other words, that the production of these
cell-seeds depends upon the adult condition of parent cells: not upon
that of the multicellular organism as a whole.
At first sight it may well appear that we have here a very formidable
array of assumptions. But Darwin ably argues in favour of each of them
by pointing to well-known analogies, drawn from the vital processes
of living cells both in the protozoa and metazoa. For example, it
is already a well-recognized doctrine of physiology that each cell
of a metazoon, or multicellular organism, though to a large extent
dependent on others, is likewise to a certain extent independent
or autonomous, and has the power of multiplying by self-division.
Therefore, as it is certain that the sexual elements (and also buds
of all descriptions) include formative material of some kind, the
first assumption—or that which supposes such formative matter to be
participate—is certainly not a gratuitous assumption.
Again, the second assumption—viz., that this particulate and formative
material is dispersed throughout all the tissues of the organism—is
sustained by the fact that, both in certain plants and in certain
invertebrated animals, a severed portion of the organism will develop
into an entire organism similar to that from which it was derived, as,
for example, is the case with a leaf of Begonia, and with portions cut
from certain invertebrated animals, such as sea-anemones, jelly-fish,
&c. This well-known fact in itself seems enough to prove that the
formative material in question must certainly admit, at all events in
many cases, of being distributed throughout all the tissues of living
organisms.
The third assumption—or that which supposes the formative material
to be especially aggregated in the sexual elements—is not so much an
assumption as a statement of obvious fact; while the fourth, fifth,
sixth, and seventh assumptions all follow deductively from their
predecessors. In other words, if the first and second assumptions be
granted, and if the theory is to comprise all the facts of heredity,
then the remaining five assumptions are bound to follow.
To the probable objection that the supposed gemmules must be of a
size impossibly minute—seeing that thousands of millions of them
would have to be packed into a single ovum or spermatozoon—Darwin
opposes a calculation that a cube of glass or water, having only one
ten-thousandth of an inch to a side, contains somewhere between sixteen
and a hundred and thirty-one billions of molecules. Again, as touching
the supposed power of multiplication on the part of his gemmules, he
alludes to the fact that infectious material of all kinds exhibits
a ratio of increase quite as great as any that his theory requires
to attribute to gemmules. Furthermore, with respect to the elective
affinity of gemmules, he remarks that “in all ordinary cases of sexual
reproduction, the male and female elements certainly have an elective
affinity for each other”: of the ten thousand species of _Compositae_,
for example, “there can be no doubt that if the pollen of all these
species could be simultaneously placed on the stigma of any one
species, this one would elect, with unerring certainty, its own pollen.”
Such, in brief outline, is Mr. Darwin’s theory of Pangenesis.
Professor Weismann’s theory of Germ-plasm is fundamentally based upon
the great distinction, in respect of their transmissibility, between
characters that are congenital and characters that are acquired. By
a congenital character is meant any individual peculiarity, whether
structural or mental, with which the individual is born. By an
acquired character is meant any peculiarity which the individual may
subsequently develop in consequence of its own individual experience.
For example, a man may be born with some malformation of one of his
fingers; or he may subsequently acquire such a malformation as the
result of accident or disease. Now, in the former case—i.e., in that
where the malformation is congenital—it is extremely probable that the
peculiarity will be transmitted to his children; while in the latter
case—i.e., where the malformation is subsequently acquired—it is
virtually certain that it will not be transmitted to his children. And
this great difference between the transmissibility of characters that
are congenital and characters that are acquired extends universally as
a general law throughout the vegetable as well as the animal kingdom,
and in the province of mental as in that of bodily organization. Of
course this general law has always been well known, and more or less
fully recognized by all modern physiologists and medical men. But
before the subject was taken up by Professor Weismann, it was generally
supposed that the difference in question was one of degree, not one of
kind. In other words, it was assumed that acquired characters, although
not so fully—and therefore not so certainly—inherited as congenital
characters, nevertheless were inherited in some lesser degree; so that
if the same character continued to be developed successively in a
number of sequent generations, what was at first only a slight tendency
to be inherited would become by summation a more and more pronounced
tendency, till eventually the acquired character might be as strongly
inherited as any other character which was _ab initio_ congenital. Now
it is the validity of this assumption that is challenged by Professor
Weismann. He says there is no evidence of any acquired characters
being in any degree inherited; and, therefore, that in this important
respect they may be held to differ from congenital characters in kind.
On the supposition that they do thus differ in kind, he furnishes a
very attractive theory of heredity, which serves at once to explain
the difference, and to represent it as a matter of physiological
impossibility that any acquired character can, under any circumstances
whatsoever, be transmitted to progeny.
But, in order fully to comprehend this theory, it is desirable first
of all to explain Professor Weismann’s views upon certain other topics
which are intimately connected with—and, indeed, logically sequent
upon—the use to which he puts the distinction just mentioned.
Starting from the fact that unicellular organisms multiply by fission
and gemmation, he argues that, aboriginally and potentially, life
is immortal. For when a protozoon divides itself into two more or
less equal parts by fission, and each of the two halves thereupon
grows into another protozoon, it does not appear that there has been
any death on the part of the living material involved; and inasmuch
as this process of fission goes on continuously from generation to
generation, there is seemingly never any death on the part of such
protoplasmic material, although there is a continuous addition to
it as the numbers of individuals increase. Similarly, in the case
of gemmation, when a protozoon parts with a small portion of its
living material in the form of a bud, this portion does not die, but
develops into a new individual; and, therefore, the process is exactly
analogous to that of fission, save that a small instead of a large part
of the parent substance is involved. Now, if life be thus immortal
in the case of unicellular organisms, why should it have ceased to
be so in the case of multicellular? Weismann’s answer is, that all
the multicellular organisms propagate themselves, not exclusively by
fission or gemmation, but by sexual fertilization, where the condition
to a new organism arising is that minute and specialized portions of
two parent organisms should fuse together. Now, it is evident that with
this change in the method of propagation, serious disadvantage would
accrue to any species if its sexual individuals were to continue to be
immortal; for in that case every species which multiplies by sexual
methods would in time become composed of individuals broken down and
decrepit through the results of accident and disease—always operating
and ever accumulating throughout the course of their immortal lives.
Consequently, as soon as sexual methods of propagation superseded
the more primitive a-sexual methods, it became desirable in the
interests of the sexually-propagating species that their constituent
individuals should cease to be immortal, so that the species should
always be recuperated by fresh, young, and well-formed representatives.
Consequently, also, natural selection would speedily see to it that all
sexually-propagating species should become deprived of the aboriginal
endowment of immortality, with the result that death is now universal
among all the individuals of such species—that is to say, among all
the metazoa and metaphyta. Nevertheless, it is to be remembered that
this destiny extends only to the parts of the individual other than the
contents of those specialized cells which constitute the reproductive
elements. For although in each individual metazoon or metaphyton
an innumerable number of these specialized cells are destined to
perish during the life, or with the death, of the organism to which
they belong, this is only due to the accident, so to speak, of their
contents not having met with their complements in the opposite sex:
it does not belong to their essential nature that they should perish,
seeing that those which do happen to meet with their complements in the
opposite sex help to form a new living individual, and so on through
successive generations _ad infinitum_. Therefore the reproductive
elements of the metazoa and metaphyta are in this respect precisely
analogous to the protozoa: potentially, or in their own nature, they
are immortal; and, like the protozoa, if they die, their death is an
accident due to unfavourable circumstances. But the case is quite
different with all the other parts of a multicellular organism. Here,
no matter how favourable the circumstances may be, every cell contains
within itself, or in its very nature, the eventual doom of death. Thus,
of the metazoa and metaphyta it is the “germ-plasms” alone that retain
their primitive endowment of everlasting life, passed on continuously
through generation after generation of successively perishing organisms.
So far, it is contended, we are dealing with matters of fact. It must
be taken as true that the protoplasm of the unicellular organisms, and
the germ-plasm of the multicellular organisms, has been continuous
through the time since life first appeared upon this earth; and
although large quantities of each are perpetually dying through
being exposed to conditions unfavourable to life, this, as Weismann
presents the matter, is quite a different case from that of all the
other constituent parts of multicellular organisms, which contain
within themselves the doom of death. Furthermore, it appears extremely
probable that this doom of death has been brought about by natural
selection for the reasons assigned by Weismann—namely, because it is
for the benefit of all species which perpetuate themselves by sexual
methods, that their constituent individuals should not live longer
than is necessary for the sake of originating the next generation, and
fairly starting it in its own struggle for existence. For Weismann has
shown, by a somewhat laborious though still largely imperfect research,
that there is throughout all the metazoa a general correlation between
the natural lifetime of individuals composing any given species and
the age at which they reach maturity, or first become capable of
procreation. This general correlation, however, is somewhat modified
by the time during which progeny are dependent upon their parents for
support and protection. Nevertheless, it is evident that this fact
tends rather to confirm the view that expectation of life on the part
of individuals has in all cases been determined with strict reference
to the requirements of propagation, if under propagation we include
the rearing as well as the production of offspring. I may observe in
passing that I do not think this general law can be found to apply to
plants in nearly so close a manner as Weismann has shown it to apply to
animals; but, leaving this consideration aside, I think that Weismann
has made out a good case in favour of such a general law with regard to
animals[3].
We have come, then, to these results. Protoplasm was originally
immortal, barring accidents; and it still continues to be immortal in
the case of unicellular organisms which propagate a-sexually. But in
the case of all multicellular organisms, which propagate sexually,
natural selection has reduced the term of life within the smallest
limits that in each given case are compatible with the performance
of the sexual act and the subsequent rearing of progeny—reserving,
however, the original endowment of immortality for the germinal
elements, whereby a _continuum_ of life has been secured from the
earliest appearance of life until the present day.
Now, in view of these results the question arises,—Why should the
sexual methods of propagation have become so general, if their effect
has been that of determining the necessary death of all individuals
presenting them? Why, in the course of organic evolution, should these
newer methods have been imposed on all the higher organisms, when
the consequence is that all these higher organisms must pay for the
innovation with their lives? Weismann’s answer to this question is as
interesting and ingenious as all that has gone before. Seeing that
sexual propagation is so general as to be practically universal among
multicellular organisms, it is obvious that in some way or another it
must have had a most important part to play in the general scheme of
organic evolution. What, then, is the part that it does play? What is
its _raison d’être_? Briefly, according to Weismann, its function is
that of furnishing congenital variations to the ever-watchful agency
of natural selection, in order that natural selection may always
preserve the most favourable, and pass them on to the next generation
by heredity. That sexual propagation is well calculated to furnish
congenital variations may easily be rendered apparent. We have only to
remember that at each union there is a mixture of two sets of germinal
elements; that each of these was in turn the product of two other sets
in the preceding generation, and so backwards _ad infinitum_ in an ever
doubling ratio. Remembering this, it follows that the germinal elements
of no one member of a species can ever be the same as those of any
other member born of different parents; on the contrary, while both
are enormously complex products, each has had a different ancestral
history, such that while one presents the congenital admixtures of
thousands of individuals in one line of descent, the other presents
similar admixtures of thousands of other individuals in a different
line of descent. Consequently, when in any sexual union two of these
enormously complex germinal elements fuse together, and constitute a
new individual out of their joint endowments, it is perfectly certain
that that individual cannot be exactly like any other individual of
the same species which has been born of different parents. The chances
must be infinity to one against any single mass of germ-plasm being
exactly like any other mass of germ-plasm; while any amount of latitude
as to difference is allowed, up to the point at which the difference
becomes too pronounced to satisfy the conditions of fertilization—in
which case, of course, no new individual is born. Hence, theoretically,
we have here a sufficient cause for all individual variations of
a congenital kind that can possibly occur within the limits of
fertility, and, therefore, that can ever become actual in living
organisms. In point of fact, Weismann believes—or, at any rate,
provisionally maintains—that this is the sole and only cause of
variations that are congenital, and therefore (according to his views)
transmissible by heredity. Now, whether or not he is right as regards
these latter points, I think there can be no question that sexual
propagation is, at all events, one of the main causes of congenital
variation; and seeing of what enormous importance congenital variation
must always have been in supplying material for the operation of
natural selection, we appear to have found a most satisfactory answer
to our question,—Why has sexual propagation become so universal among
all the higher plants and animals? It has become so because it is thus
shown to have been the condition to producing congenital variations,
which in turn constitute one of the primary conditions to the working
of natural selection.
Having got thus far, I should like to make two or three subsidiary
remarks. In the first place, it ought to be observed that this theory
touching the causes of congenital variations was not originally
propounded by Professor Weismann, but occurs in the writings of
several previous authors, and is expressly alluded to by Darwin[4].
Nevertheless, it occupies so prominent a place in Weismann’s system of
theories, and has by him been wrought up so much more elaborately than
by any of his predecessors, that we are entitled to regard it as, _par
excellence_, the Weismannian theory of variation. In the next place,
it ought to be observed that Weismann is careful to guard against the
seductive fallacy of attributing the origin of sexual propagation to
the agency of natural selection. Great as the benefit of this newer
mode of propagation must have been to the species presenting it, the
benefit cannot have been conferred by natural selection, seeing that
the benefit arose from the fact of the new method furnishing material
to the operation of natural selection, and therefore constituting
the condition to the agency of natural selection having been called
into existence at all. Or, in other words, we cannot attribute to
natural selection the origin of sexual reproduction without involving
ourselves in the absurdity of supposing natural selection to have
originated the conditions of its own activity[5]. What the causes may
have been which originally led to sexual reproduction is at present
a matter that awaits suggestion by way of hypothesis; and, therefore,
it now only remains to add that the general structure of Professor
Weismann’s system of hypotheses leads to this curious result—namely,
that the otherwise ubiquitous and (as he supposes) exclusive dominion
of natural selection stops short at the protozoa, over which it cannot
exercise any influence at all. For if natural selection depends for its
activity on the occurrence of congenital variations, and if congenital
variations depend for their occurrence on sexual modes of reproduction,
it follows that no organisms which propagate by any other modes can
present congenital variations, or thus become subject to the sway of
natural selection. And inasmuch as Weismann believes that such is
the case with all the protozoa, as well as with all parthenogenetic
organisms, he does not hesitate to accept the necessary conclusion that
in these cases natural selection is without any jurisdiction. How,
then, does he account for individual variations in the protozoa? And,
still more, how does he account for the origin of their innumerable
species? He accounts for both these things by the direct action of
external conditions of life. In other words, so far as the unicellular
organisms are concerned, Weismann is rigidly and unconditionally an
advocate of the theory of Lamarck—just as much as in the case of
all the multicellular organisms he is rigidly and unconditionally an
opponent of that theory. Nevertheless, there is here no inconsistency:
on the contrary, it is consistency with the logical requirements of
his theory that leads to this sharp partitioning of the unicellular
from the multicellular organisms with respect to the causes of their
evolution. For, according to his view, the conditions of propagation
among the unicellular organisms are such that parent and offspring
are one and the same thing; “the child is a part, and usually a half,
of its parent.” Therefore, if the parent has been in any way modified
by the action of external conditions, it is inevitable that the child
should, from the moment of its birth (i.e., fissiparous separation),
be similarly modified; and if the modifying influences continue in the
same lines for a sufficient length of time, the resulting change of
type may become sufficiently pronounced to constitute a new species,
genus, &c. But in the case of the multicellular or sexual organisms,
the child is not thus merely a severed moiety of its parent; it is the
result of the fusion of two highly specialized and extremely minute
particles of each of two parents. Therefore, whatever may be thought
touching the validity of Weismann’s deduction that in no case can
any modification induced by external conditions on these parents be
transmitted to their progeny, at least we must recognize the validity
of the distinction which he draws between the facility with which such
transmission must take place in the unicellular organisms, as compared
with the difficulty—or, as he believes, the impossibility—of its
doing so in the multicellular.
We are now in a position fully to understand Professor Weismann’s
theory of heredity in all its bearings. Briefly stated, it is as
follows. The whole organization of any multicellular organism is
composed of two entirely different kinds of cells—namely, the
germ-cells, or those which have to do with reproduction, and the
somatic-cells, or those which go to constitute all the other parts of
the organism. Now, the somatic-cells, in their aggregations as tissues
and organs, may be modified in numberless ways by the direct action
of the environment, as well as by special habits formed during the
individual lifetime of the organism. But although the modifications
thus induced may be, and generally are, adaptive—such as the increased
muscularity caused by the use of muscles, “practice making perfect”
where neural adjustments are concerned, and so on,—in no case can
these so-called acquired, or “somatogenetic,” characters exercise any
influence upon the germ-cells, such that they should reappear in the
next generation as congenital, or “blastogenetic,” characters. For,
according to the theory, the germ-cells as to their germinal contents
differ in kind from the somatic-cells, and have no other connexion or
dependence upon them than that of deriving from them their food and
lodging. So much for the somatic-cells.
Turning now to the germ-cells, these are the receptacles of what
Weismann calls the germ-plasm; and this it is that he supposes
to differ in kind from all the other constituent elements of the
organism. For the germ-plasm he believes to have had its origin in the
unicellular organisms, and to have been handed down from them in one
continuous stream through all successive generations of multicellular
organisms. Thus, for example, suppose that we take a certain _quantum_
of germ-plasm as this occurs in any individual organism of to-day. A
minute portion of this germ-plasm, when mixed with a similarly minute
portion from another individual, goes to form a new individual. But,
in doing so, only a portion of this minute portion is consumed; the
residue is stored up in the germinal cells of the new individual,
in order to secure that continuity of the germ-plasm which Weismann
assumes as the necessary basis of his whole theory. Furthermore, he
assumes that this overplus portion of germ-plasm, which is so handed
over to the custody of the new individual, is there capable of growth
or multiplication at the expense of the nutrient materials which are
supplied to it by the new soma in which it finds itself located; while
in thus growing, or multiplying, it faithfully retains its highly
complex structure, so that in no one minute particular does any part of
a many thousand-fold increase differ, as to its ancestral characters,
from that inconceivably small overplus which was first of all entrusted
to the embryo by its parents. Therefore one might represent the
germ-plasm by the metaphor of a yeast-plant, a single particle of which
may be put into a vat of nutrient fluid: there it lives and grows upon
the nutriment supplied, so that a new particle may next be taken to
impregnate another vat, and so on _ad infinitum_. Here the successive
vats would represent successive generations of progeny; but, to make
the metaphor complete, one would have to suppose that in each case the
yeast-cell was required to begin by making its own vat of nutrient
material, and that it was only the residual portion of the cell which
was afterwards able to grow and multiply. But although the metaphor
is thus necessarily a clumsy one, it may serve to emphasize the
all-important feature of Weismann’s theory—namely, the almost absolute
independence of the germ-plasm. For, just as the properties of the
yeast-plant would be in no way affected by anything that might happen
to the vat, short of its being broken up or having its malt impaired,
so, according to Weismann, the properties of the germ-plasm cannot be
affected by anything that may happen to its containing soma, short of
the soma being destroyed or having its nutritive functions disordered.
Such being the relations that are supposed to obtain between the soma
and its germ-plasm, we have next to observe what is supposed to happen
when, in the course of evolution, some modification of the ancestral
form of the soma is required in order to adapt it to some change on the
part of its environment. In other words, we have to consider Weismann’s
views on the _modus operandi_ of adaptive development, with its result
in the origination of new species.
Seeing that, according to the theory, it is only congenital variations
which can be inherited, all variations subsequently acquired by the
intercourse of individuals with their environment, however beneficial
such variations may be to these individuals, are ruled out as regards
the species. Not falling within the province of heredity, they
are blocked off in the first generation, and therefore present no
significance at all in the process of organic evolution. No matter how
many generations of eagles, for instance, may have used their wings for
purposes of flight; and no matter how great an increase of muscularity,
of endurance, and of skill, may thus have been secured to each
generation of eagles as the result of individual exercise; all these
advantages are entirely lost to progeny, and young eagles have ever to
begin their lives with no more benefit bequeathed by the activity of
their ancestors than if those ancestors had all been barn-door fowls.
The only material which is of any count as regards the species, or with
reference to the process of evolution, are fortuitous variations of the
congenital kind. Among all the numberless congenital variations, within
narrow limits, which are perpetually occurring in each generation of
eagles, some will have reference to the wings; and although these
will be fortuitous, or occurring indiscriminately in all directions,
a few of them will now and then be in the direction of increased
muscularity, others in the direction of increased endurance, others
in the direction of increased skill, and so on. Now each of these
fortuitous variations, which happens also to be a beneficial variation,
will be favoured by natural selection; and, because it likewise happens
to be a congenital variation, will be perpetuated by heredity. In the
course of time, other congenital variations will happen to arise in
the same directions; these will be added by natural selection to the
advantage already gained, and so on, till, after hundreds and thousands
of generations, the wings of eagles have become evolved into the
marvellous structures which they now present.
Such being the theory of natural selection when stripped of all
remnants of so-called Lamarckian principles, we have next to consider
what the theory means in its relation to germ-plasm. For, as before
explained, congenital variations are supposed by Weismann to be due
to new combinations taking place in the germ-plasm as a result of
the union in every act of fertilisation of two complex hereditary
histories. Well, if congenital variations are thus nothing more
than variations of germ-plasm “writ large” in the organism which is
developed out of the plasm, it follows that natural selection is
really at work upon these variations of the plasm. For, although it is
proximately at work on the congenital variations of organisms after
birth, it is ultimately, and through them, at work upon the variations
of germ-plasm out of which the organisms arise. In other words,
natural selection, in picking out of each generation those individual
organisms which are by their congenital characters best suited to their
surrounding conditions of life, is thereby picking out those peculiar
combinations or variations of germ-plasm, which, when expanded into a
resulting organism, give that organism the best chance in its struggle
for existence. And, inasmuch as a certain overplus of this peculiar
combination of germ-plasm is entrusted to that organism for bequeathing
to the next generation, this to the next, and so on, it follows that
natural selection is all the while conserving that originally peculiar
combination of germ-plasm, until it happens to meet with some other
mass of germ-plasm by mixing with which it may still further improve
upon its original peculiarity, when, other things equal, natural
selection will seize upon this improvement to perpetuate, as in the
previous case. So that, on the whole, we may say that natural selection
is ever waiting and watching for such combinations of germ-plasm as
will give the resulting organisms the best possible chance in their
struggle for existence; while, at the same time, it is remorselessly
destroying all those combinations of germ-plasm which are handed over
to the custody of organisms not so well fitted to their conditions of
life.
It only remains to add that, according to Weismann’s theory in its
strictly logical form, combinations of germ-plasm when once effected
are so stable that they would never alter except as a result of
entering into new combinations. In other words, no external influences
or internal processes can ever change the hereditary nature of any
particular mixture of germ-plasm, save and except its admixture with
some other germ-plasm, which, being of a nature equally stable, goes
to unite with the first in equal proportions as regards hereditary
character. So that really it would be more correct to say that any
given mass of germ-plasm does not change even when it is mixed with
some other mass—any more, for instance, than a handful of sand can be
said to change when it is mixed with a handful of clay.
Consequently, we arrive at this curious result. No matter how many
generations of organisms there may have been, and therefore no matter
how many combinations of germ-plasm may have taken place to give rise
to an existing population, each existing unit of germ-plasm must
have remained of the same essential nature or constitution as when
it was first started in its immortal career millions of years ago.
Or, reverting to our illustration of sand and clay, the particles
of each must always remain the same, no matter how many admixtures
they may undergo with particles of other materials, such as chalk,
slate, &c. Now, inasmuch as it is an essential—because a logically
necessary—part of Weismann’s theory to assume such absolute stability
or unchangeableness on the part of germ-plasm, the question arises, and
has to be met, What was the origin of those differences of character
in the different germ-plasms of multicellular organisms which first
gave rise, and still continue to give rise, to congenital variations
by their mixture one with another? This important question Weismann
answers by supposing that these differences originally arose out of the
differences in the unicellular organisms, which were the ancestors of
the primitive multicellular organisms. Now, as before stated, different
forms of unicellular organisms are supposed to have originated as so
many results of differences in the direct action of the environment.
Consequently, according to the theory, all congenital variations which
now occur in multicellular organisms, are really the distant results
of variations that were aboriginally induced in their unicellular
ancestors by the direct action of surrounding conditions of life.
I think it will be well to conclude by briefly summarising the main
features of this elaborate theory.
Living material is essentially, or of its own nature, imperishable;
and it still continues to be so in the case of unicellular organisms
which propagate by fission or gemmation. But as soon as these primitive
methods of propagation became, from whatever cause, superseded
by sexual, it ceased to be for the benefit of species that their
constituent individuals should be immortal; seeing that, if they
continued to be so, all species of sexually-reproducing organisms
would sooner or later have come to be composed of broken-down and
decrepit individuals. Consequently, in all sexually-reproducing or
multicellular organisms, natural selection set to work to reduce the
term of individual lifetimes within the narrowest limits that in the
case of each species were compatible with the procreation and the
rearing of progeny. Nevertheless, in all these sexually-reproducing
organisms the primitive endowment of immortality has been retained
with respect to their germ-plasm, which has thus been continuous,
through numberless generations of perishing organisms, from the first
origin of sexual reproduction till the present time. Now, it is the
union of germ-plasms which is required to reproduce new individuals
of multicellular organisms that determines congenital variations on
the part of such organisms, and thus furnishes natural selection with
the material for its work in the way of organic evolution—work,
therefore, which is impossible in the case of unicellular organisms,
where variation can never be congenital, but always determined by
the direct action of surrounding conditions of life. Again, as the
germ-plasm of multicellular organisms is continuous from generation to
generation, and at each impregnation gives rise to a more or less novel
set of congenital characters, natural selection, in picking out of
each generation the congenital characters which are of most service to
the organisms presenting them, is really or fundamentally at work upon
those variations of the germ-plasm which in turn give origin to these
variations of organisms that we recognize as congenital. Therefore,
natural selection has always to wait and to watch for such variations
of germ-plasm as will eventually prove beneficial to the individuals
developed therefrom, who will then transmit this peculiar quality of
germ-plasm to their progeny, and so on. Therefore also—and this is
most important to remember—natural selection as thus working becomes
the one and only cause of organic evolution in all the multicellular
organisms, just as the direct action of the environment is the one and
only cause of it in the case of all the unicellular organisms. But
inasmuch as the multicellular organisms were all in the first instance
derived from the unicellular, and inasmuch as their germ-plasm is
of so stable a nature that it can never be altered by any agencies
internal or external to the organisms presenting it, it follows that
all congenital variations are the remote consequences of aboriginal
differences on the part of unicellular ancestors. And, lastly, it
follows also that these congenital variations—although now so entirely
independent of external conditions of life, and even of activities
internal to organisms themselves—were originally and exclusively
due to the direct action of such conditions on the lives of their
unicellular ancestors; while even at the present day no one congenital
variation can arise which is not ultimately due to differences
impressed upon the protoplasmic substance of the germinal elements,
when the parts of which these are now composed constituted integral
parts of the protozoa, which were directly and differentially affected
by their converse with their several environments.
Again, if for the sake of distinctness we neglect all these
far-reaching deductions from his theory of _heredity_ whereby Weismann
constructs this elaborate theory of _organic evolution_, and fasten
our attention only upon the former, we may briefly summarize the
fundamental difference between his theory of _heredity_ and Darwin’s
theory of _heredity_ thus.
Darwin’s theory of heredity is the theory of _Pangenesis_: it
supposes that _all_ parts of the organism _generate_ anew in every
individual the formative material which, when collected together in the
germ-cells, constitutes the potentiality of a new organism; and that
this new organism, when developed, resembles its parents simply because
_all_ the formative material in each of the parents has been thus
_generated_ by, and collected from, _all_ parts of their respective
bodies. Weismann’s theory of heredity, on the other hand, is the
theory of the _Continuity of Germ-plasm_: it supposes that _no_ part
of the parent organism generates _any_ of the formative material which
is to constitute the new organism; but that, on the contrary, this
material stands to all the rest of the body in much the same relation
as a parasite to its host, showing a life independent of the body,
save in so far as the body supplies to it appropriate lodgement and
nutrition; that in each generation a small portion of this substance is
told off to develop a new body to lodge and nourish the ever-growing
and never-dying germ-plasm—this new body, therefore, resembling its
so-called parent body simply because it has been developed from one and
the same mass of formative material; and, lastly, that this formative
material, or germ-plasm, has been continuous through all generations of
successively perishing bodies, which therefore stand to it in much the
same relation as annual shoots to a perennial stem: the shoots resemble
one another simply because they are all grown from one and the same
stock.
CHAPTER II.
LATER ADDITIONS TO WEISMANN’S SYSTEM UP TO THE YEAR 1892.
I HAVE now furnished as complete a _résumé_ as seems desirable for
present purposes of Weismann’s theory of germ-plasm, considered both
as a theory of heredity and as a sequent theory of organic evolution.
But before proceeding to examine this elaborate system as a whole, I
must devote another chapter to a further statement of certain later
additions to—and also emendations of—the system as it was originally
propounded. These additions and alterations have reference only to
the theory of heredity: they do not affect the theory of organic
evolution as originally deduced therefrom. Moreover they have all been
due to our more recently acquired knowledge touching the morphology
and physiology of cell-nuclei: it is for the purpose of bringing his
theory of germ-plasm into accord with these results of later researches
that Weismann has thus modified the theory as it originally stood.
For my own part, I do not see that very much is gained by these
newer additions and modifications; but, be this as it may, they are
certainly very complicated, and on this account I have thought it best
to devote a separate chapter to their consideration. Furthermore,
not only in the opinion of Weismann himself, but also in that both of
his friends and foes, the main question with which his later essays
are concerned—viz., as to whether the nucleus of a cell is the only
part of a cell which is concerned in the phenomena of heredity—is
regarded as of fundamental importance to his entire edifice. Hence,
although I cannot myself perceive that the indisputable importance of
this question to any speculations on the subject of heredity is of
such special or vital significance to Weismann’s theory, it becomes
necessary for me to supply this further chapter for the purpose of
presenting the further developments of his theory.
First of all, Weismann has of late years considerably modified his
original view touching the relation of germ-cells to body-cells.
For while he originally supposed the fundamental distinction in
kind to obtain as between the whole contents of a germ-cell and the
whole contents of a somatic-cell, he now regards this distinction as
obtaining only between the nucleus of a germ-cell and the nucleus of
a somatic-cell. In other words, he regards the whole of a germ-cell,
with the exception of its nucleus, as resembling the whole of any other
cell, with the exception of _its_ nucleus. It is the nucleus of a
germ-cell alone that contains germ-plasm: all the rest of such a cell
being “nutritive, but not formative.”
This transference of the peculiar or hereditary powers of a germ-cell
from the cell as a whole to the nucleus, necessitates certain
emendations of the original theory of germ-plasm. In particular,
the broad distinction between the whole contents of a germ-cell
as “germ-plasm,” and the whole contents of a somatic-cell as
“somato-plasm,” is now discarded; and in its stead we have all nuclear
matter (whether of germ-cells or somatic-cells) comprised under the one
denomination of “nucleo-plasm,” in contradistinction to all the other
protoplasm of a cell, which is called “cytoplasm.” Hence Weismann now
regards the cytoplasm of a germ-cell as identical with the cytoplasm
of all other cells. Its function is merely that of “nourishing” the
nucleus, while, on the other hand, it is “controlled” by the nucleus as
to its own growth, shape, size, and eventual division.
But it is evident that the nucleo-plasm of a germ-cell must differ from
the nucleo-plasm of a somatic-cell, in that it not only “controls” the
growth, &c. of its own cell, but likewise presents all the additional
characters peculiar to a germ-cell. That is to say, the nucleo-plasm
of a germ-cell resembles the nucleo-plasm of a somatic-cell in that
it is nourished by, and exercises control over, the cytoplasm of
its own particular cell; but it differs from the nucleo-plasm of a
somatic-cell in admitting of fertilization, in the capability of
reproducing an entire organism, in the endowing of that organism with
all its hereditary characters, and, lastly, in providing for its own
reproduction in the next generation.
Thus it is evident, as Weismann puts it, that the nucleo-plasm of
a germ-cell must be of _two kinds_—one being concerned with the
formation and control of the germ-cell only, while the other has
to do with the construction of an entire future organism, and the
subsequent reproduction thereof. But not only so; for at each stage
in the construction of this future organism, all the somatic-cells,
as successively constructed, must likewise contain nucleo-plasm in
two kinds—one having to do only with the formation and control of its
own individual cell, and the other having to do with the formation
of the future somatic-cells, which will have to follow in the course
of ontogeny. Therefore, in order to designate this second kind of
nucleo-plasm (whether in a germ-cell or a somatic-cell) Weismann
borrows from Nägeli the term “idio-plasm[6],” or rather, I should say,
he uses the term “nucleo-plasm” when he is speaking of all the contents
of a nucleus indiscriminately, while he uses the term “idio-plasm” when
he has occasion to speak specially of the two kinds of nucleo-plasm now
before us.
Hence, the nuclear contents (nucleo-plasm) of every cell, whether
germinal or somatic, present two substances, which we may, in the
absence of any better terms supplied by Weismann himself, respectively
designate “idio-plasm-A” and “idio-plasm-B.” Idio-plasm-A is the
substance which has to do only with the formation and control of the
individual cell in which it resides, like a mollusc in its shell.
Idio-plasm-B is the substance out of which future cells are to be
formed and controlled, when in due course either of ontogeny or
phylogeny this idio-plasm-B becomes converted into idio-plasm-A,—i.e.,
into each subsequently developing tissue or organism, as the case may
be. I say ontogeny or phylogeny, and tissue or organism, because, where
a _germ-cell_ is concerned, idio-plasm-B is capable of reproducing
entire organisms of its own and of subsequent generations; whereas,
in the case of all _somatic_-cells, idio-plasm-B is capable only
of reproducing, stage by stage, some greater or less number of the
cells which are to construct the single organism of which they form a
part. Or, otherwise expressed, in the particular case of a germ-cell
idio-plasm-B is germ-plasm, and therefore is alone capable of producing
an entire organism of somatic-cells, while it is likewise alone capable
of reproducing successive organisms; for it alone contains the carriers
of heredity[7].
Thus, idio-plasm-B of an unsegmented germ-nucleus is germ-plasm. But
as soon as the germ-nucleus has undergone its first nuclear division,
its nucleo-plasm is no longer germ-plasm, inasmuch as each of the
half-portions is now no longer capable of reproducing an entire
organism—unless it be in the case of identical twins. Similarly in
the second nuclear division, each of the four resulting idio-plasms-B
is still further removed from the pristine character of germ-plasm;
and so on through all successive stages of segmentation. Hence
these successive nuclear divisions must indicate a partitioning and
re-partitioning of the original idio-plasm-B (germ-plasm) into the
idio-plasms-B severally distinctive of all the various cells of the
soma.
Now, it is evident that not _all_ the idio-plasm-B of a germ-cell which
thus passes over into the nuclei of somatic-cells can be represented
by the idio-plasm-B of those cells. At every stage of successive
cell-formation a certain part of the original idio-plasm-B of the
germ-cell must become the idio-plasm-A of somatic-cells distinctive
of that stage. For, supposing that at its differentiation stage 99
the original germ-plasm (now somatic-idio-plasm-B of 99th stage) has
reached a phase of ontogeny where the formation of tissue _m_ has
next to be followed by the formation of tissue _n_, then there still
remain the further differentiation stages 101, 102, 103, &c., to be
provided for, which, when their time arrives, will go to form the
still later tissues _o_, _p_, _q_, &c. Consequently the idio-plasm-B
of stage 100 cannot be _all_ consumed in making the tissue _n_. There
must be a residual portion which will afterwards be called upon to
form successively the idio-plasm-A of _o_, _p_, _q_, &c. Where, then,
is this residual portion of idio-plasm posited? Clearly it must be
posited in the nuclei of _n_. Thus it is that, as we began by stating,
all the nuclei of any given tissue _n_ really contain two kinds of
substance,—(1) their own idio-plasm-A, which was part of idio-plasm-B
of the preceding tissue, _m_; and (2) the idio-plasm-B, which is
destined to become idio-plasms-A of succeeding tissues _o_, _p_, _q_,
&c. Thus it follows also that the more the original idio-plasm-B is
differentiated into these successive formations of idio-plasms-A the
less of it remains for further differentiation, till, at the last
stage of ontogeny, all the original idio-plasm-B (germ-plasm) has
been thus changed into idio-plasms-A severally distinctive of all the
somatic-tissues _a_, _b_, _c_—_x_, _y_, _z_,—save only the portion
of it which has been carried through all these ontogenetic stages in a
wholly _un_differentiated condition, for the purpose of securing the
_phylogenetic_ production of the next generation. And this, of course,
is secured by the portion of undifferentiated germ-plasm in question
being deposited in the nuclei of germ-cells, at whatever stage of the
ontogeny these may be formed.
Finally, it is evident that, _at each stage_ of the differentiation of
idio-plasm-B into idio-plasms-A, the portion concerned must be capable
of self-multiplication to an almost incalculable extent,—yet this only
as idio-plasm-B of the particular kind required for constructing the
idio-plasm-A which is appropriate to the particular stage. Such is a
necessary deduction from the terms of Weismann’s theory, inasmuch as we
know that at each of the ontogenetic stages there is an incalculable
multiplication of cells belonging to that stage—cells, the “cytoplasm”
of which necessarily presupposes for its formation its own appropriate
idio-plasm in both kinds, and this in similarly increased quantities.
* * * * *
From the above theory it follows that an explanation can be given of
the healing of wounds (as in ourselves), of the regeneration of lost
parts (as the limb of a newt), or even of the reproduction of an entire
organism from a mere fragment of somatic-tissue (as in the cases
already alluded to at the commencement of this chapter—viz. the leaf
of Begonia, portions of sea-anemones, jelly-fish, &c.). For in all
these cases of repair, regeneration, and what may be called _somatic
reproduction_, we have only to suppose that not all the idio-plasm-B of
any given ontogenetic stage is consumed in the formation of that stage,
and therefore that the residue is passed on to the later stages _in a
latent condition_. It will then be available at any time to re-develop
tissue corresponding to that particular stage, should that particular
tissue happen to be lost by accident or disease. For example, if some
of the idio-plasm-B of the very first ontogenetic stage, or true
germ-plasm, should thus be passed on in an undifferentiated condition
through the somatic-tissues subsequently formed at later ontogenetic
stages, then we can understand why an _entire_ organism is reproduced
from a fragment of these tissues—or of those among which particles of
such residual and undifferentiated germ-plasm happen to be scattered.
Similarly, if idio-plasm-B of the ontogenetic stage at which a limb is
formed be not all consumed in constructing the limb, then the limb, if
afterwards lost, will be reconstructed, although an entire organism
will not be reproduced from a fragment of somatic-tissue. And similarly
also with the mere repair of injuries, where the only overplus of
idio-plasm-B is that of idio-plasm-B belonging to the very last stages
of ontogeny.
But, it is almost needless to observe, this kind of transmission of
idio-plasm-B from one stage of ontogeny in an unaltered condition
to subsequent stages, is not to be confused with the other kind of
transmission previously referred to, whereby idio-plasm-B of one
stage becomes successively transformed into the idio-plasms-A of
successive stages. In the former case, at whatever stage of ontogeny
the transmission may start from, the idio-plasm-B from that stage lies
dormant, and is never destined to undergo further differentiation,
unless the results of accident or disease should call upon it to do
so. In the latter case, on the other hand, the idio-plasm-B of any
given stage is passed on to the next stage for the express purpose of
transforming itself into the idio-plasms-A of that and, in due order,
of all subsequent stages.
* * * * *
It will be observed that all this elaboration of the original theory
of germ-plasm—an elaboration which is largely derived from the
speculative writings of Nägeli—serves no other purpose than that of
indicating what Professor Weismann now regards as the most probable
_mode_ in which germ-plasm undergoes its modification into the various
kinds of somatic-cells. For, inasmuch as the idio-plasms-B of all
somatic-cells are originally derived from that of the germ-cell,
and inasmuch as each expends its formative energies exclusively in
constructing and controlling the cells which, as idio-plasms-A, they
respectively inhabit, it is still the germ-plasm of the original
germ-cell that is finally converted into the various tissues which
together constitute the soma—notwithstanding that, in order thus to
become transmuted into body-substance, or somato-plasm,it must pass
through the sundry intermediate stages of idio-plasm-B, idio-plasm-A,
and cytoplasm, of any given ontogenetic stage. Hence I do not see that
it makes any substantial difference to Weismann’s theory of heredity,
whether we speak of germ-plasm being converted into “somato-plasm,”
or into “idio-plasm” _plus_ “somatic-idio-plasm,” _plus_ “cytoplasm.”
But as Weismann himself thinks that it does make some great difference
whether we adhere to his original generic term “somato-plasm,” or adopt
his newer and more specific terms as just enumerated, I append _in
extenso_ the most recent exposition of his views upon this subject[8].
Before quitting this somewhat complicated addition to the original
theory of germ-plasm, I must briefly allude to the descriptions and
illustrations of karyokinesis which were given in Part I of _Darwin and
after Darwin_, for the prospective benefit of any general readers who
might afterwards be sufficiently interested in Weismann’s speculations
to desire a statement of the main facts on which this further
development of his theory rests. It seemed undesirable to burden the
present volume with an account of recent investigations so well known
to naturalists, while, on the other hand, it was clearly desirable
that such an account should be given somewhere, if the speculations in
question were to be rendered intelligible to anybody else. Therefore I
must here request those of my readers who are not already acquainted
with the matter to consult pp. 128-134 of Part I. It will there be
seen how enormously complex are the visible processes which take
place in the nucleus of a germ-cell (and likewise of a somatic-cell),
preparatory to its division; and therefore, supposing that the nucleus
alone contains the material concerned in the phenomena of heredity,
it appears that no small corroboration is lent to Weismann’s views by
these histological observations. And, more particularly, if we suppose
with him that the material in question is restricted to that portion of
the segregating nuclear matter which is called the “nuclear thread[9],”
in the formation of the “loops” or “rods” of this substance we seem to
have presented a visible expression of the marshalling of “the carriers
of heredity,” and the successive passage of the originally generalized
“germ-plasm” of the germ-cell into the ever more and more specialized
“nucleo-plasms” of the somatic-cells. Indeed, the new theory of
heredity, when thus brought into relation with the new results of
histological observation, appears so well to fit the latter, that one
would be sorry to find the coincidence unmeaning, or the theory false.
But, without passing any criticism, it is sufficient to note that the
question whether or not the theory is true—and therefore correctly
interprets the phenomena of karyokinesis,—must depend chiefly on
whether it be eventually proved that the “nuclear thread” is indeed
the only part of a germ-cell, or even the only part of a tissue-cell,
which is concerned in controlling the phenomena of heredity on the one
hand, and of ontogeny on the other. Into this question, however, I do
not propose to enter. It will be enough to assume, for the sake of
argument, that Weismann’s view of the matter will eventually prove to
be true. At the same time, we must remember that at present this view
as to the nuclear thread being the sole repository of the material of
heredity is merely hypothetical.
* * * * *
We now arrive at the last of those features in Weismann’s theory of
heredity, the importance of which necessitates mention in such a mere
statement of the theory as the present chapter is concerned with.
According to Weismann’s own view of his theory, two objections have to
be met. In the first place, there is the objection that all individuals
_which are born of the same parents_ are not exactly alike, as the
theory might have expected they would be, seeing that the admixture of
identical germ-plasms has been concerned in the formation of the whole
progeny. In the second place, and quite apart from this objection,
there is the difficulty that, if every act of fertilization essentially
consists in a fusion of one mass of germ-plasm belonging to a male
germ-cell with another mass belonging to a female germ-cell, at each
generation the mass of germ-plasm contained in an egg-cell must be
doubled—with the result that ova must progressively increase in size
during the course of phylogeny. But ova do not thus progressively
increase in size. Therefore, if the imperishable nature of germ-plasm
is to be theoretically sustained, it is necessary to show some means
whereby ova and spermatozoa are able to get rid of at least one half of
their respective germ-plasms in each generation—i.e., before each act
of impregnation. Weismann meets both these difficulties by an appeal to
the following facts.
It is well known that the ripe ovum extrudes two minute particles
of protoplasmic substance, which are called polar bodies[10]. These
both proceed from the nucleus of the ovum, but are not formed
simultaneously. For the first polar body is really one half of the
original nucleus of the cell, and therefore is formed by the first
segmentation of this nucleus. The second polar body, on the other
hand, is one half of the remaining nucleus, and is similarly formed
by the second segmentation. Hence, when both polar bodies have been
extruded from the ovum, only one quarter of the original nuclear matter
remains. So far, of course, the facts prove too much for Weismann’s
theory, because the theory wants to get rid of only one half of the
original nuclear matter before impregnation, _if all the nuclear matter
be germ-plasm_. Therefore Weismann concludes that all the original
nuclear matter of the ripe ovum is not germ-plasm, but that only one
half of it is so, while the other half—or that half which goes to
constitute the first polar body—is idio-plasm-A, which, as we have
already seen, the egg-cell shares in common with all other cells. It
is merely “ovogenetic”: its function is that of constructing the ovum,
_quâ_ cell: it has nothing whatever to do with the germ-plasm which the
particular cell contains. Therefore, having discharged its function of
constructing this cell, it is itself discharged from the cell as the
first polar body.
The nucleus of the fully-formed ovum having thus got rid of all
its superfluous idio-plasm-A by throwing off the first polar body,
is supposed henceforth to consist of pure germ-plasm (i.e., of
idio-plasm-B belonging to the first ontogenetic stage), and one half
of this is next got rid of by the second segmentation in the form of
the second polar body. Therefore, according to the theory and so far as
the problems of heredity are concerned, we need not any further trouble
ourselves about the first polar body. But it will at once be seen that
by the interpretation which Weismann puts upon the second polar body,
and also, of course, upon the extrusion of some of its nuclear matter
by the male cell, he meets both the difficulties against his theory of
germ-plasm which we are now engaged in considering.
That he thus meets the second of those difficulties—i.e., concerning
the otherwise perpetual accumulation of germ-plasm—is evident without
explanation. That he likewise meets the first—i.e., concerning the
non-resemblance of individuals born of the same parents—is scarcely
less evident. For it is hardly conceivable that such a complex mass
of germ-plasms as the nucleus of a fertilized ovum must be could ever
present in any two eggs precisely the same proportional representation
of the “carriers of heredity,” after one half of each set had been thus
discharged from each egg. Therefore, if the second polar body removes
from each egg one half of the ancestral germ-plasms, “every egg will
contain a somewhat different combination of hereditary tendencies, and
thus the offspring which arise from the different germ-cells of the
same mother can never be identical[11].
Such, then, is Weismann’s theory of the physiological meaning of
polar bodies. And as the bearing of this particular theory on his
more general theory of heredity does not appear to me a vitally
intimate one, I think my subsequent examination of the main theory
will be simplified if I now proceed at once to an examination of the
subordinate one. For by doing this I shall hope to show that the
bearings just mentioned are of much less importance than he represents
them to be; and, therefore, that we may hereafter proceed to consider
his theory of heredity without any special reference to his theory of
polar bodies.
To begin with, as regards the first polar body, one would like to
know more clearly why it is necessary that this residuum of merely
“ovogenetic idio-plasm” (or idio-plasm-A of the egg-cell) has to be got
rid of before the germ-plasm can proceed to discharge its physiological
functions. Seeing that both these (hypothetically) very different
materials occur in the self-same nucleus, some very delicate mechanism
must be needed for their separation; and it is not apparent why such a
mechanism should have been evolved, rather than what would have been
the simpler plan of adapting the germ-plasm to hold its own against the
idio-plasm-A, even if one could see that any interference between these
very different substances is in any way probable. For my own part, at
all events, I cannot see why this microscopical atom of “ovogenetic
idio-plasm” should not simply be left to be absorbed among the millions
of cells that afterwards go to form the foetus.
Again, as regards the second polar body, Weismann’s theory of it is
framed to explain, (_a_) how the excess of germ-plasm is got rid of
in each ontogeny, and (_b_) why the offspring of the same parents
do not all precisely resemble one another. These, be it observed,
are the only two functions which Weismann’s theory of polar bodies
subserves in relation to his theory of germ-plasm. But, it appears
to me, neither of these functions is necessary, in so far as any
requirements of the latter theory are concerned. For surely, polar
bodies or no polar bodies, there is already a mechanism at work in
each ontogeny which is of itself sufficient to discharge both these
functions, and so to anticipate both the supposed difficulties which
the subsidiary theory is adduced to meet. The very essence of ontogeny,
as a process, itself consists in a continuous succession of nuclear
divisions—and this not only as regards somatic-cells, but also as
regards germ-cells. Now, in the great majority of organisms, there is
an infinitely greater number of germ-cells (both male and female) than
can possibly be required either for the purpose of getting rid of any
excess of germ-plasms in the nucleus of each cell, or of preventing
the germ-plasms of any one germ-cell precisely resembling those of any
other. If every plant or animal produced only a single female-cell or a
single male-cell, then indeed we might require from Professor Weismann
a demonstration of some special mechanism to secure the expulsion of
half its ancestral germ-plasms; since otherwise the single female-cell
or male-cell would have to increase its dimensions in each successive
generation. But, as matters actually stand, nature seems to have made
much more than ample provision for preventing the undue accumulation
of ancestral germ-plasms in any individual germ-cell, by enormously
multiplying; through continuous division and subdivision, the _number_
of germ-cells in each ontogeny. And similarly, of course, as regards
the different aggregations of ancestral germ-plasms which are left
for distribution among these innumerable germ-cells. “If one group
of ancestral germ-plasms is expelled from one egg, and a different
group from another egg, it follows that no two eggs can be exactly
alike as regards their contained hereditary tendencies.” Granted; but
this consideration applies equally to the original segregation of
germ-plasms in the multiplying eggs of each ontogeny—for it follows
from the theory of germ-plasm that the most primitive egg-cell in each
ontogeny must have contained all the ancestral germ-plasms which are
afterwards distributed among its innumerable progeny of egg-cells.
And, as far as the facts of “individual variation” are concerned, I do
not see why the differential partitioning of “ancestral idio-plasms”
should be any better secured by nuclear division of a mature germ-cell
than by that of an immature. Less so, indeed; for the wonder is that
during the many-thousand-fold division of an immature ovum so precise
a distribution of these “ancestral idio-plasms” is maintained, as is
proved to be maintained (on the theory of germ-plasm) by the facts of
heredity.
However, Weismann takes a widely different view of the matter. For
while he allows that “such an early reducing division would offer
advantages in that nothing would be lost, for both the daughter
nuclei would (? might) become eggs, instead of one of them being lost
as a polar body”—while he allows this, he nevertheless rejects the
possibility of “such an early reducing division.” But I do not see that
the reasons which he assigns for this rejection of it are adequate.
First, he says that if this were the way in which the superfluous
germ-plasm of each generation were got rid of, _far too much_ provision
has been made for the purpose,—seeing that the practically indefinite
number of nuclear divisions which the immature germ-cells undergo
would cause a much “greater decrease of the ancestral idio-plasms
of each than could afterwards be compensated by the increase due
to fertilization.” But this rejoinder is of cogency only if it be
supposed that at each nuclear division of an immature ovum, “the
ancestral idio-plasms” (germ-plasm) are incapable of the power of
self-multiplication which soon afterwards becomes one of its most
essential characters. Why, then, should we suppose this substance to be
totally incapable of increase in the multiplying ova of ontogeny, when
at the same time we are to suppose the same substance capable of any
amount of increase in the multiplying ova of phylogeny? To this obvious
question no answer is supplied: in fact the question is not put.
Secondly, Weismann says that in parthenogenetic ova only one polar body
is extruded. This he regards as equivalent to the first polar body of a
fertilizable ovum (i.e., as composed of ovogenetic nuclear substance);
and hence he argues that the second polar body of a fertilizable ovum
must be regarded as composed of germ-plasm. But even supposing that he
is right as to the fact that parthenogenetic ova invariably extrude but
one polar body, his argument from this fact would only be available
after we had already accepted his view touching the character of the
second polar body. So long as this view is itself the subject of
debate, he cannot prove it by the fact in question. In other words,
unless we have already agreed that the second polar body has the
function which Weismann assigns to it, we cannot accept the fact which
he adduces as furnishing any evidence of his view touching the function
of the second polar body.
For these reasons I cannot see that the subordinate theory of
polar bodies is required in the interests of the general theory of
germ-plasm. The difficulties which it is adduced to meet do not appear
to me to be any difficulties at all. Therefore, in now proceeding
to consider what in my opinion are the real difficulties which lie
against the major theory of germ-plasm, I shall not again allude to the
minor—and, in this connexion, superfluous—theory of polar bodies.
Such, then, is Professor Weismann’s theory of heredity in its original
and strictly logical form. In the course of our examination of it
which is to follow in Chapter III and IV, we shall find that in almost
every one of its essential features, as above stated, the theory has
had to undergo—or is demonstrably destined to undergo—some radical
modification. But I have thought it best to begin by presenting the
whole theory in its completely connected state, as it is in this
way alone that we shall be able to disconnect what I regard as the
untenable parts from the parts which still remain for investigation at
the hands of biological science. Such, indeed, is the only object of
my “Examination of Weismannism.” For, rightly or wrongly, it appears
to me that the unquestionable value of his elaborate speculations is
seriously discounted by certain oversights with regard to matters of
fact, and not a few inconsistencies touching matters of theory. In
displaying both these defects, I am not without hope that the result
may be that of inducing Professor Weismann so to modify his system of
theories as to strengthen the whole by removing its weaker parts.
CHAPTER III.
WEISMANN’S THEORY OF HEREDITY (1891).
WE now proceed to examine Weismann’s theory of germ-plasm, and as this
in its various developments has now become a highly complex theory, we
had best begin by marking out the lines on which the examination will
be conducted.
As I have already pointed out, the Weismannian system is not concerned
only with the physiology of reproduction: it is concerned also—and
in an even larger measure—with the doctrine of descent. The theory
of germ-plasm as a whole is very much more than a theory of heredity;
it is a new theory of evolution. The latter, indeed, is deduced from
the former; but although the two are thus intimately related, they
are nevertheless not mutually dependent. For the relationship is such
that the new theory of evolution stands upon the basis supplied by
the new theory of heredity, and although it follows from this that
if the latter were disproved the former would collapse, it does not
follow that if the former were to be found untenable the latter must
necessarily be negatived. Hence, for the sake of clearness, and also
for the sake of doing justice to both theories, we had best deal
with them separately. The present chapter, then, will be devoted to
examining Weismann’s theory of heredity, while the ensuing chapter will
be concerned with his sequent theory of evolution.
Again, Weismann’s theory of heredity stands on his fundamental
postulate—the continuity of germ-plasm; and also on a fact well
recognized by all other theories of heredity, which he calls the
stability of germ-plasm. But his sequent theory of evolution stands
not only on this fundamental postulate, and on this well-recognized
fact; it requires for its logical basis two further postulates—viz.,
that germ-plasm has been _perpetually_ continuous “since the first
origin of life,” and _unalterably_ stable “since the first origin of
sexual propagation.” That these things are so, a very few words will be
sufficient to prove.
Any theory of heredity which supposes the material of heredity to
occupy a more or less separate “sphere” of its own, is not obliged
further to suppose that this material has _always_ been thus isolated,
or even that it is now _invariably_ so. There have been one or two
such theories prior to Weismann’s, and they were founded on the
well-known fact of congenital characters being at any rate _much more_
heritable than are acquired characters. But it has not been needful
for these theories to assume that the “continuity” thus postulated
has been _perpetually_ unbroken. Even if it has been frequently to
some extent interrupted, all the facts of _heredity_ could be equally
well comprised under such theories—and this even if it be supposed
that acquired characters are but rarely, or never, transmitted to
progeny. For, in as far as the continuity may have been interrupted,
it does not follow that the acquired characters (body-changes),
which by hypothesis caused the interruption, must be inherited
by progeny exactly as they occurred in the parents. Or, in other
words and adopting Weismann’s terminology, _so far as the facts of
heredity are concerned_, there is no reason why germ-plasm should not
frequently have had its hereditary qualities modified by some greater
or less degree of commerce with somatic-tissues, and yet never have
reproduced in progeny the identical acquired characters which caused
the modification of germ-plasm in the parents: some other and totally
different characters might with equal—or even more—likelihood have
been the result, as we shall see more clearly a few pages further on.
Why, then, does Weismann so insist upon this continuity of germ-plasm
as _perpetual_ “since the origin of life”? It appears to me that his
only reason for doing so is to provide a basis, not for his theory
of heredity, but for his additional theory of evolution. It is of
no consequence to the former that germ-plasm should be regarded as
thus perpetual, while it is of high importance to the latter that the
fundamental postulate of continuity should be supplemented by this
further postulate of the continuity as thus perpetual.
Similarly as regards the postulate of the stability of germ-plasm as
absolute. It is enough for all the requirements of Weismann’s theory of
heredity that the material basis of heredity should present a merely
_high degree_ of stability, such as the facts of atavism, degeneration,
&c. abundantly prove that it possesses. For his sequent theory of
evolution, however, it is necessary to postulate this stability as
_absolute_ “since the first origin of sexual reproduction.” Otherwise
there would be no foundation for any of the distinctive doctrines
which go to constitute this theory.
It may not be immediately apparent that Weismann’s theory of heredity
is not _per se_ concerned with either of these two additional
postulates of the continuity of germ-plasm as _perpetual_, and the
stability of germ-plasm as _absolute_; while both are logically
necessary to his further theory of evolution. On this account, and
also for the sake of clearness in all that is to follow, we had best
begin by comparing his theory of heredity with those of his principal
predecessors—Darwin and Galton.
* * * * *
For the purposes of this comparison we may start by again alluding to
the fact, that even in the multicellular organisms reproduction is not
confined to the sexual methods. Many kinds of invertebrated animals
will reproduce entire organisms from the fragments into which a single
organism has been chopped: plants of various kinds can be propagated
indefinitely by cuttings, grafts, and buds, or even by leaves, as we
have already observed in Chapter I. Now, when the whole organism is
thus reproduced from a severed portion of somatic-tissue, it reproduces
its sexual elements. Whence, then, in such cases are these elements
derived? Obviously they are not derived immediately from the sexual
organs—or even from the sexual cells—of their parents: they are
derived from the somatic-cells of a single parent, if we choose to
retain this term; and therefore, as Strasburger pointed out soon after
Weismann’s theory was published, it seems as if such facts are in
themselves destructive of the theory. How, then, does Weismann meet
them? As we have already seen in Chapter II, he meets them in the
only way they can be met on the lines of his theory—viz., by those
newer amendments of his theory which suppose that in all these cases
the germ-plasm is _not_ confined to the specially sexual cells, but
occurs also in the nuclear substance of those somatic-cells which thus
prove themselves capable of developing into entire organisms. In other
words, the sexual elements which develop during what I have previously
called this “somatic reproduction” of multicellular organism, are
supposed to be derived from the sexual cells of ancestors, not indeed
immediately (for this they plainly are not), but mediately through
the somatic-tissues of their a-sexual parent. Now, in view of this
extension, the theory of germ-plasm becomes somewhat closely allied
to that of pangenesis. For example, when the fragment of a leaf of
_Begonia_ is laid upon moist soil, there strikes root, and grows a new
_Begonia_ plant capable of sexual reproduction, Darwin supposes the
explanation to be that what he calls “formative material” occurs in all
cells of the leaf, while Weismann supposes the explanation to be that
what he calls “germ-plasm “ occurs in all—or at any rate in most—of
the cells of the leaf. So that, except as regards the terms employed,
the two theories are identical in their mode of viewing this particular
class of phenomena.
Moreover by thus allowing, in his second essay on Heredity, that
germ-plasm need not be restricted to the specially sexual cells, but
in some cases, at any rate[12], may occur distributed in full measure
of reproductive efficiency throughout the general tissues of the
organism, Weismann cannot refrain from taking the further step of
supposing that the germ-plasm, like the gemmules of Darwin, is capable
of any amount of multiplication _in the general cellular tissues of
plants_—seeing that plants can be propagated by cuttings, buds, &c.,
indefinitely. And this, as we have seen, Professor Weismann, in his
second essay, does not shrink from doing. Moreover, although I cannot
remember that he has anywhere expressly said so, it is obvious that the
allied phenomena of regeneration and repair admit of explanation by his
hypothesis of “ontogenetic grades,” after the manner already stated
in Chapter II. Indeed, it is evident that in no other way can these
phenomena be brought within the range of his theory. But from this
it follows that not only in the case of organisms which are capable
of somatic reproduction is the formative nucleo-plasm (idio-plasm-B)
diffused throughout the somatic-tissues: on the contrary, it must be
_universally_ diffused throughout _all_ the somatic-cells of _all_
living organisms; and whether as it there occurs it is capable of
reproducing entire organisms, single organs, single tissues, or a mere
cicatrix, depends only on the “ontogenetic grade” of differentiation
which this diffused nucleo-plasm has (or has not) previously undergone.
Moreover, as we have already seen, at whatever ontogenetic grade of
differentiation it may be present in a given somatic-tissue, it must
there be capable of indefinite self-multiplication. Therefore, in all
these respects this “formative nucleo-plasm” (or idio-plasm-B) of
Weismann precisely resembles the “formative material” (or gemmules) of
Darwin.
Lastly, as De Vries has pointed out[13], there must be at least as
many divisions and subdivisions in the substance of germ-plasm, as
there are differences between the somatic organs, tissues, and even
cells, to which germ-plasm eventually gives rise—no matter through
how many ontogenetic grades of idio-plasm it may first have to pass.
Or, in other words, we must accept, as the material basis of heredity,
ultimate particles[14] of germ-plasm, which are already differentiated
into as many diverse categories as there are differences between all
the constituent parts of the resulting soma; for, as shown in the
Appendix, no change in the facts of the case has been shown by simply
changing the original term “germ-plasm” into “idio-plasm,” wherever
the phenomena of ontogeny are concerned. It may be convenient, for
the sake of presenting newer additions to the theory, to restrict the
term “germ-plasm” to “idio-plasm of the first ontogenetic stage”; but
as idio-plasms of all subsequent ontogenetic stages are supposed to
be ultimately derived from this idio-plasm of the first stage, it is
evident that the particulate differences in question must already have
been present in the so-called “undifferentiated idio-plasm of the first
ontogenetic stage.” Unless we are to have a mere juggling with words,
we cannot put into our successive idio-plasms any particles of kinds
differing from those which are contained in the original germ-plasm.
Therefore I say that, notwithstanding this change of terminology,
Weismann must continue to assume, as the material basis of heredity,
ultimate particles of germ-plasm which are already differentiated into
as many diverse categories as there are differences between the parts
of the resulting soma—although, of course, these ultimate particles
need not be nearly so numerous _in each of their categories_ as they
afterwards become by self-multiplication while forming each of the
resulting tissues.
But this is precisely what the theory of pangenesis supposes; so that
I see no reason why these ultimate particles of germ-plasm should
not be regarded as “gemmules,” so far as their _size_, _number_,
and _function_ are concerned. In point of fact, they differ from
gemmules only in respect to their _origin_: they are not particles
derived from somatic-cells of the preceding generation, but particles
derived from germ-plasm of the preceding generation. Or, to state
the difference in another form, if we regard the sexual elements as
constituting the physiological centre of the organism, then the theory
of germ-plasm supposes these ultimate carriers of heredity to originate
at this centre, and then to travel centrifugally; while the theory of
pangenesis supposes them to originate at the periphery, and then to
travel centripetally.
This point of difference, however, arises from the deeper ones,
which—having now exhausted the points of agreement—we must next
proceed to state.
If, as we have seen, “formative material” and “germ-plasm” agree in
being particulate; in constituting the material basis of heredity; in
being mainly lodged in highly specialized, or germinal, cells; in being
nevertheless also distributed throughout the general cellular tissues,
where they are alike concerned in all processes of regeneration,
repair, and a-sexual reproduction; in having an enormously complex
structure, so that every constituent part of the future organism is
already represented in them by corresponding particles; in being
everywhere capable of a virtually unlimited multiplication, without
ever losing their hereditary endowments; in often carrying these
endowments in a dormant state through a number of generations, until
at last they reappear again in what we recognize as reversions to
ancestral characters;—if in all these most important respects the
two substances are supposed to be alike, it may well appear at first
sight that there is not much room left for any difference between them.
And, in point of fact, the only difference that does obtain between
them admits of being stated in two words,—Continuity, and Stability.
Nevertheless, although thus so few in number, these two points of
difference are points of great importance, as I will now proceed
briefly to show.
If the substance which constitutes the material basis of heredity
has been _perpetually continuous_, in the sense of never having
had any of its hereditary endowments in any way affected by the
general body-tissues in which it resides, the following important
consequences, it will be remembered, arise. The process of organic
evolution must have been exclusively due to a natural selection of
favourable variations occurring within the limits of this substance
itself; and therefore the so-called Lamarckian factors can never have
played any part at all in the evolution of any but the unicellular
organisms. On the other hand, if this substance has not been thus
perpetually continuous, but more or less formed anew at each ontogeny
by the general body-tissues in which it resides, natural selection has
probably been in some corresponding degree assisted in its work of
organic evolution by the Lamarckian factors, with the result that the
experiences of parents count for something in the congenital endowments
of their offspring. So much for the first of the two differences
between germ-plasm and gemmules, or the difference which arises from
the perpetual continuity of germ-plasm.
Touching the second difference, or that which arises from the
_absolute stability_ of germ-plasm, it will be remembered how from
this character there arises another important chain of consequences.
Namely, individual variations of the congenital kind can only be due
to admixtures of different masses of germ-plasm in every act of sexual
fertilization; natural selection is therefore dependent, for the
possibility of its working, upon the sexual methods of propagation;
hence, natural selection is without any jurisdiction among the
unicellular organisms, where the Lamarckian factors hold exclusive
sway; and hence, also, the multicellular organisms are ultimately
dependent upon this absolute stability of their germ-plasm for all the
progress which they have made in the past, as well as for any progress
which they may be destined to make in the future.
Thus we see that the two points of difference between germ-plasm and
gemmules are not merely of great importance as regards the particular
problem which is presented by the phenomena of heredity: they are of
still greater importance as regards the general theory of evolution.
For if these two qualities of perpetual continuity and absolute
stability can be proved to belong to the material basis of heredity,
the entire theory of evolution will have to be reconstructed from its
very foundation—and this quite apart from the more special question
as to the transmission of acquired characters. Therefore we shall
presently have to consider these two alleged qualities with the care
that they demand, as having been seriously suggested by so eminent a
naturalist as Professor Weismann. But, before proceeding to do so, I
must briefly compare his theory with that of Mr. Galton.
“Stirp” resembles both “germ-plasm” and “gemmules” in all the respects
which have above been named as common to the two latter. But it differs
from gemmules and further resembles germ-plasm in all the following
particulars. It is derived from the stirp of proceeding generations,
and constitutes the sole basis of heredity. Only a part of it, however,
is consumed in each ontogeny—the residue being handed over to
“contribute to form the stirps of the offspring,” where it undergoes
self-multiplication at the expense of the nutriment supplied to it
from the somatic system of the offspring, and so on through successive
generations. Again, stirp is concerned in all processes of regeneration
and repair, in the same centrifugal manner as germ-plasm is so
concerned. Furthermore, the influence of sexual propagation in the
blending of hereditary qualities of the stirp is recognized, while the
principle of panmixia, or the cessation of selection, is entertained,
and shown to invalidate the evidence of pangenesis which Darwin
derived from the apparently transmitted effects of use and disuse in
our domesticated animals[15]. Lastly, it is clearly stated that on
the basis supplied by this “theory of heredity,” it becomes logically
possible to dispense with the Lamarckian principles _in toto_, leaving
natural selection as the sole known cause of organic evolution through
a perpetual continuity of stirp, together with individual variations of
the same, whether by sexual admixture or otherwise.
So far, then, there is not merely resemblance, but virtual identity,
between the theories of stirp and germ-plasm. Disregarding certain
speculative details, the coincidence is as complete as that between a
die and its impress. But although the two theories are thus similar
in _logical construction_, they differ in their interpretations
of _biological fact_. That is to say, although Galton anticipated
by some ten years all the main features of Weismann’s theory of
heredity[16], and showed that, as a matter of form, it was logically
intact, he refrained from concluding on this account that it must be
the true theory of heredity. He argued, indeed, that in the main it
was probably the true theory; but he guarded his presentation of it
by not undertaking to deny that there might still be some degree of
intercommunication between the material basis of heredity in stirp,
and the somatic tissues of successive organisms. The construction
of a theory which, as a matter of theory, could dispense with the
Lamarckian principles _in toto_, was seen to be a very different
thing from proving, as a matter of fact, that these principles are
non-existent—and this, even though it was seen that a recognition of
the principle of panmixia must be taken to have considerably attenuated
the _degree_ of their operation as previously estimated by Darwin
in the theory of pangenesis. In short, after pointing out that the
doctrine of stirp might very well adopt the position which about a
decade later was adopted by the doctrine of germ-plasm—namely, that
of altogether _supplanting_ the doctrine of gemmules,—Galton allowed
that this could be done only as a matter of formal speculation; and
that, as a matter of real interpretation of the facts of nature, it
seemed more judicious to stop at _modifying_ the doctrine of gemmules,
by provisionally retaining the hypothesis of gemmules, but assigning to
their agency a greatly subordinate _rôle_. Or to quote his own words:—
The conclusion to be drawn from the foregoing arguments is, that we
might almost reserve our belief that the structural [i. e., “somatic”]
cells can react on the sexual elements at all, and we may be confident
that at the most they do so in a very faint degree; in other words,
that acquired modifications are barely, if at all, inherited, in
the correct sense of that word. If they were not heritable, then
the second group of cases [i.e., those of acquired as distinguished
from congenital characters] would vanish, and we should be absolved
from all further trouble; if they exist, in however faint a degree,
a complete theory of heredity must account for them. I propose, as
already stated, to accept the supposition of their being faintly
heritable, and to account for them by a modification of Pangenesis[17].
Seeing, then, that Galton did not undertake to deny a possibly slight
influence of somatic-tissues on the hereditary qualities of stirp, it
follows that he did not have to proceed to those drastic modifications
of the general theory of descent which Weismann has attempted. Stirp,
like germ-plasm, is _continuous_; but, unlike germ-plasm, it is not
_necessarily_ or _absolutely_ so. Again, stirp, like germ-plasm,
is _stable_; yet, unlike germ-plasm, it is not _perpetually_ or
_unalterably_ so. Hence we hear nothing from Galton about our having to
explain the unlikeness of our children to ourselves by variations in
our protozoan ancestors; nor do we meet with any of those other immense
reaches of deductive speculation which, in my opinion, merely disfigure
the republication of stirp under the name of germ-plasm.
Now, I allude to these, the only important points of difference between
stirp and germ-plasm, for the sake of drawing prominent attention to
the fact that it makes a literally immeasurable difference whether we
suppose the material basis of heredity to be _perpetually_ continuous
and _unalterably_ stable, or whether we suppose that it is but
_largely_ continuous and _highly_ stable. In the former case, all the
far-reaching deductions which Weismann draws with reference to the
general theory of descent—or apart from the more special problem of
heredity—follow by way of logical consequence. In the latter case,
there is no justification for any such deductions. For, no matter how
faintly or how fitfully the hereditary qualities of the material in
question may be modified by the somatic-tissues in which it resides,
or by the external conditions of life to which it is exposed, these
disturbances of its absolute stability, and these interruptions of
its perpetual continuity, must cause more or less frequent changes on
the part of its hereditary qualities—with the result that specific
or other modifications of organic types need not have been solely due
to the varying admixture of such material in sexual unions on the one
hand, or to the unassisted power of natural selection on the other.
Numberless additional causes of individual variation are admitted,
while the Lamarckian principles are still allowed some degree of
play. And although this is a lower degree than Darwin supposed, their
influence in determining the course of organic evolution may still have
been enormous; seeing that their action, in whatever measure it may be
supposed to obtain, must always have been _cumulative_ on the one hand,
and _directive_ of variations in adaptive lines on the other. Or, as
Galton himself observes, in the passage already quoted, “if they exist,
_in however faint a degree_, a complete theory of heredity must account
for them.” He saw, indeed, that a most inviting _logical_ system could
be framed by denying that they can ever exist in any degree—or, in
other words, by supposing that stirp was _exactly_ the same as what was
afterwards called germ-plasm, in that it always occupied a separate
“sphere” of its own, where its continuity has been uninterrupted “since
the first origin of life.” But Galton was not seduced by the temptation
to construct an ideally logical system; and he had what I regard as the
sound judgement to abstain from carrying his theory of stirp into any
such transcendental “sphere” as that which is occupied by Weismann’s
theory of germ-plasm, in relation to the general doctrine of descent.
* * * * *
There is, then, a vast distinction between any theory of heredity
which postulates the material of heredity as highly stable and largely
continuous, and Weismann’s theory, which postulates this material as
absolutely stable and perpetually continuous. But we must next take
notice that Weismann himself has not kept this distinction in view
with the constancy which we should have expected from so forcible a
thinker. On the contrary, although in the construction of his theory
of evolution he never fails to press the postulates of _absolute_
stability and _perpetual_ continuity to their logical conclusions in
the various doctrines above enumerated (pp. 57-58), when engaged on
his more special theory of heredity he every now and then appears to
lose sight of the distinction. Indeed, he occasionally makes such large
concessions with regard to both these postulates, that, were they to be
entertained, the occupation of his critics would be gone: his theory
of heredity would become converted into Galton’s, while his theory of
evolution would vanish altogether. It is therefore necessary to quote
some of these concessions, if only to justify ourselves in subsequently
ignoring them. I will give one instance of each; but it is necessary to
preface the illustrations with a few words to mark emphatically three
very distinct cases of congenital variation—leaving aside for the
present the question whether or not they all occur in fact, as they are
held to do by one or other of the theories of heredity.
1. The case where impoverished nutrition of the body has the effect
of simply _starving_ its germinal material. This is not a case
where either the continuity or the stability of such material is
affected. Its full efficiency as “formative material” may indeed be
thus deteriorated to any extent, so that the progeny may be to any
extent puny or malformed; but this will not necessarily cause any
such re-shuffling of its “molecules” as will thereafter result in
a permanent phylogenetic change. At most it will affect only the
immediate offspring of poorly nourished parents; and natural selection
will always be ready to eliminate such inefficient individuals. This
case I will always hereafter call the case of _nutritive_ congenital
changes.
2. The case where germinal material is influenced by causes which
do effect a re-shuffling of its “molecules,” so that a permanent
phylogenetic change does result. Observe, in this case, it does not
signify whether the causes arise from external conditions of life, from
any action of the soma on its own germinal material, or from so-called
“spontaneous” changes on the part of such material itself. But the
one cause which has not been concerned in producing an hereditary
modification of this class is the mixture of “germ-plasms” in an act
of sexual union. In hereafter speaking of this case I will follow
Weismann’s terminology, and call congenital changes thus produced
_specialized_ congenital changes.
3. Lastly, we have the case of the Lamarckian factors. This precisely
resembles case 2, save that the congenital changes produced are still
more “specialized.” For while in the preceding case the re-shuffling
before mentioned may have produced a congenital change of any kind,
in the present case the congenital change produced must be of one
particular kind—viz., a reproduction by heredity of the very same
modification which occurred in the parents. “The fathers have eaten
sour grapes, and the children’s teeth are set on edge.” This would
be an extreme example of “use-inheritance,” and so of case 3. But
if the fathers had eaten sour grapes, and the children, instead of
having their teeth set on edge, were to be born with a wryneck or a
squint, then we should have a good example of case 2. In order, then,
to mark the important distinction between these two cases, I will
hereafter call the highly specialized changes due to the Lamarckian
factors—supposing such changes to be possible—_representative_
congenital changes.
These several distinctions being understood, I will proceed to furnish
the two quotations from Weismann, which are respectively illustrative
of his concessions touching his two fundamental postulates, as
previously explained.
We may fairly attribute to the adult organism influences which
determine the phyletic development of its descendants. For the
germ-cells are contained in the organism, and the external
influences which affect them are intimately connected with the state
of the organism in which they lie hid. If it be well nourished, the
germ-cells will have abundant nutriment; and, conversely, if it be
weak and sickly, the germ-cells will be arrested in their growth. It
is even possible that the effects of these influences may be more
specialized; that is to say, they may act only upon certain parts
of the germ-cells. But this is indeed very different from believing
that the changes of the organism which result from external stimuli
can be transmitted to the germ-cells, and will re-develop in the next
generation at the same time as that at which they arose in the parent,
and in the same part of the organism[18].
It will be perceived that Weismann himself here very clearly draws
all the distinctions between cases 1, 2, and 3, as above explained.
Therefore it becomes the more remarkable that he should not have
perceived how radically inconsistent it is in him thus to entertain
as “possible” congenital variations belonging to the case 2. For, as
we have now so fully seen, the theory of germ-plasm (as distinguished
from that of stirp) cannot entertain the possibility of an hereditary
and specialized change of _any_ kind as thus produced by external
conditions of life: should such a possibility be entertained, there
must obviously be an end to the _absolute stability_ of germ-plasm,
and a consequent collapse of Weismann’s theory of evolution. Either
germ-plasm is absolutely stable, or else it is but highly stable. If
it is absolutely stable, individual variations of an hereditary kind
can occur only as results of sexual admixtures of germ-plasm, and
Weismann’s theory of evolution is established. But if germ-plasm is
not absolutely stable (no matter in how high a degree it may be so)
hereditary individual variations may be produced by other causes, and
Weismann’s theory of evolution collapses. Therefore, if we are to
examine his theory of _evolution_, we can do so only by ignoring such a
passage as the one just quoted, which surrenders the postulate of the
_absolute stability_ of germ-plasm.
Again, if we are to examine Weismann’s theory of _heredity_, we must
similarly ignore such a passage as the following, where he represents
that he is similarly prepared to surrender his still more fundamental
postulate of the _perpetual continuity_ of germ-plasm.
After remarking that some of his own experiments on the climatic
varieties of certain butterflies raise such difficulties against his
whole theory of heredity that even now “he cannot explain the facts
otherwise than by supposing the passive acquisition of characters
produced by the direct influence of climate,” he goes on to remark more
generally—“We cannot exclude the possibility of such a transmission
occasionally occurring, for, even if the greater part of the effects
must be attributed to natural selection, there might be a smaller part
in certain cases which depends on this exceptional factor[19]”—i.e.,
the Lamarckian factor!
Now, it must be particularly noted that in this passage Weismann is
speaking, not as in the previous passage, of _specialized_ congenital
characters, but of _representative_ congenital characters. In other
words, he here entertains the possibility which in the passage
previously quoted he very properly rejects—namely, “that changes of
the organism which result from external stimuli can be transmitted to
the germ-cells, and will re-develop in the next generation _at the same
time as that at which they arose in the parent, and in the same part
of the organism_.” But it is evident that if the theory of germ-plasm
is undermined by the concession made in the passage thus previously
quoted, in the passage last quoted a match is put to the fuse. It
does not signify whether the particular case of the butterflies in
question will ever admit of any other explanation more in accordance
with the theory of germ-plasm: the point is that in _no_ case can this
theory entertain the possibility of causes other than admixtures of
germ-plasm in sexual unions producing hereditary changes, (A) of _any_
kind, (B) still less of a _specialized_ kind, and (C) least of all of
a _representative_ kind. For the distinguishing essence of this theory
is, that germ-plasm must always have moved, so to speak, in a closed
orbit of its own: its “sphere” must have been perpetually distinct from
those of whatever other “plasms” there may be in the constellations of
living things. So that, in such passages as those just quoted, Weismann
is not only destroying the very foundations of his general theory of
evolution, but at the same time he is identifying his more special
theory of heredity with those which had been already published by his
predecessors, and more particularly by Galton. Now, it is not Galton’s
theory that we are considering; and therefore we must hereafter
ignore those fundamental admissions, whereby Weismann every now and
again appears ready to relinquish all that is most distinctive of, or
original in, his own elaborate system of theories.
It is, indeed, impossible not to admire the candour of these
admissions, or to avoid recognizing the truly scientific spirit which
they betoken. But, at the same time, one is led to doubt whether in
making them Professor Weismann has sufficiently considered their full
import. He appears to deem it of comparatively little importance
whether or not acquired characters can sometimes and in some degrees
influence the hereditary qualities of germ-plasm, provided he can
show that _much the larger_ part of the phenomena of heredity must be
ascribed to the continuity of germ-plasm. In other words, he seems
to think that it matters but little whether in the course of organic
evolution the Lamarckian factors have played but a very subordinate
part, or whether they have not played any part at all. Moreover, I have
heard one or two prominent followers of Weismann give public expression
to the same opinion. Therefore I must repeat that it makes a literally
immeasurable difference whether we suppose, with Galton, that the
Lamarckian factors may sometimes and in some degrees assert themselves,
or whether we suppose, with the great bulk of Weismann’s writings and
in accordance with the logical requirements of _his_ theory, that they
can never possibly occur in any degree. The distinctive postulate
of his theory of heredity, and one of the two fundamental doctrines
on which he founds his further theory of evolution, is, that the
physiology of sexual reproduction cannot admit of any inversion of the
relations between “germ-plasm” and “somatic idio-plasm[20].” This is a
perfectly intelligible postulate, but it is not one with which we may
play fast and loose. Either there is such a physiological mechanism
as it announces, in which case the relations in question can never
be inverted “occasionally,” any more than rags may “occasionally”
help to construct the mill which is to form them into paper;—or else
there is no such mechanism, in which case we may have to do with
gemmules, physiological units, stirp, micellae, pangenes, plastidules,
or any of the other hypothetical “carriers of heredity” to which our
predilections may happen to incline; but the one substance with which
we certainly have not to do is germ-plasm[21].
* * * * *
After these tedious but necessary preambles, we may now proceed
to examine Professor Weismann’s postulate as to the perpetual
continuity of germ-plasm, with its superstructure in his theory of
heredity—reserving for the next chapter our examination of his further
postulate touching the absolute stability of germ-plasm, with its
superstructure in his theory of evolution.
The evidence which Weismann has presented in favour of his fundamental
postulate of the perpetual continuity of germ-plasm may be conveniently
dealt with under two heads—namely, indirect evidence as derived from
general reasoning, and direct evidence derived from particular facts.
The general reasoning is directed to show, (1) that there is no
evidence of the transmission of acquired characters; (2) that the
theory of pangenesis is “inconceivable”; and, (3) that the alternative
theory of germ-plasm is amply conceivable. Now, to the best of my
judgement, not one of these propositions is borne out by the general
reasoning in question. But as the latter is almost entirely of an _a
priori_ character, and also of a somewhat abstruse construction, I
think the patience of any ordinary reader will be saved by relegating
this part of our subject to an Appendix. Therefore, remarking only
that any one who cares to look at Appendix I ought, in my opinion, to
perceive that there is no real evidence against the transmission of
acquired characters to be derived from Weismann’s general reasoning in
this connexion, I will at once proceed to consider the evidence which
he has adduced in the way of particular facts.
* * * * *
In the first place, as one result of his brilliant researches on the
_Hydromedusae_, he has found that the generative cells occur only in
certain localized situations, which, however, vary greatly in different
species, though they are always constant for the same species. He
has also found that the varying situations in different species of
the localized or generative areas correspond, place for place, with
successive stages in a process of gradual transposition which has
occurred in the phylogeny of the _Hydromedusae_. Lastly, he has found
that in each ontogeny these successive stages of transposition are
repeated, with the result that during the individual lifetime of
one of these animals the germ-cells migrate through the body, from
what used to be their ancestral situation to what is now the normal
situation for that particular species. Such being the facts, Weismann
argues from them that the germ-cells of the _Hydromedusae_ are thus
proved to present properties of a peculiar kind, which cannot be
supplied by any of the other cells of the organism; for, if they could,
whence the necessity for this migration of these particular cells? Of
course it follows that these peculiar properties must depend on the
presence of some peculiar substance, and that this is none other than
the “germ-plasm,” which here exhibits a demonstrable “continuity”
throughout the entire phylogeny of these unquestionably very ancient
Metazoa.
The second line of direct evidence in favour of the continuity of
germ-plasm which Weismann has adduced is, that in the case of some
invertebrated animals the sexual apparatus is demonstrably separated
as reproductive cells (or cells which afterwards give rise to the
reproductive glands) at a very early period of ontogeny—so early
indeed, in certain cases, that this separation constitutes actually
the first stage in the process of ontogeny. Therefore, it is argued,
we may regard it as antecedently improbable that the after-life of the
individual can in any way affect the congenital endowments of its ova,
seeing that the ova have been thus from the first anatomically isolated
from all the other tissues of the organism.
The third and only other line of direct evidence is, that organisms
which have been produced parthenogenetically, or without admixture of
germ-plasms in any previous act of sexual fertilization, do not exhibit
congenital variations.
Taking, then, these three lines of verification separately, none of
them need detain us long. For although the fact of the migration of
germ-cells becomes one of great interest in relation to Weismann’s
theory _after the theory has been accepted_, the fact in itself does
not furnish any evidence in support of the theory. In the first
place, it tends equally well to support Galton’s theory of stirp;
and therefore does not lend any special countenance to the theory of
germ-plasm—or the theory that there cannot now be, and never can
have been, any communication at all between the plasm of the germ and
that of the soma. In the second place, the fact of such migration is
not incompatible even with the theory of pangenesis, or the theory
which supposes such a communication to be extremely intimate. There
may be many other reasons for this migration of germ-cells besides
the one which Weismann’s theory supposes. For example, the principle
of physiological economy may very well have determined that it is
better to continue for reproductive purposes the use of cells which
have already been specialized and set apart for the execution of those
purposes, than to discard these cells and transform others into a kind
fitted to replace them. Even the theory of pangenesis requires to
assume a very high degree of specialization on the part of germ-cells;
and as it is the fact of such specialization alone which is proved
by Weismann’s observations, I do not see that it constitutes any
criterion between his theory of heredity and that of Darwin—still
less, of course, between his theory and that of Galton. Lastly, in this
connexion we ought to remember that the _Hydromedusae_ are organisms
in which the specialization in question happens to be least, as is
shown by the fact that entire individuals admit of being reproduced
from fragments of somatic-tissues; so that these are organisms where we
would least expect to meet with the migration of germ-cells, were the
purpose of such migration that which Weismann suggests. This line of
evidence therefore seems valueless.
Nor does it appear to me that the second line of evidence is of any
more value. In the first place, there is no shadow of a reason for
supposing that an apparently anatomical isolation of germ-cells
necessarily entails a physiological isolation as regards their special
function—all “physiological analogy,” indeed, being opposed to such
a view, as is shown in Appendix I. In the second place, there is no
proof of any anatomical isolation, as we may likewise see in that
Appendix. In the third place, the fact relied upon to indicate such
an isolation—viz., the early formation of germ-cells—is not a fact
of any general occurrence. On the contrary, it obtains only in a
comparatively small number of animals, while it does not obtain in any
plants. In the Vertebrates, for example, the reproductive cells are
not differentiated from the somatic cells till after the embryo has
been fully formed; while in plants their development constitutes the
very last stage of ontogeny. In the fourth place, the argument, even
for what it is worth, is purely deductive; and deductive reasoning in
such a case as this—where the phenomena are enormously complex and
our ignorance unusually profound—is always precarious. Lastly, in the
fifth place, Weismann has now himself abandoned this argument. For in
one of his later essays he says:—
Those instances of early separation of sexual from somatic cells,
upon which I have often insisted as indicating the continuity of the
germ-plasm, do not now appear to be of such conclusive importance as
at the time when we were not sure about the localization of the plasm
in the nuclei. In the great majority of cases the germ-cells are not
separated at the beginning of embryonic development, but only in some
of the later stages.... It therefore follows that cases of early
separation of the germ-cells afford no proof of a direct persistence
of the parent germ-cells in those of the offspring.
The last line of direct evidence, or that derived from the alleged
non-variability of parthenogenetic organisms, is, as Professor Vines
has shown, opposed to fact. Therefore, in his later writings, Weismann
has abandoned this line of evidence also.
Upon the whole,then, we must conclude with regard to the fundamental
postulate of perpetual continuity, that there is actually no evidence
of a direct kind in its favour. And, as Weismann’s arguments of an
indirect kind are dealt with in Appendix I, it remains only to state
such evidence _per contra_ as, to the best of my judgement, appears
valid.
* * * * *
The fundamental proposition which we have been considering, and to
the further consideration of which we have now to proceed, is, in
effect, that germ-plasm differs from stirp in having been _perpetually_
restricted to a “sphere” of its own, “_since the first origin of
life_.” Criticism, therefore, must be directed to show that the
“sphere” in question has not been proved so entirely independent as
this fundamental proposition sets forth; but that, on the contrary,
there appears to be a certain amount of reciprocal action between this
sphere and that of the somatic-tissues—even though we may agree (as I
myself agree) with Galton in holding that the degree of such reciprocal
action is neither so intimate nor so constant as it was held to be
by Darwin. This, indeed, is the direction which the course of our
criticism has taken already. For it has just been shown that Weismann
has failed to adduce any facts (preceding text) or considerations
(Appendix I) in support of his fundamental proposition as above stated,
save such as proceed on a prior acceptance of the proposition itself.
The facts and considerations which he has adduced are therefore
useless as evidence in support of this proposition, although they
would admit of being explained by it supposing it to have been already
substantiated by any facts or considerations of an independent kind.
Which is merely another way of saying, as already said, that there is
no evidence in favour of the proposition.
But I am now about to argue that there _is_ evidence _against_ the
proposition. For I am about to argue, not only as heretofore that for
anything Weismann has shown to the contrary there _may_ be a certain
amount of reciprocal action between the sphere of germinal-substance
and the sphere of body-substance; but that, as a matter of fact, there
_is_ a certain amount of such reciprocal action.
Without laying undue stress on the intimate “correlation” that subsists
between the reproductive organs and all other parts of the organism,
I nevertheless think that the fact ought here to be noted. For the
changes which occur at puberty and after the reproductive functions
have ceased, as well as those which may be artificially produced by
castration, &c., prove at any rate some extremely important association
between the soma as a whole and its reproductive apparatus as a whole.
No doubt it may properly enough be answered that this proof does not
extend to the vital point of showing the association to be between
the soma as a whole, and that particular part of the reproductive
apparatus in which the “carriers of heredity” reside—namely, the ova
and spermatozoa; and, therefore, that the facts in question may be
due only to some changed conditions of nutrition on the part of the
somatic-tissues which these alterations on the part of the reproductive
glands entail. On this account we must fully allow that the facts in
question are not in themselves of any conclusive weight; but I think
they are worth mentioning, because they certainly seem to countenance
the theory which supposes some reciprocal influence as exercised by
the germinal elements on the somatic-tissues and _vice versa_, rather
than they do the theory which supposes the germinal elements and the
somatic-tissues to have always occupied totally different “spheres.”
Here, however, is a stronger class of facts. It has not unfrequently
been observed, at any rate in mammals, that when a female has borne
progeny to a male of one variety, and subsequently bears progeny to a
male of another variety, the younger progeny presents a more or less
unmistakable resemblance to the father of the older one. Now, this
is a fact to which Weismann has nowhere alluded; and therefore I do
not know how he would meet it. But, as far as I can see, it can be
explained only in one or other of two ways. Either there must be some
action of the spermatic element on the hitherto unripe ovum, or else
this element must exercise some influence on the somatic-tissues of
the female, which in their turn act upon the ovum[22]. Now, I do not
deny that the first of these possibilities might be reconcilable with
the hypothesis of an absolute continuity of germ-plasm; for it is
conceivable that the life of germ-plasm is not coterminous with that
of the spermatozoa which convey it, and hence that, if the carriers of
heredity, after the disintegration of their containing spermatozoa,
should ever penetrate an unripe ovum, the germ-plasm thus introduced
might remain dormant in the ovum until the latter becomes mature, and
is then fertilized by another sire. In this way it is conceivable that
the hitherto dormant germ-plasm of the previous sire might exercise
some influence on the progeny of a subsequent one. But it seems clear
that the second of the two possibilities above named could not be thus
brought within the hypothesis of an absolute continuity of germ-plasm.
Therefore it seems that the school of Weismann must adopt the first,
to the exclusion of the second. Unfortunately for them, however, there
is another (and clearly analogous) fact, which goes to exclude the
first possibility, and most definitely to substantiate the second. For,
in the case of plants, where there can be no second progeny borne by
the same “ovary,” but where we happen to be able to see that a marked
effect is sometimes produced on the somatic-tissues of the mother by
the pollen of the father, there can be no question as to the male
element being able to exercise a direct influence on the soma of the
female. Consequently, whatever we may think with regard to the case of
animals, the facts with regard to plants are in themselves enough to
sustain the only position with which we are concerned—viz., that the
male element is capable of directly modifying the female soma.
The facts with regard to plants are these. When one variety fertilizes
the ovules of another, not unfrequently the influence extends beyond
the ovules to the ovarium, and even to the calyx and flower-stalk, of
the mother plant. This influence, which may affect the shape, size,
colour, and texture of the somatic-tissues of the mother, has been
observed in a large number of plants belonging to many different
orders. The details of the matter have already been dealt with by
Darwin, in the eleventh chapter of his work on _Variation_, &c.; and
this is what he says. The italics are mine.
The proofs of the action of foreign pollen on the mother-plant
have been given in considerable detail, because this action is of
the highest theoretical importance, and because it is in itself
a remarkable and apparently anomalous circumstance. That it is
remarkable under a physiological point of view is clear, for the male
element not only affects, in accordance with its proper function, the
germ, but at the same time various parts of the mother-plant, _in the
same manner as it affects the same parts in the seminal offspring
from the same two parents_. We thus learn that an ovule is not
indispensable for the reception of the influence of the male element.
Darwin then proceeds to show that this direct action of the male
element on the somatic tissues of another organism is not so rare or
anomalous as it at first sight appears; for in the case of not a few
flowers it comes into play as a needful preliminary to fertilization.
Thus, for instance:—
Gärtner gradually increased the number of pollen grains until he
succeeded in fertilizing a Malva, and has proved that many grains
are first expended in the development, or, as he expresses it, in
the satiation, of the pistil and ovarium. Again, when one plant
is fertilized by a widely distinct species, it often happens that
the ovarium is fully and quickly developed without any seeds being
formed; or the coats of the seeds are formed without any embryo being
developed therein.
So much, then, in proof of the direct action of the male element on the
somatic-tissues of another organism. It remains to show that a similar
action may be exercised by this element on the somatic tissues of its
own organism. This has been proved by Hildebrand, who found “that
in the normal fertilization of several Orchideae, the action of the
plant’s own pollen is necessary for the development of the ovarium; and
that this development takes place not only long before the pollen tubes
have reached the ovules, but even before the placentae and ovules have
been formed”; so that with these orchids the pollen acts directly on
their own ovaria, as a preliminary to the formation of the ovules which
are subsequently to be fertilized.
It is to be regretted that Professor Weismann has not given us his
opinion upon this whole class of facts, for assuredly they appear
directly to contradict his theory. The theory is, “that the germ-plasm
and the somato-plasm have always occupied different spheres”: the fact
is, that the germ-plasm may directly act upon the somato-plasm, both
within and beyond the limits of the same organism.
* * * * *
Hitherto we have been considering certain very definite facts, which
seem to prove that the germinal elements are able directly to affect
the somatic-tissues. We have next to consider such facts as seem to
prove the opposite side of a reciprocal relationship—viz., that the
somatic-tissues are able directly to affect the germinal elements.
And here there are two distinct lines of evidence to be distinguished.
Firstly, in certain cases—exceptional it is true, but this does not
signify—somatic-tissues have been found capable of modifying the
hereditary endowments of germinal elements by means of simple grafting.
This line of evidence has also been disregarded both by Weismann and
his followers; but it is nevertheless an important one to consider.
For, if it be the case that the somatic-tissues of an organism A,
by being merely grafted on-those of organism B, can so affect the
germinal elements of B as to cause their offspring to resemble A—or,
contrariwise, if the somatic-tissues of A can thus act on B—then,
although it may not be properly said that any “acquired characters”
have been transmitted from A to the progeny of B, (or _vice versa_,)
such an a-sexual transmission of alien characters, in its relation to
the theory of germ-plasm, is scarcely less awkward than are certain
facts which they appear to prove.
Secondly, that acquired characters may be transmitted to progeny by
the more ordinary methods of sexual propagation (Lamarckian factors).
This second line of evidence will be fully and independently dealt with
in future chapters, specially devoted to the subject. Therefore we have
here to consider only the first.
Now, the force of this first line of evidence will become apparent,
if we reflect that the only way in which the facts can be met by
Weismann’s theory, would be by supposing that the somatic germ-plasms
which are respectively diffused through the cellular tissues of the
scion and the graft become mixed in some such way as they might have
been, had the hybrid been due to seminal propagation instead of to
simple grafting. But against this, the only interpretation of the facts
which is open to the theory, there lies the following objection, which
to me appears insuperable.
Where sexual cells are concerned there is always a definite arrangement
to secure penetration of the one by the other, and we can see the
necessity for such an arrangement in order to effect an admixture
of their nuclear contents, where alone germ-plasm is supposed by
Weismann’s theory to reside. But in tissue-cells, which have not
been thus specialized, it would be difficult to believe that nuclear
contents can admit of being intimately fused by a mere apposition of
cell-walls. For not only are the nuclear contents of any two such cells
thus separated from one another by two cell-walls and two masses of
“cytoplasm”; but it is not enough to suppose that in order to produce
a graft-hybrid only two of these somatic-cells need mix their nuclear
contents, as we know is all that is required in order to produce
a seminal hybrid by means of sexual cells. On the contrary, in the
former case most, if not all, the somatic-cells which are brought into
apposition by the graft must be supposed thus to mix their nuclear
contents at the plane of the graft; for otherwise the hybrid would
not afterwards present equally the characters of stock and scion.
Now, there may be hundreds of thousands of such cells, and therefore
it seems impossible that the facts of graft-hybridization can be
reconciled with the theory of germ-plasms[23].
The third line of evidence against this theory—i.e., the evidence in
favour of the transmission of acquired characters—is to constitute the
subject-matter of future chapters. Therefore it will here be sufficient
to adduce only one fact of this kind. And I select it because it is one
that has been dealt with by Weismann himself. In one of his more recent
statements he says:—
The distinguished botanist De Vries has proved that certain
constituents of the cell body—e. g., the chromatophores of
Algae—pass directly from the maternal ovum to the daughter organism,
while the male germ-cells generally contain no chromatophores. Here it
appears possible that a transmission of somatogenetic variation has
occurred[24].
Now although, as Weismann goes on to observe, “in these lower plants,
the separation between somatic and reproductive cells is slight,”
in the facts to which he alludes we appear to have good evidence of
an influence exercised by somatic cells upon the germinal contents
of reproductive cells. And if such an influence is capable of being
exercised in the case of “these lower plants,” it follows that
there is no such _absolute_ separation between somatic tissues and
germ-plasm as Weismann’s theory requires. Moreover it follows that,
if the essential distinction between germ-plasm and somato-plasm (or
“somatic idio-plasm”) is thus violated at the very foundation of the
multicellular organisms, there ceases to be any _a priori_ reason for
drawing arbitrary limits, either as to the level of organization at
which such “transmission of somatogenetic variation has occurred,” or
as to the degree of detail into which it may extend. Both these matters
then stand to be tested by observation; and the burden of proof lies
with the school of Weismann to show at what level of organization,
and at what degree of representation, somatogenetic changes cease to
reproduce themselves by heredity.
Passing on, then, to higher levels of organization, and therefore to
higher degrees of representation, I shall endeavour to show that this
burden of proof cannot be discharged. For I shall endeavour to show,
not merely, as just shown, that there ceases to be any _a priori_
reason for drawing arbitrary limits with respect either to levels of
organization or to degrees of representation, but that, as a matter of
fact, there are no such limits as the passage above quoted assigns.
On the contrary, I believe there is as good evidence to prove the not
unfrequent transmission of acquired (“somatogenetic”) characters
among the higher plants—and even among the higher animals—as there
is of the occurrence of this phenomenon in the case of the Alga just
mentioned. But in order to do this evidence justice, I shall have to
take a new point of departure and consider as a separate question
the transmissibility of acquired characters. Meanwhile, and as far
as Weismann’s theory of heredity is concerned, it is enough to have
shown,—if I have been successful in doing so,—that not only is
there no evidence to sustain his fundamental postulate touching the
material of heredity having always occupied a separate “sphere” of
its own “since the first origin of life”; but that there is good
evidence to prove the contrary. For whether or not the reciprocal
action of “somato-plasm” and “germ-plasm” can ever proceed to the
extent of causing acquired characters to be inherited (so as to produce
“representative congenital changes”), all that is distinctive in this
theory must be regarded as barren speculation, unless it can be shown
that the foregoing facts have failed to prove such a reciprocal action
as ever occurring in any lower degree (so as to produce “specialized
congenital changes”).
CHAPTER IV.
EXAMINATION OF WEISMANN’S THEORY OF EVOLUTION (1891).
HAVING now considered germ-plasm as perpetually continuous, we have
next to regard it as unalterably stable.
First, let it be noted that these two fundamental and distinctive
postulates of the whole Weismannian system are so intimately connected
as to be in large measure mutually dependent. For, on the one hand, if
germ-plasm has not been perpetually continuous since the first origin
of life, it cannot have been absolutely stable “since the first origin
of sexual propagation”: every time that its hereditary characters are
modified by its containing soma (whether or not representatively so),
its stability has been so far upset. On the other hand, if germ-plasm
has not been absolutely stable, it cannot have been perpetually
continuous “since the first origin of life.” As often as its stability
has been upset, its “molecular structure” has been modified by causes
_ab extra_, as distinguished from mixtures of germ-plasms in sexual
unions. Therefore, it can no longer have been continuous in the sense
of having borne an ineffaceable record of all congenital variations,
_due to sexual unions_, throughout the entire phylogeny of the
Metaphyta and Metazoa. At most it can have been continuous only in the
attenuated sense, that however much and however often its hereditary
characters may have been modified by somatic changes on the one hand or
by changes in the external conditions of life on the other, they can
never have been thus modified _representatively_, as supposed by the
theory of pangenesis.
From which it follows that, while examining in our last chapter
Weismann’s doctrine of the perpetual continuity of germ-plasm, we
have been indirectly examining also his companion doctrine of the
unalterable stability of germ-plasm. Nevertheless, for the sake of
doing justice to both these doctrines, I have thought it desirable to
examine each on its own merits, without prejudice arising from our
criticism of the other. To such a separate and independent examination
of the doctrine of unalterable stability we will, therefore, now
proceed.
* * * * *
As we have already and repeatedly seen, this doctrine of the
unalterable or absolute stability of germ-plasm “since the first origin
of sexual propagation” is a logically essential part of Weismann’s
theory of evolution, or of his system of hypotheses considered as a
whole. It is so because upon this doctrine depends his reference of
individual variations in the Metazoa to an ultimate origin in the
Protozoa, the significance of sexual reproduction in the theory of
natural selection, &c., &c. Therefore this doctrine of the absolute
stability of germ-plasm is enunciated by Weismann, not merely for the
purpose of meeting any one class of facts, such as those of atavism,
persistence of rudimentary organs, &c. The doctrine is enunciated for
the purpose of constituting one of the foundation-stones of his general
theory of evolution. We have now to consider how far the quality of
this stone renders it trustworthy as a basis to build upon.
In the first place, we can scarcely fail to perceive that this doctrine
of the absolute stability of germ-plasm is not only gratuitous, but
intrinsically improbable. That the most complex material in nature
should likewise be the most stable is opposed to all the analogies of
nature, and therefore to all the probabilities of the case.
Again, the germ-plasm, as it originally occurred (and still exists) in
unicellular organisms, is supposed to be exactly the same _kind_ of
material as now occurs in the germ-cells of multicellular organisms.
Yet the very same theory which supposes so absolute a stability on the
part of germ-plasm when located in germ-cells (or diffused through
somatic-cells), likewise supposes so high a degree of variability on
the part of germ-plasm when not thus located, as to represent that all
individual variations which have ever taken place in the unicellular
organisms—and all the innumerable species of such organisms which
have arisen therefrom—have been due to the direct action of external
conditions of life; or, in other words, to the _instability_ of
germ-plasm. The very same substance which at one time and in one place
is supposed to be so absolutely unchangeable, at another time and in
another place is supposed to be highly susceptible of change.
Lastly—and this is, perhaps, the most curious part of the whole
matter—the place where germ-plasm is supposed to be unchangeable is
not the place where it is most likely to be so, but the place where
it is least likely. For germ-plasm as it occurs in the germ-cells of
multicellular organisms must have a constitution greatly more complex
even than that which it has in unicellular organisms—seeing that in
the former case, and by hypothesis, it bears a living record of the
whole phylogeny of the Metaphyta and Metazoa in all their innumerable
branchings. And not only so, but when germ-plasm occurs in germ-cells
it becomes exposed to much greater vicissitudes: its environment has
become vastly more complex, as well as greatly more liable to change
with the changing conditions of life of the many mutable species in
which it resides, and on the individual somas of which it now depends
for its nourishment. So that, altogether, we have here on merely
_a priori_ grounds about as strong a case against this doctrine of
absolute stability as it is well conceivable that on merely _a priori_
grounds a case can be.
Turning next to arguments _a posteriori_, let us begin by considering
those which Weismann has adduced in support of the doctrine.
First, he alleges that there is a total absence of variability on the
part of all organisms which have been produced parthenogenetically, or
from unfertilized ova. We may look in vain, he says, for any individual
differences on the part of any multicellular organisms, which have been
brought into existence independently of the blending of germ-plasms in
a previous act of sexual union. Now, unquestionably, if this statement
could be corroborated by sufficiently extensive observation, the fact
would become one of immense significance—so much so, indeed, that of
itself it would go far to neutralize all antecedent objections, and
to verify his theory as to sexual propagation being the sole cause
of congenital variation. But seeing that the alleged fact stands so
entirely out of analogy with the phenomena of bud-variation (which will
be alluded to later on), it is highly improbable, even on antecedent
grounds; while Professor Vines has refuted the statement on grounds of
actual fact. Thus, speaking of the _Basidiomycetes_, he says—
These Fungi are not only entirely a-sexual, but it would appear that
they have been evolved in a purely a-sexual manner from a-sexual
ascomycetous or æcidiomycetous ancestors. The Basidiomycetes, in fact,
afford an example of a vast family of plants, of the most varied form
and habit, including hundreds of genera and species, in which, so far
as minute and long-continued investigation has shown, there is not,
and probably never has been, any trace of a sexual process[25].
Here, then, we have actual proof of “hereditary individual variations”
among a-sexually propagating organisms, sufficient in amount to
have given origin, not merely to “individual differences,” but to
innumerable species, and even genera. Consequently Weismann allows
that the criticism abolishes this line of evidence in favour of the
absolute stability of germ-plasm[26]. Consquently, also, we must now
add, in whatever measure the alleged fact would have corroborated
the theory had it been proved to be a fact, in that measure is the
theory discredited by proof that it is not a fact. For, if the theory
were sound, this particular fact would certainly have admitted of
demonstration: therefore the proof that it is not a fact—but the
reverse of a fact—amounts at the same time to a disproof of the
theory[27].
The only other line of evidence to be adduced in favour of the absolute
stability of germ-plasm is that which is furnished by the high
antiquity of some specific types, by the facts of atavism, and by the
persistency of vestigial organs. But this line of evidence is as futile
as the other. Nobody has ever questioned that hereditary characters
are persistently stable as long as they are persistently maintained by
natural selection; and this, according to Weismann himself, must have
been the case with all long-enduring species: these, therefore, fail to
furnish any evidence of the _inherent_ stability of germ-plasm, which
is the only point in question.
Again, as regards the facts of atavism, nobody is disputing these
facts. What we are disputing is whether the _degree_ of inherent
stability which they unquestionably prove can be rationally regarded
as such that it may endure, not merely for such a comparatively small
number of generations as these facts imply, but actually for any
number of generations, or through the practically infinite series of
generations that now intervene between the higher metazoa and their
primeval parentage in the protozoa. Clearly, the ratio between these
two things is such that no argument derived from the facts of atavism
can be of any avail for the purposes of this Weismannian doctrine.
Lastly, as regards vestigial organs, the consideration that,
surprisingly persistent as they unquestionably are, nevertheless they
do eventually disappear, seems to prove that the power of heredity
does in time become exhausted, even in cases most favourable to
its continuance. That it should thus become finally exhausted is
no more than Darwin’s theory of perishable gemmules, or Galton’s
theory of a not absolutely stable stirp, would expect. But the fact
is irreconcilable with Weismann’s theory of an absolutely stable
germ-plasm.
Hence, we can only conclude that there is no evidence in favour of the
hypothesis that germ-plasm has been unalterably stable “since the first
origin of sexual propagation”; while the suggestion that it may have
been so is on antecedent grounds improbable, and on inductive grounds
untenable. It only remains to add that the _degree_ of stability has
been proved in not a few cases to be less than even the theory of
gemmules might anticipate. Many facts in proof of this statement might
be given, but it will here suffice to quote one, which I select because
it has been dealt with by Professor Weismann himself.
Professor Hoffmann has published an abstract of a research, which
consisted in subjecting plants with normal flowers to changed
conditions of life through a series of generations. In course of
time, certain well-marked variations appeared. Now, in some cases
such directly-produced variations were transmitted by seed from the
affected plants; and therefore Weismann acknowledges,—“I have no
doubt that the results are, at any rate in part, due to the operation
of heredity.” Hence, whether these results be due to the transmission
of somatogenetic characters (“representative changes”), or to the
direct action of changed conditions of life on the germ-plasm itself
(“specialized changes”), it is equally certain that the hereditary
characters of the plants were congenitally modified to a large extent,
within (at most) a few generations. In other words, it is certain that,
if there be such a material as germ-plasm, it has been proved in this
case to have been highly unstable. Therefore, in dealing with these
and other similar facts, Weismann himself can only save his postulate
of continuity by surrendering for the time being his postulate of
stability[28].
If to this it be replied that Hoffmann’s facts are exceptional—that
Gärtner, Nägeli, De Candolle, Peter, Jordan, and others, did not find
individual variations produced in plants by changed conditions of life
to be inherited,—the reply would be irrelevant. It does not require
to be proved that all variations produced by changed conditions of
life are inherited. If only some—even though it be but an extremely
small percentage—of such variations are proved to be inherited, the
many millions of years that separate the germ-plasm of to-day from its
supposed origin in the protozoa, must have furnished opportunities
enough for the occurrence of such variations to have obliterated, and
re-obliterated numberless times, any aboriginal differences in the
germ-plasms of incipiently sexual organisms. Moreover, it is probable
that when further experiments shall have been made in this direction,
Hoffmann’s results will be found not so exceptional as they at present
appear. Mr. Mivart, for example, has mentioned several instances[29];
while there are not a few facts of general knowledge—such as the
modifications undergone by certain Crustacea as a direct result of
increased salinity of the water in which they live—that will probably
soon be proved to be facts of the same order. But here attention
must be directed to another large body of facts, which are of high
importance in the present connexion.
The phenomena of what is called bud-variation in plants are phenomena
of not infrequent occurrence, and they consist in the sudden appearance
of a peculiarity on the part of a shoot which develops from a single
bud. When such a peculiarity arises, it admits of being propagated, not
only by cuttings and by other buds from that shoot, but sometimes also
by seeds which the flowers of the shoot subsequently produce—in which
case all the laws of inheritance that apply to congenital variations
are found to apply also to bud-variation. Or, as Darwin puts it, “there
is not any particular in which new characters arising by bud-variation
can be distinguished from those due to seminal variation”; and,
therefore, any theory which deals with the latter is bound also to take
cognizance of the former. Now, as far as I can find, there is only one
paragraph in which Weismann alludes to bud-variation, and what he there
says I do not find very easy to understand. Therefore I will quote the
whole paragraph _verbatim_.
I have not hitherto considered budding in relation to my theories,
but it is obvious that it is to be explained, from my point of view,
by supposing that the germ-plasm which passes on into a budding
individual consists not only of the unchanged germ-plasm of the
first ontogenetic stage, but of this substance altered, so far as
to correspond with the altered structure of the individual which
arises from it—viz., the rootless shoot which springs from the stem
or branches. The alteration must be very slight, and perhaps quite
insignificant, for it is possible that the differences between the
secondary shoots and the primary plant may depend chiefly on the
changed conditions of development, which takes place beneath the earth
in the latter case, and in the tissues of the plant in the former.
Thus we may imagine that the idio-plasm [? of that particular bud],
when it develops into a flowering shoot, produces at the same time
the germ-cells which are found in the latter. We thus approach an
understanding of Fritz Müller’s observation; for if the whole shoot
which produces the flower arises from the same idio-plasm which also
forms its germ-cells, we can readily understand why the latter should
contain the same hereditary tendencies which were previously expressed
in the flower which produced them. The fact that variations may
occur in a single shoot depends on the changes explained above, which
occur in the idio-plasm during the course of its growth, as a result
of the varying proportions in which the ancestral idio-plasms may be
contained in it.[30]
The meaning here appears to be twofold. For there are only two ways
of explaining the phenomena of bud-variation. Either they are due
to the influence of external conditions acting on the particular
bud in question, or else they are due to so-called “spontaneous”
changes taking place within the bud itself. Possibly it may be
both, but at least it must be either. Well, in the above passage,
Weismann appears to assume that it is both. For at the beginning of
the passage he speaks of the “germ-plasm of the first ontogenetic
stage” becoming “altered so far as to correspond with the altered
structure of the individual which arises therefrom,” and he goes
on to say that the alteration “may depend chiefly on the changed
conditions of development”—that is, as I understand, the influence
of external conditions. But at the end of the paragraph he says that
“the changes which occur in the idio-plasm during the course of its
growth” in the sporting bud, are due to “the varying proportions in
which the ancestral idio-plasms may be contained in it.” Thus, I
take it, Weismann here entertains both explanations of the phenomena
in question: he appears to regard these phenomena as partly due to
peculiar admixtures of ancestral idio-plasms in the bud itself (or
“spontaneous” variation), but partly also to an alteration of the
germ-plasm by its changed condition of development (or variation caused
by external conditions).
However, it is but of little consequence whether or not this is the
meaning which Weismann intends to convey. For the point we are coming
to is, that, whatever he intends to convey, “from the point of view”
of the theory of germ-plasm, there is only _one_ interpretation
possible. It is not open to Weismann (as it was to Darwin, or even to
Galton,) to entertain _both_ the explanations, whether separately or
in conjunction. For germ-plasm (unlike gemmules, or even stirp) must
be held always and everywhere _unalterably_ stable: else the whole
superstructure of Weismann’s theory of evolution falls to the ground.
We cannot consent to his retaining this theory on the one hand, and,
on the other, explaining bud-variation by “germ-plasm of the first
ontogenetic stage” becoming altered “chiefly by changed conditions of
development.” Even if it were true that “the alteration must be very
slight, if not quite insignificant,” there would here be a rift in the
lute, which must finally stop any further harping on the subject of
Evolution.
From the point of view of this theory, then, there is only one
interpretation open,—viz., that a bud-variation is ultimately due to
a peculiar admixture of germ-plasms in the seed from which the bud was
ultimately derived. But the objections to entertaining this as even
a logically possible explanation of the phenomena in all cases, is
insuperable.
In the first place, such a variation, when it does arise, is usually
a variation of an extremely pronounced character; therefore it is
very far from supporting Weismann’s view, that the “alteration”
of germ-plasm which is needed to produce it “must be very slight,
and perhaps quite insignificant.” In most cases where it occurs
bud-variation presents so extreme a departure from the normal type,
that no other kind of variation can be fitly compared with it in this
respect. In particular, the degree of variation is usually very much
greater than that which customarily obtains in congenital variations
of the ordinary kind; and, therefore, if these be supposed due to
particular admixtures of germ-plasm in sexual propagation, much more
must those admixtures which give rise to sporting buds be characterized
by peculiarities of no “insignificant” order. And much more, therefore,
ought they to assert themselves in sister-buds developed from the
same individual seed (ovule), than we find to be the case with any
sister-organisms which are developed from different individual seeds.
Yet, in the second place, so far is this from being the case, that
the most remarkable feature connected with bud-variation—next to the
suddenness and extreme amount of the variation itself—is the usually
isolated nature of its occurrence. There may be thousands of other
buds on the same plant, and yet it is one bud alone that deviates so
suddenly and so widely from its ancestral characters. Nay, more, a
single bud-variation may—and usually does—occur in plants which are
habitually propagated by cuttings and graftings; so that there may not
only be thousands, but millions of buds all derived from one original
seed, and all for many years remaining perfectly true to their parent
type, with the single exception of the sporting bud, which, while it
departs so widely from that type, is usually capable of transmitting
its extraordinary characters indefinitely by a-sexual, and not
infrequently also by sexual, methods. So that, altogether, it seems
impossible to suppose that in millions and millions of sister-buds,
which through years and years exhibit no variation, a highly peculiar
admixture of germ-plasm (which was originally present in the
parent-seed) should have been latent; that it should then suddenly
become so patent in a single bud, after which it never occurs in any
other bud, save in the progeny of the sporting one.
On the whole, then, while it thus seems impossible to attribute
all cases of bud-variation to mixtures of germ-plasms in sexual
propagation, the theory of germ-plasm is unable to entertain any
other explanation, on pain of surrendering its postulate touching the
unalterable stability of germ-plasm, on which the Weismannian theory of
evolution is founded.
So much for Weismann’s evidence touching the extreme, or virtually
everlasting, stability of germ-plasm. We have seen that this evidence
is not merely of a very poor character _per se_, or on antecedent
grounds; but that it is directly negatived as evidence by the a-sexual
origin of species in the plants alluded to by Professor Vines; by
certain facts which prove so high a degree of instability on the
part of this hypothetical substance, that in some cases it admits of
being very considerably modified in the course of only two or three
generations by exposure to changed conditions of life; while in
other cases it may “sport,” so as to produce “hereditary individual
variations,” which are much more pronounced than any of those that
ordinarily result from a blending of hereditary qualities in an act of
sexual union.
* * * * *
It will be well to conclude our examination of Weismann’s system by
stating exactly the effect produced on his theory of evolution by the
foregoing disproof of its fundamental postulate—the absolute stability
of germ-plasm.
Clearly, in the first place, if germ-plasm has not been absolutely
stable “since the first origin of sexual propagation,” the hereditary
characters of germ-plasm may have been modified any number of times,
and in always accumulating degrees. It matters not whether the
modifications have been due mainly to external or to internal causes.
It is enough to have shown that modifications occur. For, it will
be remembered, the doctrine of the absolute stability of germ-plasm
is, that inasmuch as the “molecular” structure of germ-plasm cannot
be affected either from without or from within, the only source of
“hereditary individual variations” is to be found in admixtures of
germ-plasms taking place in sexual fertilization. Slight “molecular”
differences having been originally impressed upon different masses of
germ-plasm when these were severally derived from their unicellular
sources, so unalterable has been the stability of germ-plasm ever
since, that these slight “molecular” differences have never been
in any degree effaced; and although in sexual unions they have for
untold ages been obliged to mix in ever-varying proportions, they
still continue—and ever must continue—to assert themselves in each
ontogeny. Therefore, as Weismann himself formulates this astonishing
doctrine,—“The origin of hereditary individual variations cannot
indeed be found in the higher organisms, the Metazoa and Metaphyta;
but is to be sought for in the lowest—the unicellular organisms.” Or
again,—“The formation of new species, which among the lower Protozoa
could be achieved without amphigony, could only be attained by means of
this process in the Metazoa and Metaphyta. It was only in this way that
hereditary individual differences could arise and persist[31].”
Now this doctrine is the most distinctive, as it is the most original
feature in Weismann’s system of theories. That it is of interest as an
example of boldly carrying the premises of a theory to their logical
termination, no one will deny. But as little can it be denied that the
very stringency of this logical process brings the theory itself into
collision with such facts as those which have now been stated, and
which, as far as I can see, are destructive of the theory—or, at any
rate, of all that side of the theory which depends on the doctrine of
absolute stability.
Take, for instance, the sequent doctrine that natural selection
is inoperative among the unicellular organisms. Here, indeed, we
have another of those doctrines which are so improbable on merely
antecedent grounds, that their presence might well be deemed a source
of irremediable weakness to the whole theory of evolution of which
they form integral, or logically essential, parts. For seeing that
the rate of increase in most of the unicellular organisms is quite
as high as—and in most cases very much higher than—the rate that
obtains in any of the multicellular, it becomes on merely antecedent
grounds incredible that the struggle for existence should here _not_
lead to any survival of the fittest. When, for instance, we learn from
Maupas that a single Stylonichia is potentially capable of yielding a
billion descendants within a week, we should need some extraordinarily
good evidence to make us believe that as regards this organism natural
selection is inoperative. But the point at present is that, quite apart
from all general and _a priori_ considerations of this kind, Weismann’s
doctrine that unicellular organisms cannot be influenced by natural
selection must be abandoned. For this doctrine followed deductively
from the premiss that in the multicellular organisms congenital
variations can only be due to admixtures of germ-plasms in acts of
sexual fertilization; so that, in the absence of such admixtures,
there could be no material for natural selection to work upon. But
now we have found that this premiss must be given up; and, therefore,
the deduction with respect to the unicellular organisms falls to the
ground. Although it is true that the unicellular organisms propagate
by fission, and although we grant, for the sake of argument, that they
never propagate by way of sexual unions—even so this can no longer be
taken to argue that none of their innumerable species owe their origin
to natural selection. And, although it is probably true that the sexual
methods of propagation constitute one source of hereditary individual
variation among the multicellular organisms, there is no vestige of any
independent reason for supposing that this is the _only_ source of such
variation; while the sundry facts which have now been given amount to
nothing short of a demonstration to the contrary[32].
Lastly, and as regards the multicellular organisms, it is evident
that Weismann’s essay _On the Significance of Sexual Reproduction in
the Theory of Natural Selection_ must be cancelled. For, apart from
the contradictory manner in which this matter has been stated (pp.
70, 93, notes), and apart also from the consideration that other
and quite as probable reasons have been suggested for the origin of
sexual reproduction, there is the fact that Weismann’s theory is no
longer tenable after the above destruction of its logical postulate
in the absolute stability of germ-plasm. For, in the absence of
this postulate, there is no basis for the theory that admixtures of
germ-plasms in sexual reproduction furnish the sole means whereby
heritable variations can be supplied for the working of natural
selection.
_Summary._
THE theory of germ-plasm is not only a theory of heredity: it is
also, and more distinctively, a theory of evolution. As a theory of
heredity it is grounded on its author’s fundamental postulate—the
continuity of germ-plasm; and, further, on a fact well recognized
by all other theories of heredity, which he expresses by the term
stability of germ-plasm. But as a theory of evolution it requires two
additional postulates for its support—viz., that germ-plasm has
been _perpetually_ continuous “since the first origin of life,” and
_absolutely_ stable “since the first origin of sexual reproduction.”
It is clear that these two additional postulates are not needed
for his theory of heredity, but only for his additional theory of
evolution. There have been other theories of heredity, prior to this
one, which, like it, have been founded on the postulate of “continuity”
(in Weismann’s sense) of the substance of heredity; but it has not
been needful for any of these theories to postulate further that this
substance has been _always_ thus isolated, or even that it is now
_invariably_ so. For even though the isolation be frequently invaded
by influences of body-changes on the congenital characters of this
substance, it does not follow that the body-changes must be transmitted
to offspring exactly as they occurred in parents. They may produce in
offspring what we have agreed to call “specialized” hereditary changes,
even if they never produce “representative” hereditary changes,—i.e.,
the transmission of acquired characters. But it is essential to
Weismann’s theory of _evolution_ that body-changes should not exercise
a modifying influence of _any_ kind on the ancestral endowments of this
substance; hence, for the purposes of this further theory he has to
assume that germ-plasm presents, not only _continuity_, but continuity
_unbroken since the first origin of life_.
Similarly as regards his postulate of the stability of germ-plasm
as _absolute_. It is enough for all the requirements of his theory
of heredity, that the substance in question should present the high
degree of stability which the facts of atavism, persistence of
vestigial organs, &c., prove it to possess. But for his further
theory of evolution it is necessary to make this further postulate of
the stability of germ-plasm _as undisturbed since the first origin of
sexual propagation_: otherwise there would be no logical foundation for
any of the distinctive doctrines which go to constitute that theory.
Thus much understood, we proceeded to examine the theory of germ-plasm
in each of its departments separately—i.e., first as a theory of
heredity, and next as a theory of evolution. And we begun by comparing
it as a theory of heredity with the preceding theories of Darwin and
Galton. In the result we found that germ-plasm resembles gemmules in
all the following respects. It is particulate; constitutes the material
basis of heredity; is mainly lodged in highly specialized cells; is
nevertheless also distributed throughout the general cellular tissues,
where it is concerned in all processes of regeneration, repair, and
a-sexual reproduction; presents an enormously complex structure,
in that every constituent part of a potentially future organism is
represented in a fertilized ovum by corresponding particles; is
everywhere capable of virtually unlimited multiplication, without ever
losing its hereditary endowments; is often capable of carrying these
endowments in a dormant state through a long series of generations,
until at last they reappear again in what we recognize as reversions.
Such being the points of resemblance, the only points of difference
may be summed up in the two words—continuity, and stability. For,
as regards continuity, while Darwin’s theory supposes the substance
of heredity to be more or less formed anew in each generation by
the body-tissues of that generation, Weismann’s theory regards
this substance as owing nothing to the body-tissues, further than
lodgement and nutrition. Therefore, while the theory of gemmules can
freely entertain the doctrines of Lamarck, the theory of germ-plasm
excludes them as physiologically impossible, in all cases where
sexual reproduction is concerned. Again, as regards stability, while
Darwin’s theory simply accepts the fact of such a degree of stability
appertaining to the substance of heredity as the phenomena of atavism,
&c. prove, Weismann’s theory postulates the stability of this substance
as absolute. But, as we have now so often seen, he does so in order to
provide a hypothetical basis for his further theory of evolution. In as
far as his theory of heredity is concerned, there is no reason why it
should differ from Darwin’s in this respect.
Again, comparing Weismann’s theory of heredity with that of Galton, we
found that germ-plasm resembles stirp in all the points wherein we have
just seen that it resembles germ-plasm. Or, otherwise stated, all three
theories are thus far coincident. But germ-plasm resembles stirp much
more closely than it does gemmules, seeing that the theory of stirp is
founded on the postulate of “continuity” in exactly the same manner
as is the theory of germ-plasm. In point of fact, the only difference
between these two theories consists in the two further postulates
presented by the latter—viz., that the “continuity” in question has
been unbroken since the origin of life, while the “stability” in
question has been uninterrupted since the origin of sexual propagation.
But seeing that both these additional postulates have reference to
Weismann’s theory of evolution, we may say that his theory of heredity
is, as regards all essential points, indistinguishable from that of
Galton.
The truly scientific attitude of mind with regard to the problem of
heredity is to say, as Galton says, “that we might almost reserve our
belief that the structural [i.e., somatic] cells can react on the
sexual elements at all, and we may be confident that at most they do
so in a very faint degree; in other words, that acquired modifications
are barely, if at all, _inherited_, in the correct sense of that word.”
But for Weismann’s further theory of evolution, it is necessary to
postulate the two additional doctrines in question; and it makes a
literally immeasurable difference to the theory of evolution whether
or not we entertain these two additional postulates. For no matter how
faintly or how fitfully the substance of heredity may be modified by
somatic tissues, by external conditions of life, or even by so-called
spontaneous changes on the part of this substance itself, numberless
causes of congenital variation are thus admitted, while even the
Lamarckian principles are hypothetically allowed some degree of play.
And although this is a lower degree than Darwin supposed, their
influence in determining the course of organic evolution may still have
been enormous; seeing that their action in any degree must always have
been _directive_ on the one hand, and _cumulative_ on the other.
Having thus pointed out the great distinction between the theories
of stirp and of germ-plasm, it became needful to note that Weismann
himself is not consistent in observing it. On the contrary, in some
passages he apparently expresses himself as willing to resign both
his distinctive postulates—continuity as _perpetual_, and stability
as _absolute_. But it is evident that such passages must be ignored
by his critics, because, although as far as his theory of heredity is
concerned they betoken an approach to the less speculative views of
Galton, any such approach is proportionally destructive of his theory
of evolution. It must not be supposed that I am taking an ungenerous
advantage of these occasionally fundamental concessions. On the
contrary, one cannot but admire the candour which they display. But,
as I have said, it is necessary for us to ignore them, if only in
order to examine the Weismannian theory of germ-plasm as a distinctive
theory at all. And more than this. Seeing that his theory of heredity
differs from Galton’s chiefly in being further an elaborate theory
of evolution (founded on the two additional postulates in question),
my main object has been to show the enfeeblement of the former which
Weismann has caused by his addition of the latter. If he were to
express his willingness to abandon his theory of evolution for the
sake of strengthening his theory of heredity by identifying its
main features with those of Galton’s, personally I should have no
criticism to pass. Indeed, I was myself one of the first evolutionists
who called in question the Lamarckian factors; and ever since the
publication of Galton’s theory of heredity at about the same time, I
have felt that in regard to its main principles—or those in which
it agrees with Weismann’s—it is probably the true one. But I can
nowhere find that Weismann is thus prepared to surrender his theory of
evolution. Occasionally he plays fast and loose with the two additional
postulates on which this theory is founded; but he does so without
appearing to perceive the speculative impossibility of any longer
sustaining his temple of evolution if he were to remove its pillars of
germ-plasm.
Ignoring, then, these inconsistencies, we proceeded to examine
separately, and on their own respective merits, the two distinctive
postulates of the theory of germ-plasm—_perpetual_ continuity since
the first origin of life, and _absolute_ stability since the first
origin of sexual propagation.
It does not appear to me that very much has to be said, either for
or against the former postulate, on merely antecedent grounds, or
grounds of general reasoning. Therefore I relegated to an Appendix
my examination of what Weismann has argued on these grounds, while
in the text I considered only what he has advanced as evidence _a
posteriori_. Here, as we saw, he has developed three distinct lines
of verification—viz. (A) the migration of germ-cells in some of
the _Hydromedusae_,(B) the early separation of germ-cells in the
ontogeny of certain Invertebrata, and (C) the alleged invariability of
organisms which are produced parthenogenetically. But we have seen,
with respect to (A), that the specialized character of germinal cells
is a fact which every theory of heredity must more or less recognize;
and, therefore, that the migration of these cells, wherever it may be
found to occur, does not lend any peculiar countenance to Weismann’s
theory. There may be many reasons for such migration other than the one
which this theory assigns; while the reason which it does assign is
rendered improbable by the consideration that in the _Hydromedusae_
the material of heredity is already and richly diffused throughout the
general tissues. (B) and (C) are both contrary to fact; and, therefore,
in whatever measure they would have corroborated the theory had they
proved to be true, in that measure must they be held to discountenance
the theory now that they have been shown to be false.
It appears, then, that there is no evidence in support of the postulate
of the _perpetual_ continuity of germ-plasm. There is nothing to show
the necessary non-inheritance of acquired characters. The only evidence
which one can recognize as good, is that which makes equally in
favour of the theory of stirp—or rather, of the well-known fact that
congenital characters are at any rate much _more_ heritable than are
acquired: which, it is needless to repeat, is a widely different thing
from proving—or even rendering probable—the _absolute_ restriction of
germ-plasm to a separate “sphere” of its own “since the origin of life.”
But now, although there is no evidence in support of this postulate,
there is no small amount of evidence against it. For this evidence
goes to indicate that no small amount of reciprocal action habitually
takes place between body-tissues and germinal elements: indeed it seems
almost to prove that the orbits of germ-plasm and somato-plasm are not
mutually exclusive, but touch and cut each other to a considerable
extent. The evidence in question, it will be remembered, is derived
from the effects of puberty, senility, castration, &c.; the occasional
effect of pollenization on the somatic tissues of plants; the influence
which a stock occasionally exercises upon a scion, or _vice versa_,
which proves the possibility of a transmission of hereditary characters
by a mere grafting together of somatic tissues; the direct evidence
given by De Vries that in certain Algae constituents of cellular tissue
pass immediately from the maternal ovum to the daughter organism; and
the evidence, both direct and indirect, which remains to be given
on a larger scale in my subsequent volume, where we shall have to
challenge the validity of Weismann’s fundamental postulate touching
the non-occurrence of Lamarckian factors in any of the multicellular
organisms.
It must here again be noticed that in those passages where he concedes
the possibly “occasional” transmission of acquired characters
Weismann is annihilating his own theory, root and branch. Thus,
for example, in allusion to De Vries’ observation just mentioned,
he says that we cannot exclude the possibility of “changes being
induced by external conditions in the organism as a whole, and
then communicated to the germ-cells after the manner indicated in
Darwin’s hypothesis of pangenesis.” But it is obvious that the
theory of germ-plasm _must_ “exclude the possibility of such a
transmission occasionally occurring”; for the very essence of that
theory consists in its postulating a difference between germ-plasm
and the general body-substance _in kind_, such that there never _can_
be any “communication” from the one to the other “after the manner
indicated by Darwin’s hypothesis of pangenesis.” Any prevarication
over this point amounts simply to abandoning the theory of germ-plasm
altogether, and opening up a totally distinct issue—namely, the
_relative importance_ of natural selection and the Lamarckian factors
in the process of organic evolution. It may be perfectly true—and I
myself believe it is perfectly true—that Darwin attributed too large
a measure of importance to the Lamarckian factors; but whether or
not he did so is quite a different question from that which obtains
between his theory of pangenesis and Weismann’s theory of germ-plasm.
The former question is whether we are to “_modify_” the theory of
pangenesis, so as to constitute it the theory of stirp; the latter
question is whether we are to “_abolish_” the theory of pangenesis, in
favour of its logical antithesis, the theory of germ-plasm. And this
question remains to be dealt with in my next volume.
* * * * *
Coming then, lastly, to the companion postulate of germ-plasm as
absolutely stable since the first origin of sexual propagation, we had
to observe that, unlike the one we have just been considering, there
is an immensely strong presumption against it on merely antecedent
grounds. That the most complex substance in nature should likewise
be the most stable substance with regard to complexity of “molecular
structure”; that the greater its complexity becomes the greater becomes
its stability, so that while in the comparatively simple unicellular
organisms it is eminently susceptible of modification by external
conditions, it entirely ceases to be thus susceptible when it becomes
evolved into the incomparably more complex and immensely more varied
structures which form the bases of heredity in the multicellular
organisms—where, also, it must come into ever more and more
intricate as well as more and more diverse relations with the external
world;—all this is, I repeat, well nigh incredible. At any rate,
speaking for myself, I should require some enormous weight of evidence
to balance so enormous an antecedent improbability, or before I could
regard such a doctrine as meriting any serious attention.
What, then, is the evidence that has been adduced? We have found
that this evidence is _nil_. On the other hand, we have found that
the evidence against the doctrine is abundantly sufficient to
annihilate the doctrine—and this quite apart from all the antecedent
considerations just alluded to. For not only have we the sundry facts
of bud-variation, a-sexual origin of species, &c., which contradict
the doctrine; but we have also the results of direct experiment, which
prove that the alleged stability of germ-plasm may be conspicuously
upset by slight changes in the external conditions of life. So that
both from within and from without the stability which is alleged in
theory admits of being overturned by facts.
And here, in order to avoid all possible confusion, I must ask it once
more to be noted that there is not, and never has been, any question
touching the _high degree_ of stability which is exhibited by whatever
substance it is that constitutes the material basis of heredity.
But this is a widely different thing from supposing the stability
_absolute_, so that it can never have been affected in any degree since
the first origin of multicellular organisms, or in any of the millions
of species into which these organisms have ramified. And the fact that
in some cases we are actually able to observe a change of congenital
characters as resulting from some “spontaneous” change in the
hereditary material itself (as in bud-variation), or from some change
in the external conditions of life (as in Hoffmann’s experiments)—this
fact is more than is required in order finally to overthrow the
intrinsically untenable doctrine which is in question.
Now, with the collapse of this doctrine there collapses also the
important chain of deductions therefrom, which together constitute
Weismann’s new theory of evolution. In particular, that natural
selection is the exclusive means of modification among all the Metazoa
and Metaphyta, while it is as exclusively ruled out with respect to
all the Protozoa and Protophyta; that individual variations among the
former can only be determined by sexual unions, while among the latter
they can only be determined by the direct action of the environment;
that the origin of congenital variability in all the Metazoa and
Metaphyta is to be sought, and can only be found, in variations which
occurred millions of years ago in the Protozoa and Protophyta; that
the “significance of sexual propagation” is to be found in the view,
that by this means alone can congenital variations have been ever since
produced; &c., &c.
* * * * *
Upon the whole then, it appears to me that both the fundamental
postulates of the theory of germ-plasm are unsound. That the substance
of heredity is largely continuous and highly stable I see many and
cogent reasons for believing. But that this substance has been
uninterruptedly continuous since the origin of life, and absolutely
stable since the origin of sexual propagation, I see even more and
better reasons for disbelieving. And inasmuch as these two latter,
or distinctive, postulates are not needed for Weismann’s theory of
heredity, while they are both essential to his theory of evolution,
I cannot but regret that he should thus have crippled the former by
burdening it with the latter. Hence my object throughout has been
to display, as sharply as possible, the contrast that is presented
between the brass and the clay in the colossal figure which Weismann
has constructed. Hence, also, my emphatic dissent from his theory of
evolution does not prevent me from sincerely appreciating the great
value which attaches to his theory of heredity. And although I have
not hesitated to say that this theory is, in my opinion, incomplete;
that it presents not a few manifest inconsistencies, and even logical
contradictions; that the facts on which it is founded have always
been facts of general knowledge; that in all its main features it was
present to the mind of Darwin, and distinctly formulated by Galton;
that in so far as it has been constituted the basis of a more general
theory of organic evolution, it has clearly proved a failure:—such
considerations in no wise diminish my cordial recognition of the
services which its distinguished author has rendered to science by his
speculations upon these topics. For not only has he been successful
in drawing renewed and much more general attention to the important
questions touching the transmissibility of acquired characters, the
causes of variation, and so on; but even those parts of his system
which have proved untenable are not without such value as temporary
scaffoldings present in relation to permanent buildings. Therefore, if
I have appeared to play the _rôle_ of a hostile critic, this has only
been an expression of my desire to separate what seems to me the grain
of good science from the chaff of bad speculation. And the candour
which Professor Weismann has always displayed towards criticism of this
character enables me to hope with assurance, that I have said nothing
which he himself will regard as inconsistent with high admiration of
his work as a naturalist, or of his originality as a philosopher.
CHAPTER V.
WEISMANNISM UP TO DATE (1893).
HITHERTO we have been considering Professor Weismann’s system as it
stood prior to the publication of his most recent works on _Amphimixis_
and _The Germ-plasm_, in 1891 and 1893 respectively. These later and
highly elaborate essays present considerable modifications of the
system, as it stood when the foregoing criticism was written. But,
for reasons already stated in the Preface, it appears to me desirable
to leave that criticism as it was originally constructed, and to
supply this further chapter for the purpose of dealing with the large
alterations of, and important additions to, the theory of germ-plasm,
which the maturer thought of its gifted author has led him to announce.
A few general remarks may be most conveniently made at the outset.
In the first place, these recent publications present the advantage
over their predecessors of being systematic treatises, instead of more
or less independent papers. On this account they present a logical
sequence of thought, which renders the task of examination much less
difficult than it was in the case of the first volume of the _Essays_.
In the second place, as a result of his more matured reflection,
Professor Weismann has himself perceived a considerable number of the
difficulties and objections which I have set forth in the preceding
chapters. And not only has he thus anticipated many of my criticisms;
but, as a result of doing so, he has changed not a few of the most
important parts of his previous system, with the result of greatly
improving it.
But, in the third place, notwithstanding that his remarkable power
of speculative thinking is everywhere united with adequate knowledge
in the sundry branches of biological science with which it deals, I
confess to a serious doubt whether it has not been permitted to enjoy
an undue amount of liberty. If only they can be laced together by a
thread of logical connexion, hypotheses are added to hypotheses in such
profusion as we are acquainted with in the works of metaphysicians, but
which has rarely been approached in those of naturalists. The whole
mechanism of heredity has been now planned out in such minuteness of
detail and assurance of accuracy, that in reading the account one is
reminded of that which is given by Dante of the topography of Inferno.
For not only is the “sphere” of germ-plasm now composed of nine
circles (molecules, biophores, determinants, ids, idants, idio-plasm,
somatic-idioplasm, morpho-plasm, apical-plasm), but in most of these
regions our guide is able to show us such strange and interesting
phenomena, that we return to the fields of science with a sense of
having been indeed in some other world. Or, to change the metaphor, if
it be the case that “a true scientific judgement consists in giving a
free rein to speculation with one hand, while holding ready the break
of verification with the other,” I think it must be admitted that, in
as far as he has erred, Professor Weismann has done so by driving a
chariot which is unprovided with any break at all.
Hence, fourthly, it is needless to follow, even in epitome, the
innumerable windings of these never-ending speculations. For, on the
one hand, it would be impossible to do so without adding an unduly
extended chapter to our already tediously prolonged consideration of
Weismann’s views; while, on the other hand, we should have to deal
merely with matters of comparative detail. The additions which have
been made to his theory by his most recent publications are chiefly
concerned with the matter just alluded to—viz., a minute elaboration
of the hypothetical mechanism of heredity, in accordance with the
general theory of germ-plasm. Without question this elaboration is
everywhere thoughtful, and often highly ingenious; but until the
general theory in question shall have been satisfactorily grounded,
it seems premature to supply so immense a design of purely deductive
construction. Beautiful though it may be in its imposing elevation,
this drawing of “the architecture of germ-plasm” must be regarded
as a work of artistic imagination rather than as one of scientific
generalization. From the latter point of view it is at most a temple
_in posse_, and even if it is ever to be realized _in esse_, we cannot
allow the actual building to begin until we are much more sure than
anybody is at present entitled to be touching the foundations on which
it is proposed to rear so great an edifice.
Again, and fifthly, even if Weismann should ever be able to satisfy
us upon this matter, or fully to demonstrate his basal proposition
touching the perpetual continuity of germ-plasm, there would still be
a far cry between accepting this sufficiently simple proposition and
supposing that there is any adequate reason for entertaining so complex
a scheme of the structure of germ-plasm. No doubt Weismann himself
would be quite ready to admit, that from his basal proposition of the
continuity of germ-plasm it is logically possible to construct many
other designs of the architecture of germ-plasm, besides the one which
he has so beautifully drawn. And although most of such alternative
designs would doubtless embody some one or other of the features which
are presented by his own, no one could say which features common to any
two of the designs represent the facts. For in the case of all alike
there would be a necessary absence of verification: the architects
would all and equally have to acknowledge that their imposing pictures
of “the palace of truth” were but imaginary. Such, in my opinion, has
been the case with all theories of the ultimate mechanism of heredity
hitherto published; but the difference between them and Weismann’s
theory in this respect is, that while most of the others have not gone
into speculative details further than was necessary as a means of
substantiating their basal postulates, Weismann’s, as now developed in
_The Germ-plasm_, is mainly concerned with such speculative details as
an end, or object, _per se_.
But, it may be replied, by thus constructing an ideal mechanism of
heredity Weismann is greatly strengthening his fundamental postulate
of the continuity of germ-plasm, because he shows how all the main
facts of heredity, and allied phenomena, admit of being explained
if once the postulate be accepted. If this were urged, however, I
should have two remarks to offer. The first is that Weismann, in
constructing his ideal mechanism, has gone very much further in the way
of elaboration than can possibly be required for this purpose. So much
further, indeed, that his purpose has evidently been the constructing
of his ideal mechanism, as I have just said, for its own sake, and
not for the sake of substantiating its basal proposition by showing
how well the latter can be made to work in explaining the phenomena
of heredity, &c. Moreover—and this is my second remark—however well
the basal proposition may be made to work in this respect, we must
not be deceived into supposing that such a fact is equivalent to a
substantiation of the proposition. This proposition—the continuity of
germ-plasm—is the inverse of that which constitutes the basis of the
theory of pangenesis. For while the latter assumes that in the last
resort it is always somatic tissues which produce the substance of
heredity, the former simply inverts the terms of this assumption, and
holds that it is always the substance of heredity which produces the
somatic tissues. Now, in all cases where one theory consists in thus
simply inverting the terms of another, it will be found that the facts
which they both seek to explain lend themselves equally to explanation
by either, up to some certain and usually distant point, where a
crucial test becomes possible. Take, as an example, the geocentric
and heliocentric theories of the solar system. Here the question was
whether the earth moved round the sun, or _vice versa_; and so many
of the facts of observation lent themselves equally well to either
interpretation, that it was very many centuries before the crucial
tests were forthcoming. So, in the present instance, the question is as
to whether the carriers of heredity move from body-cells to germ-cells,
or _vice versa_; and it is because the theory which sustains the latter
view has merely to invert the terms of the one which takes t
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