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 the former,
that so many of the facts of observation lend themselves equally well
to both—as we have seen in chapter III (pp. 56-59).
Lastly, yet another reason for not considering in any detail Professor
Weismann’s intricate speculations on the ultimate mechanism of heredity
is, that by so doing I should have found it impossible to avoid
obscuring the main issues. For even Professor Weismann himself, by the
extreme care which he has taken in fully presenting his scheme of this
ultimate mechanism, has not found it practicable to keep distinctly
before our view the relative insignificance of such details, as
compared with the fundamental importance of his original postulates.
Hence, I have deemed it best in the present chapter to restrict our
attention to the changes which he has recently made in these the
foundations of his entire system.
For these reasons, then, I will mention only those main features in the
“architecture of germ-plasm” which it is necessary to understand for
the purposes of the following criticism touching the general theory of
germ-plasm in the most recent phase of its evolution.
* * * * *
To begin with, Weismann has now seen the desirability of ceasing to
designate the ultimate “carriers of heredity” by the term “molecules.”
Indeed, in these later volumes he has fully anticipated my remarks
touching the use of this term in his previous “Essays[33].” The result
of his more mature reflection may be presented in epitome thus.
A number of “molecules,” in the proper or chemical sense of the word,
go to form a “biophore,” which is the ultimate unit of living substance.
A number of “ biophores” go to form a “determinant,” which is a special
element in the germ-plasm, capable of directing the ontogeny of such
and such a group of cells as is independently variable from the germ
onwards.
A number of “determinants” go to form an “id,” which is the same
hypothetical body as Weismann has hitherto designated by the term
“ancestral germ-plasm.” That is to say, it is a group of determinants
indissolubly united in phylogeny, and therefore transmitted by heredity
as one complex whole. Ids are, perhaps, microscopically visible; and,
if so, they probably correspond to the small granules (microsomata),
which are familiar to the histologist in the structure of chromosomes.
A number of “ids” go to form an “idant,” which is a chromosome, or
chromatin fibre[34].
In my opinion the most important advance which Weismann has made in
his theory by means of this scheme has reference to the third of these
divisions—the determinant. It is a matter of observation that every
cell of a multicellular organism does not vary independently: it
appears to be always the case that in the phenomena of variation a
smaller or a larger _group_ of cells is concerned. Now there must be
something that determines the similar and simultaneous variation of
such a whole group of cells; and, in all cases where such a variation
is congenital, it is certain that this something must be contained in
the substance of heredity. So far, I think, we must all agree, whether
or not we regard this substance as “germ-plasm.” In other words,
whether we regard the carriers of heredity as proceeding centrifugally
(germ-plasm) or centripetally (gemmules), it seems to me that we ought
to accept Weismann’s doctrine of determinants. Indeed, pathologists
have already furnished a foreshadowing of such a doctrine in regard to
the phenomena presented by certain diseases, such as cancer; but it is
an important step to have extended the idea from pathology to biology
in general—and, at the same time, to have given it a more definite
shape than it has hitherto presented. In Weismann’s hands it serves to
render more conceivable—if not also more intelligible—that process
of marshalling cell-formations, which, be our theories what they may,
is assuredly the most distinctive and remarkable fact of ontogenetic
organization.
Again, as regards the id, I do not see how any one can attentively
read Professor Weismann’s discussion without acknowledging that, if we
once accept his doctrine of determinants, his sequent doctrine of ids
becomes a logical necessity.
On the other hand, however, I do not see that such is the case with
respect to idants; and still less do I see any reason for identifying
the latter with chromosomes—even assuming that chromosomes are the
visible repositories of the carriers of heredity[35].
Referring the reader to Weismann’s own exposition for a full account of
these and many other additions to his general theory of germ-plasm, I
will at once proceed to consider the alterations or emendations of that
theory which have been published in his last two volumes, and which,
as we shall find, have in large measure anticipated some of the most
important points in the foregoing criticism. Therefore in the following
criticism I will consider _seriatim_ what he has now said touching all
these points, and conclude by offering some general remarks on the
resulting position of his general system of theories up to the present
date.
Pursuing the same method of criticism as that adopted in the preceding
chapters, we will first consider the further modifications of
Weismann’s theory of heredity, and next those of his theory of organic
evolution.
_Weismann’s theory of Heredity_ (1893).
First of all, Weismann has now profoundly modified his theory of polar
bodies. For, owing to certain more recent researches of Professor O.
Hertwig, he very candidly allows:—“My previous interpretation of the
first polar body as the removal of ovogenetic nucleo-plasm from the egg
must fall to the ground: about this there is no possible doubt[36].”
He now regards both polar bodies as concerned in the same function of
removing superfluous germ-plasm. Therefore one-half of his previous
theory is abandoned: “the ovogenetic idio-plasm” is now supposed to be
simply absorbed in the course of ontogeny, as I had suggested in one
of the preceding chapters (pp. 42-46). The consequence is that he has
now nothing to oppose to the view which is likewise there suggested
(pp. 43-44)—viz., that his whole theory of polar bodies is rendered
needless and improbable by the fact that the very mode in which ova
are produced renders ample provision for the removal of any amount of
superfluous germ-plasm which the theory of germ-plasm may require.
It is needless to say, after what has already been said in the pages
just referred to, that in my opinion Professor Weismann has improved
his main system of theories by dropping this part of his subordinate
and, for the most part, separate theory of polar bodies. I only wish he
could have seen his way to dropping the whole.
* * * * *
Again, he has now fully considered the phenomena of repair,
regeneration, reproduction from somatic tissues, budding, and
graft-hybridization.
Touching the four former he takes the view which I have supposed that
he would (p. 53). As regards the latter, he fully accepts the fact of
an occasional transmission of characters from one species or variety of
plant to another by mere grafting[37]. But, although the explanation
which he gives of this fact may pass muster so far as the only case
which he deals with in detail is concerned, I do not see how it can do
so to many others. For the case which he considers is that of _Cystisus
adami_, where a bud of one species of Laburnum having been inserted
in the wood of another produced a shoot which presented intermediate
characters; and these have ever since been propagated by cuttings.
Weismann’s interpretation of the facts here is, “that they were due to
an abnormal kind of amphimixis, so that the idants of both species were
combined in the apical cell of the first shoot[38].” Now, although this
explanation may well apply to a case of graft-hybridization by means
of buds, it obviously cannot do so to any case where hybridization is
produced by the grafting of woody tissues. For here there is no “apical
cell” in the question; and therefore the difficulties which I have
adduced on page 82 remain. Possibly Weismann may dispute the fact of
hybridization in any of these cases; but, as he has not expressly done
so, I will not go into the question of evidence[39].
* * * * *
One important addition to this side of Weismann’s system has been made
in order to meet the class of difficulties which are presented by
the apparent inheritance of certain climatic variations, as already
mentioned on pp. 67-8. For example, his own butterflies seemed to
render definite proof of somatogenetic variations caused by changed
conditions of life being transmitted to progeny. Therefore, it will
be remembered, Weismann candidly admitted, “even now I cannot explain
the facts otherwise than by supposing a passive acquisition of
characters produced by the direct influence of climate”—i.e., an
exactly _representative_ copying in progeny of characters acquired
by parents. I have already quoted these words in order to show their
logical inadmissibility as used by Weismann. He cannot be allowed thus
to entertain the Lamarckian factors and at the same time to maintain
his theory of germ-plasm, which excludes them as physiologically
impossible. Doubtless he was himself aware of this, for he immediately
added that “new experiments will be necessary to afford the _true_
explanation[40].”
The explanation, however, which he now gives is not based on any
new experiments, but on a new suggestion to the effect that all
such seemingly conclusive instances of the inheritance of acquired
characters are, in truth, illusory. This suggestion is that “Many
climatic variations may be due wholly or in part to the simultaneous
variation of corresponding determinants in some parts of the soma, and
in the germ-plasm of the reproductive cells.[41]” For example, if, as
Weismann now supposes, determinants of the same kinds occur in the
somatic tissues as well as in the germ-cells, when a particular spot
occurs on a butterfly’s wing, it has been due to a particular kind of
determinant which in the course of ontogeny was transmitted from the
germ-cell for the express purpose of controlling the size and colour
of the spot. But a residue of precisely similar determinants was
reserved in the germ-cell (germ-plasm), for the purpose of determining
a precisely similar spot in the next generation. Hence, if a rise of
temperature, or any other external change, is capable of so acting
on the determinant in the soma as to cause it to impart an abnormal
colour to the spot when formed, a similar change is likely to be
simultaneously effected in the corresponding determinants which are
lying dormant in the germ-plasm. Therefore, when the latter become
active in the ontogeny of the next generation, they will produce spots
presenting the same variations as those of the preceding generation.
Obviously, however, there would not be here any transmission of
acquired characters. The change would be “specialized,” but not
“representative.”
No doubt we have here a sufficiently ingenious method of circumventing
an awkward class of facts. But I should like to make two observations
with regard to it.
In the first place, the suggestion is highly speculative, and has
been advanced solely for the sake of saving the theory of germ-plasm.
There are no facts adduced in its favour, and it could scarcely be
entertained as in the least degree probable by any one who has not
already accepted the theory in question. Hence, unless we are to
embark on a course of circular reasoning, we must refuse to accept the
explanation of hereditary climatic variation now offered, until it
shall have been fully corroborated by the experimental enquiry to which
Weismann says he is now submitting it.
My second observation is, that the suggestion is not new; but appears
to have been derived from Professor Weismann’s recent study of Mr.
Galton’s _Theory of Heredity_. At all events, the suggestion is there
presented with sufficient lucidity, thus:—
It is said that the structure of an animal changes when he is placed
under changed conditions; that his offspring inherit some of his
change; and that they vary still further on their own account, in
the same direction, and so on through successive generations, until
a notable change in the congenital characteristics of the race has
been effected. Hence, it is concluded that a change in the personal
structure has reacted on the sexual elements. For my part, I object to
so general a conclusion, for the following reasons. It is universally
admitted that the primary agents in the processes of growth,
nutrition, and reproduction are the same, and that a true theory of
heredity must so regard them. In other words, they are all due to the
development of some germinal matter, variously located. Consequently,
when similar germinal matter is everywhere affected by the same
conditions, we should expect that it would be everywhere affected in
the same way. The particular kind of germ whence the hair sprang, that
was induced to throw out a new variety in the cells nearest to the
surface of the body under certain changed conditions of climate and
food, might be expected to throw out a similar variety in the sexual
elements at the same time. The changes in the germs would everywhere
be collateral, although the moments when any of the changed germs
happened to receive their development might be different[42].
This allusion to Mr. Galton’s _Theory of Heredity_ leads me to consider
what Professor Weismann has said with regard to it in this latest
publication, where, for the first time, he has dealt with it. In my
opinion he has done but scant justice to the views of his predecessor,
and therefore I will occupy some considerable space in seeking to
justify this opinion.
As already stated, from the time that Mr. Galton published his theory
I have felt that in its main contention it presents a probably true
solution of the main problem of heredity—viz., to account for the
contrast between congenital and acquired characters in respect of
transmissibility. And this solution, as likewise already stated, was
substantially identical with that which Professor Weismann published
in the next decade. Indeed, the only important difference between
these two theories of heredity is, that while Weismann’s excludes on
deductive grounds the physiological possibility of the inheritance of
acquired characters, Galton’s more judiciously leaves to be determined,
by subsequent enquiry of the inductive kind, the question whether
acquired characters are ever transmitted in faint degrees, or whether
they are never transmitted at all. In addition to this important
difference, however, there are certain others which seem to me of very
little consequence, inasmuch as they have reference to speculations
on the ultimate mechanism of heredity, or the intimate morphology and
physiology of the carriers of heredity—speculations which it would be
absurd to suppose can be other than purely conjectural. Therefore in
my previous criticism I did not allude to these subordinate points of
difference, but stated merely, in general terms, that Galton’s view
of the ultimate mechanism in question was such as to leave room for
the possibility of the occasional transmission of acquired characters.
And in this respect, it still seems to me, his theory has an advantage
over that of Weismann. No doubt the latter is a much more elaborate and
highly finished piece of work; but beauty of ideal construction is no
guarantee of scientific truth—as we shall presently find exemplified
in a striking manner with regard to Weismann’s theory of evolution.
And if his theory of heredity, in its final shape, is a much more
precise, detailed, and logically coherent structure than any which
has ever been framed in this department of biological thought, there
is all the more reason to scan critically the fundamental postulate
on which it rests. Hence I cannot help feeling that it will be time
enough to consider minor differences between the two theories when the
physiological possibility of the occasional transmission of acquired
characters, as entertained by Galton’s theory, shall have been ruled
out as demonstrably opposed to fact.
Seeing, however, that Professor Weismann thinks otherwise, and appears
to attach as much importance to differences concerning deductive
_minutiae_ as he does to those concerning fundamental principles,
I will here contrast the two theories somewhat more in detail than
heretofore, and with special reference to what he has now himself said
touching their relationship.
It will be remembered that the primary or fundamental difference
just alluded to is, that while the theory of germ-plasm postulates
an _absolute_ continuity, the theory of stirp postulates but a
_partial_ continuity, of the substance of heredity. Hence, according
to Weismann’s view, we must go back to the unicellular organisms for
the origin of this substance in the multicellular; and we must regard
use-inheritance as physiologically impossible. On the other hand,
according to Galton’s view, there is no necessity for us to do either
of these things. The origin of stirp is to be found in the somatic
tissues of the multicellular organisms themselves. Nevertheless, this
theory differs greatly from pangenesis, in that the former supposes the
origin of hereditary substance to be mainly given in the _phylogeny_
of any group of multicellular organisms, while the latter supposes
it to be given mainly in each _ontogeny_, Galton’s theory is, that
in each ontogeny only a small part of the stirp derived from parents
is consumed in making the new organism—the larger part being handed
over in trust for passing on to the next generation, in the same way
as Weismann supposes to be the case with germ-plasm. Darwin’s theory,
on the other hand, does not entertain any such notion of “continuity”
in the substance of heredity from germ-cell to germ-cell of parent
and offspring; it supposes that in each successive generation the
germ-cells are _wholly_ supplied with their germinal material from
somatic-cells of each individual organism. Or, adopting our previous
terminology, the three theories may be ranked thus.
The particulate elements of heredity all proceed centripetally from
somatic-cells to germ-cells (gemmules): the inheritance of acquired
characters is therefore habitual.
These particulate elements proceed for the most part, though not
exclusively, from germ-cells to somatic-cells (stirp): the inheritance
of acquired characters is therefore but occasional.
The elements in question proceed exclusively in the centrifugal
direction last mentioned (germ-plasm): the inheritance of acquired
characters is therefore impossible[43].
Such being the fundamental points of difference between these three
theories of heredity, we have now to consider more particularly those
which obtain between Galton’s and Weismann’s.
The general doctrine of gemmules (i. e. somatic-cell-germs) is
accepted by Galton; but instead of supposing, as Darwin supposed, that
these minute bodies freely circulate through all the body tissues,
so that some of them are absorbed from all the somatic-cells by
the germ-cells, and there constitute the entire mass of hereditary
material out of which the offspring will afterwards be formed, Galton
supposes that gemmules circulate with comparative difficulty, and that
only comparatively few of them gain access to the germ-cells in each
generation. Hence, characters acquired in the individual lifetime
are much less heritable than those which are called congenital. For
congenital characters are due to the “continuity” of stirp through
numberless generations in the phylogeny of the organism; hence such
characters are represented by a vastly greater number of equivalent
hereditary elements. Weismann, on the other hand, rejects the doctrine
of gemmules _in toto_.
Again, according to Galton’s view, “individual [congenital] variation
depends upon two factors; the one is the variability of the germ[44]
and of its progeny; the other is that of all kinds of external
circumstances, in determining which out of many competing germs, of
nearly equal suitability, shall be the one that becomes developed.
The variability of germs under changed conditions, and that of their
progeny, may be small, but it is indubitable; absolute uniformity being
scarcely conceivable in the condition and growth, and, therefore, in
the reproduction of any organism. The law of heredity goes no further
than to say, that like _tends_ to produce like; the tendency may be
very strong, but it cannot be absolute[45].”
Here, of course, there is a wide difference between stirp and
germ-plasm. For while Galton does not entertain amphimixis among
the “factors” of congenital variation, Weismann, as we are now well
aware, has hitherto regarded it as the sole cause of such variation.
Nevertheless, as we shall presently find, Weismann has now greatly
modified his views upon this point, and does entertain, in _The
Germ-plasm_, both the “factors” mentioned by Galton. Hence, the
difference between the two theories in question with regard to this
matter is not nearly so wide as it was prior to the publication of
Weismann’s last work.
The next most important point of difference between the theories of
stirp and germ-plasm has reference to the mechanism of ontogeny.
According to Galton, this is simply a struggle between all the
carriers of heredity composing the stirp of a fertilized ovum. It
is not, however, a struggle for existence, but what may be called a
struggle for development. In the fertilized ovum all the carriers
of heredity are, to begin with, in a “latent” condition; but of
this enormous multitude of “germs” or “gemmules,” only a very small
proportional number are destined to become “patent”—i. e., developed
into the tissue-cells composing the new organism. The vast majority
of the gemmules, or those which fail to be thus developed, go to
constitute the stirp of the new organism when this has been formed by
the development of the comparatively few successful gemmules. Thus much
understood, the following quotation will be fully intelligible.
My argument is this: Of the two groups of germs, the one consisting
of those that succeed in becoming developed and in forming the bodily
structure, and the other consisting of those that remain continually
latent, the latent vastly preponderates in number. We should expect
the latent germs to exercise a corresponding predominance in matters
of heredity, unless it can be shown that, on the whole, the germ that
is developed into a cell becomes thereby more fertile than if it had
remained latent. But the evidence points the other way. It appears
both that the period of fertility is shorter, and the fecundity even
during that period is less in the germ that becomes developed into a
cell, than they are in the germ that remains latent. Much less then
would the entire bodily structure, which consists of a relatively
small number of these comparatively sterile units, successfully
compete in matters of heredity with the total effect of the much more
numerous and more prolific units which are in a latent form[46].
Thus, Galton’s theory of the mechanism of geny is a theory of
struggle; and this constitutes a point of difference on which Weismann
lays much stress in his latest work. For, as we know, Weismann regards
the mechanism of ontogeny as characterized by a peaceful succession
of “stages,” which are “pre-determined from the germ onwards”; and
in his latest work this idea of orderly sequence has been further
elaborated in his doctrine of “determinants.” In short, to adopt
their own metaphors, while Galton tells us that the mechanism of
ontogeny is like that of a political election, where rival candidates
compete to “represent” the nation (stirp) in Parliament (individual
organism); Weismann likens it to the mechanism of a well-drilled army,
where ultimate carriers of heredity (privates) are banded together
in companies, regiments, battalions, &c., under the command of
corresponding officers (determinants).
Lastly, there is yet one further point of difference between stirp and
germ-plasm, which is thus stated by Weismann:—
Galton’s idea is only conceivable on the presupposition of the
occurrence of sexual reproduction, while the theory of the continuity
of the germ-plasm is entirely independent of any assumption as to
whether each primary constituent is present in the germ _singly_
or in numbers. According to my idea, the active and the reserve
germ-plasm contain precisely similar primary constituents, gemmules,
or determinants; and on this the resemblance of a child to its
parent depends. The theory of the continuity of the germ-plasm, as I
understand it, is not based on the fact that each “gemmule” necessary
for the construction of the soma is present many times only, so that
a residue remains from which the germ-cells of the next generation
may be formed: it is founded on the view of the existence of a
special adaptation, which is inevitable in the case of multicellular
organisms, and which consists in the germ-plasm of the fertilized
egg-cell becoming doubled primarily, one of the resulting portions
being reserved for the formation of germ-cells[47].
These being the main points of difference between the theories of stirp
and of germ-plasm to which Professor Weismann has alluded, I will now
proceed to consider them separately, in reverse order to that in which
they have been here stated.
The point of difference last mentioned need not detain us long, because
it seems to me one of very little importance. “Whether each primary
constituent is present in the germ singly or in numbers” cannot greatly
signify, so long as both theories agree that, sooner or later, they
must be present plurally. Galton supposes them to be thus present
from the first (i. e. in the unfertilized ovum), while Weismann
supposes them to be so only as a result of their self-multiplication
at a somewhat later stage (i. e. in the segmenting ovum, and onwards
throughout the procreative life of the individual). Doubtless Weismann
does not suppose that they ever become so numerous as Galton imagines;
but the whole question is so highly speculative that I do not see how
any useful purpose can be served by debating it. Nor do I see why
Weismann should conclude that “Galton’s idea is only conceivable on the
presupposition of the occurrence of sexual reproduction.” It is true
that Galton has discussed exclusively the case of sexual reproduction;
but I cannot perceive that any of his ideas are inapplicable to
a-sexual.
Touching the question whether the phenomena of ontogeny had best
be ascribed to a competition among a vast number of “germs,” or to
a strictly ordered evolution of a comparatively small number of
“determinants,” a considerable array of arguments might be adduced
in support of either view. Thus, Galton might well maintain that
his interpretation of the observable facts is most in accordance
with the general analogies supplied by organic nature as a whole.
The ancient aphorism of Heraclitus, “Struggle is the father, king,
and lord of all things,” has been in large measure justified by
Darwin and his followers, at any rate within the range of biology.
Not only have we the “struggle for existence” where “the origin of
species” is concerned; but Roux has well argued, in his remarkable
work on _Der Kampf der Theile im Organismus_, that the principle of
“struggle” is concerned to an equally important extent as between all
the constituent parts of the same individual. But if this is so—if
every tissue-cell of the organism owes its maintenance to success
in a general contest for nutriment, &c.,—do we not find at least a
probability that it owes its origin as a visible cell to a similar
success in a similarly general contest among the invisible elements
from which tissue-cells are developed? Nay, does it not seem well
nigh incredible that when this selection-principle is seen to be the
governing cause of evolution everywhere else, it should cease to play
any part at all just at the place where we are unable to see what is
going on? As we are agreed that this “father of all things” is of prime
importance in phylogeny—to say nothing of physiology, psychology, and
sociology,—must we not deem it absurd to suppose that it is supplanted
in ontogeny by the opposite principle of absolute peace?
On the other hand, Weismann adduces many forcible considerations
_per contra_; so that, in the result, I deem it best to dispose of
the question with two general remarks. The first is, that the rival
views are not necessarily incompatible. Each may present one aspect
of the truth. Weismann’s doctrine of determinants may be—and, to
the best of my judgement, must be—sound; but this does not hinder
that Galton’s doctrine of struggling “germs” may be so likewise.
For, as we have already seen, these germs present the same compound
character which belong to determinants; in fact I do not suppose
that Galton would object to identifying them with determinants. On
the other hand, I do not see why Weismann should object to supposing
that similar determinants compete among themselves for ontogenetic
development. Indeed, he has already argued, in his suggestive theory of
“germ-tracts,” that it is usually only one among a number of similar
determinants which does succeed in achieving such development—or, as
he expresses it, which “becomes active.” But what is it that causes
this activity? Surely it must be some superiority on the part of the
active determinant over its passive companions. And, if so, it is the
selection-principle that is here at work. In fact, he has himself laid
no small stress on what he calls “the struggle of the determinants of
the two parents in ontogeny,” and has even supplied a long section on
“the Struggle of the Ids in Ontogeny.” Therefore I do not see why he
should so emphatically dissent from Galton’s view upon this matter as
he does in his work on _The Germ-plasm_[48].
My second remark is a brief one—viz., that the whole question is of
so very speculative a character, that I cannot see the smallest use in
debating it.
The only remaining point of difference between strip and germ-plasm
is the one referring to stability. Needless to say, Galton is at one
with Weismann in recognizing a high degree of stability on the part
of the substance of heredity; but the agreement extends only so far
as is necessitated by the facts of atavism, &c. Indeed, he does not
even mention—although he perhaps implies—what Weismann has called
amphimixis as among the factors of individual congenital variation.
Weismann, on the other hand, has hitherto regarded amphimixis as the
sole cause of all such variations. But, as we shall presently find,
in his recent work on _The Germ-plasm_ he has now greatly modified
his views upon this subject, and, in fully recognizing the “factors”
of variability to which Galton alludes, has correspondingly lessened
the difference between germ-plasm and stirp. But this is a point
which can be better dealt with when we come to consider the important
modifications which in this respect the theory of germ-plasm has
undergone.
* * * * *
The only other matter which has to be mentioned in connexion with
Weismann’s theory of heredity is, that in _The Germ-plasm_ he has for
the first time given us his views upon the influence of a previous sire
on the progeny of a subsequent one by the same dam. The phenomena in
question, which I have already detailed in pp. 77-9, 110, he designates
by the term “telegony.” The analogous phenomena in plants he calls,
following Focke, “xenia.”
With regard to telegony, he adopts, almost precisely, the position
which I surmised that he would. That is to say, he first disputes
the alleged facts, and then argues that, even if they be facts, they
admit of being explained on the theory of germ-plasm by supposing that
some of the germ-plasm from the first sire penetrates the unripe ova
which are afterwards fertilized by the second[49]. The only difference
between his views and my own upon this matter is, therefore, as follows.
Supposing that the phenomena alleged ever occur in fact, I have said
that the only way of explaining them would seem to be, “that the life
of ‘germ-plasm’ is not conterminous 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
ontogeny of the embryo[50].”
Now, this is substantially the position which Weismann takes up: only
instead of supposing that it is the “carriers of heredity” of the first
sire which gain access to the unripe ovum “after the disintegration
of their containing spermatozoa,” he supposes that it is one of the
spermatozoa which does so before its disintegration has commenced. Of
course there is here no difference in principle, but only a question
touching the mode in which the access is presumably effected. But,
as regards this question, I retain my original opinion. For, while I
can see no theoretical difficulty in supposing that “the carriers of
heredity,” when set free by the disintegration of their containing
spermatozoa, may reach the unripe ova while still embedded in the
depths of the ovary, I do see a difficulty, amounting almost to a
physiological impossibility, in supposing that a whole spermatozoon
can perform such a feat. From all that we know about the powers and
functions of spermatozoa in the vertebrata, it appears simply absurd to
imagine that these bodies are able to penetrate the dense coating of an
ovary, and then delve their way through the stroma. There is, indeed, a
remarkable investigation which was published a year or two ago by Mr.
Whitman[51] which appears to prove that in certain leeches the male
injects his seminal fluid into any part of the body of the female,
and that the spermatozoa then reach the ova by wandering about her
general tissues until some of them happen to hit upon her ovary. But in
this case the spermatozoa are specially adapted to perform such acts
of penetration—being spear-like bodies provided with a sharp point.
Hence, if Weismann should quote this instance, it would not tend to
support his view, seeing that the spermatozoa of mammals do not exhibit
any such specializations of structure; and therefore, before any one of
them can effect fertilization, must wait for the ovum to mature, reach
the surface of the ovary, and rupture its follicle.
But, as already observed, it does not signify, so far as we are here
concerned with the matter, in what precise manner the telegonous
influence may be supposed to be exercised—provided that it may be so
_directly_, and not necessarily through first having to influence the
whole material organism. Therefore I quite agree with Weismann that
the facts—supposing them to be facts—are quite as explicable by the
theory of germ-plasm as by that of pangenesis[52].
Again, with respect to xenia, Weismann writes:—
As such eminent botanists as Focke, and more recently De Vries, have
expressed much doubt with regard to these observations—or rather
interpretations—we must wait until these cases have been critically
re-investigated before attempting to account for them theoretically.
The chief difficulty we should meet with in any such explanation would
be due to the fact that we are here concerned with the influence of
the _germ-plasm_ of the sperm-cell on a tissue of another plant which
only constitutes _a part_ of this plant. It would thus be necessary to
assume that all the determinants of this germ-plasm are not active,
and that only those take effect which determine the nature of the
fruit.
Now, it does not appear that De Vries has looked into the matter on
his own account, as he merely refers to what Focke has said. And this
amounts merely to showing the dubious character of some half-dozen
cases which Focke gives as those which alone have fallen within his
cognizance. Why he does not mention any of the numerous cases which are
quoted by Darwin, I do not understand. Nor can I understand why he does
not consider what seem to be the particularly conclusive facts given on
p. 80,—i. e., where xenia appears to constitute “a needful preliminary
to fertilization.” But the whole matter is one for botanists to deal
with, and if any doubt attaches to it, at least the grounds of such
doubt should be fully stated. Still more, in my opinion, should the
matter be freed from any such doubt. The question—if there be a
question—is one of great interest from a merely physiological point
of view, while in relation to the fundamental problems of heredity
its importance is immense. Surely, then, any competent botanist who
disputes the facts ought to test them by way of experiment.
But, be this as it may, I must call prominent attention to the
following very remarkable words wherewith Weismann concludes the
passage above quoted. For he there says, that even supposing there
were no doubt as to the facts or their interpretation, “the chief
difficulty” which they would oppose to the theory of germ-plasm
would be, “that we are here concerned with the influence of the
_germ-plasm_ of the sperm-cell on a tissue of another plant which only
constitutes _a part_ of this plant.” In other words, Weismann now
freely entertains the possibility of a direct action of germ-plasm on
the somatic tissues, even though these belong to another individual!
Thus he now concedes the only point for the establishment of which I
adduced the phenomena of xenia, in Chapter III: the whole of one side
of that “reciprocal action between the sphere of germinal-substance
and the sphere of body-substance,” which I contended for on pp. 76-85,
is now conceded; and although it is the less important side, its
surrender goes far to weaken the doctrine of a perpetual isolation
of germinal-substance to a “sphere” of its own. If we suppose that
the germinal substance of one organism may thus directly act upon the
somatic tissues of another, and that changed conditions of life are
able to produce simultaneously an acquired character in the soma and a
precisely identical character as congenital in the germ (pp. 129-30),
we are plainly inviting ourselves to abandon the complex explanation
of living material in “two kinds,” where one is capable in all sorts
of ways of communicating with the other, while the possibility of any
reciprocal action is excluded. For the simpler hypothesis of living
material as all of one kind encounters no such antinomies. So long
as one kind of this material was supposed to be as distinct from the
other as a parasite is distinct from its host, there was not so much to
choose between the theory of germ-plasm and that of gemmules in this
respect of simplicity. But the more that the former theory has had to
be adjusted to facts, the greater has its complexity become, until now
its own author is obliged to make so many additional assumptions for
the purpose of mantaining it, that we begin to wonder how long it can
continue to support the weight of its accumulating difficulties.
* * * * *
So much for the main modifications which have this year been made in
Weismann’s postulate of the perpetual continuity of germ-plasm. We
must next consider the changes which he has effected in his companion
postulate of the absolute stability of germ-plasm.
_Weismann’s Theory of Evolution (1893)._
Of far more importance than any of the alterations which Professor
Weismann has recently made in his theory of heredity, are those
whereby he has modified his sequent theory of evolution. For while,
as we have just seen, his work on _The Germ-plasm_ leaves the former
theory substantially unaltered,—although largely added to in matters
of detail,—it so profoundly modifies the latter that careful readers
will find no small difficulty in ascertaining how much of it has been
allowed to remain. I will consider only the main modifications, and
these I will take separately.
* * * * *
It will be remembered that one distinctive feature in Weismann’s theory
of evolution has hitherto been, that the unicellular organisms differ
from the multicellular in the following important particulars.
1. There being no division in unicellular organisms between germ-cells
and somatic-cells, there is no possibility in them of the occurrence of
amphimixis.
2. Consequently, there is no possibility in them of congenital
variations, in the sense that these occur in multicellular organisms.
3. Hence the only causes of individual variation and of the origin
of species in the unicellular organisms are the Lamarckian factors,
just as in the multicellular the only cause of these things is natural
selection.
4. Hence, also, the unicellular organisms are potentially immortal,
while the multicellular have acquired mortality for certain adaptive
reasons.
But now, with the exception of No. 4, all these positions have been
abandoned. For, chiefly on account of the beautiful researches of
Maupas, Weismann has come to perceive that no real distinction can be
drawn between an act of sexual union in the multicellular organisms,
and an act of conjugation in the unicellular. Amphimixis, therefore,
is now held by him to occur equally in both these divisions of organic
nature, with the consequence that the Protozoa and Protophyta owe their
individual variations, and therefore the origin of their innumerable
species, as exclusively to the action of natural selection as is the
case with the Metazoa and Metaphyta. In fact, the term “amphimixis” has
been coined in express relation to these very points.
It will be seen, however, that this important change of view merely
postpones the question as to the origin of amphimixis, if the object of
this process be that which Weismann supposes—viz., the providing of
material in the way of congenital variations on which natural selection
can act. Therefore he is obliged to assume that there now are, or once
have been, organisms of a less organized character than even the lowest
of the unicellular forms—organisms, that is to say, which possess no
nucleus, but are wholly composed of undifferentiated bioplasm. These
most primitive organisms it must have been that were not subject to any
process of natural selection, but, in virtue of an exclusive action of
the Lamarckian factors upon their protoplasmic substance, gave rise to
individual variations which subsequently gave rise to a unicellular
progeny—when the process of natural selection was immediately
inaugurated, and thereafter entirely superseded the Lamarckian factors.
Or, to state the matter in Weismann’s own words:—
My earlier views on unicellular organisms as the source of individual
differences, in the sense that each change called forth in them
by external influences, or by use and disuse, was supposed to be
hereditary, must therefore be dismissed to some stage less distant
from the origin of life. I now believe that such reactions under
external influences can only obtain in the lowest organisms which are
without any distinction between nucleus and cell-body. All variations
which have arisen in them, by the operation of any causes whatever,
must be inherited, and their hereditary individual variability is due
to the direct influence of the external world.... If I am correct in
my view of the meaning of conjugation as a method of amphimixis, we
must believe that all unicellular organisms possess it, and that it
will be found in numerous low organisms, in which it has not yet been
observed[53].
It is not very clear, at first sight, how Professor Weismann, after
having thus abandoned the propositions 1, 2, and 3, as above stated,
manages to retain his former view as given in No. 4. Nevertheless
he does so, by representing that a unicellular organism, even
though it present such a considerable degree of organization as we
meet with in the higher Protozoa, still resembles a germ-cell of a
multicellular organism, in that it consists of all the essential
constituents of a germ-cell, including germ-plasm in its nucleus. And
inasmuch as a germ-cell is potentially immortal, so it must be with
a unicellular organism; in the one case, as in the other, the design
of the structure is that its contained germ-plasm shall fuse with
the germ-plasm contained in the nucleus of another individual cell,
when the life of both will be preserved. For my own part, however, I
cannot see that in either case the _cell_, as distinguished from its
contained _germ-plasm_, is thus shown to be potentially immortal. On
the contrary, it appears to me a mere accident of the case that in a
unicellular organism the immortal substance (germ-plasm) is contained
in a single cell, which is at the same time a _free_ cell, and, as
such, is denominated an “organism.” We might just as well call a
germ-cell an “organism,” whether as an ovum it happens to be embedded
in a mass of somatic-cells, or as a locomotive spermatozoon it happens
to be free. In fact Weismann himself appears to recognize this. But,
if so, it is surely a distinction without a difference to say that
unicellular organisms are immortal, while multicellular are mortal.
For in neither case is the organism immortal, while in both cases it
is the germ-plasm (i.e., the substance of heredity) that is so. Where
the cell containing the germ-plasm happens to be a free cell, it is
called an “organism”; but whether it be a germ-cell or a protozoan,
it alike ceases to be a cell when it has given origin to a multitude
of other cells, whether these happen to be other germ-cells (_plus_
somatic-cells) or other protozoan cells. In short, _quâ_ cell, all
cells are mortal: it is only the substance of heredity which some cells
contain that can be said, in any sense of the term, to be immortal. For
the immortality in question does not belong to unicellular organisms
as such, but to the germ-plasm which they contain. And from this it
follows that, as the immortality of germ-plasm is one and the same
thing as the continuity of germ-plasm, by alleging an immortality as
belonging to the unicellular organisms, Weismann is merely restating
his fundamental postulate. Hence, also, he is but denying, in a
somewhat round-about way, the occurrence of spontaneous generation.
I conclude, therefore, that his sole remaining distinction between the
unicellular and the multicellular organisms is but illusory, or unreal.
And, with regard to the great change which he has thus effected in his
system by expressly abolishing all the other distinctions, I have only
to say that in my opinion he has thereby greatly improved his system.
For he has thus relieved it of all the formidable difficulties which he
had needlessly created for himself, and which I have already enumerated
in the foregoing pages (88-89). In his ever-shifting drama of evolution
the unicellular organisms have left the stage _en masse_, and, so far
as they are concerned, we are all as we were before the curtain rose.
* * * * *
But of even more importance than this fundamental change of view
with regard to the unicellular organisms, is a further and no less
fundamental change with regard to the multicellular. That such is the
case will immediately become apparent by a simple statement of the
fact, that Weismann has now expressly surrendered his postulate of the
absolute stability of germ-plasm!
We have already seen that, even in the first volume of his _Essays_,
there were some passages which gave an uncertain sound with regard to
this matter. But as they seemed attributable to mere carelessness on
the part of their author, after quoting a sample of them, I showed
it was necessary to ignore such inconsistent utterances—necessary,
that is, for the purpose of examining the theory of germ-plasm as
even so much as a logically coherent system of ideas[54]. For we have
seen that if any doubt were to be entertained touching the _absolute_
stability of germ-plasm “since the first origin of sexual propagation,”
a corresponding measure of doubt would be cast on Weismann’s theory of
congenital variation as _solely_ due to amphimixis, with the result
that his whole theory of evolution would be similarly rendered dubious.
Since then, however, he has gone very much further in this direction.
First, in reply to Professor Vines he says (1890):—
I am at present inclined to believe that Professor Vines is correct
in questioning whether sexual reproduction is the only factor which
maintains Metazoa and Metaphyta in a state of variability. I could
have pointed out in the English edition of my “Essays” that my views
on this point had altered since their publication; my friend Professor
de Bary, too early lost to science, had already called my attention to
those parthenogenetic Fungi which Professor Vines justly cites against
my views; but I desired, on grounds already mentioned, to undertake no
alteration in the essays[55].
Next, in his essay on _Amphimixis_ (1892), there are several passages
to somewhat the same effect; while, lastly, in his _Germ-plasm_ (1893),
the fundamental postulate in question is, as I have said, expressly
surrendered. For example, we have in the following words the final
conclusions of his recent arguments. Speaking of amphimixis, he says:—
_It is not the primary cause of hereditary variation._ By its means
those specific variations which already exist in a species may
continually be blended in a fresh manner, but it is incapable of
giving rise to new variations, even though it often appears to do
so.... _The cause of hereditary variation must lie deeper than this.
It must be due to the direct effects of external influences on the
biophores and determinants._[56]
These quotations are enough to show that Weismann has now abandoned
his original theory of congenital variations being exclusively due to
amphimixis, and adopts in its stead the precisely opposite view—viz.,
that the origin of all such variations must be ascribed to the direct
influence of causes acting on germ-plasm from without. Up to the
present year the very essence of the whole Weismannian theory of
evolution has been that, owing to the stability of germ-plasm since
the first origin of sexual propagation, “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,” because “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[57].”
But about the beginning of the present year we have this fundamental
doctrine directly contradicted in such words as:—
_The origin of a variation_ is equally independent of selection
and amphimixis, and is due to the constant occurrence of slight
inequalities of nutrition in the germ-plasm[58].
This complete reversal of his previous doctrine brings Weismann into
line with Darwin, who long ago gave very good reasons for the following
conclusion:—
Those authors who, like Pallas, attribute all variability to the
crossing either of distinct races, or to distinct individuals
belonging to the same race but somewhat different from each other, are
in error; as are those authors who attribute all variability to the
mere act of sexual union [_amphimixis_][59].
And again:—
These several considerations alone render it probable that variability
of every kind is directly or indirectly caused by changed conditions
of life. Or, to put it under another point of view, if it were
possible to expose all the individuals of a species during many
generations to absolutely uniform conditions of life, there would be
no variability[60].
Hence, Darwin was disposed to find the main, if not the only, causes of
congenital variations in circumstances depending for their efficacy on
the _instability_ of what Weismann calls germ-plasm. And the noteworthy
fact is, that Weismann has now adopted this view, to the destruction
of his originally fundamental postulate touching the _stability_ of
germ-plasm since the first origin of sexual propagation.
* * * * *
By such a right-about-face man[oe]uvre, Weismann has placed his
critics in a somewhat difficult position. For, in the first place, it
is only towards the close of _The Germ-plasm_ that the man[oe]uvre
is executed, and then only in a few sentences such as I have just
quoted—italicized, it is true, but otherwise so slightly emphasized
that, as Professor Hartog has observed, no one of his reviewers has
noticed it[61]. In the second place, he nowhere expressly recognizes
the effects upon his theory of evolution, which necessarily follow
from the change. And, lastly, the manner in which he endeavours to
underpin that theory after having thus removed its logical foundation
in his former postulate of the absolute stability of germ-plasm, is so
peculiar that it is hard to epitomize his reasoning with due regard to
brevity.
Speaking for myself, I can only say that my first impulse, after
reading the sentences above quoted, was to cancel the whole of Chapter
IV, as well as all those parts of Chapters I and III where the
Weismannian theory of evolution is alluded to; and then to start anew
with a bare statement that this theory had now been wholly discarded by
its author. But after due consideration it seemed desirable to leave
the criticism as it was originally written, not only on account of the
reasons already stated in the Preface, but still more because I found
it would be impracticable to start a new criticism of the greatly
modified theory of evolution without introducing many and lengthy
parts of the old one, for the purpose of showing how the most recent
theory had been arrived at. Hence, seeing that my previous criticism
was far from having been rendered obsolete by the large changes which
had taken place in Weismann’s system of theories, I concluded that it
was best to retain what I had written, and add the present paragraphs
for the purpose of dealing exclusively with the changes in question.
In now proceeding to do this, I think it is needless to occupy space
by giving the reasons which have caused Weismann thus to abandon his
doctrine of the universal stability of germ-plasm since the first
origin of sexual propagation, and to substitute the precisely opposite
doctrine of its universal instability. It is enough to say that
these reasons all arise by way of logical necessity from the further
working out in _The Germ-plasm_ of his theory of heredity—or, more
correctly, from the additions which he has there made to his previous
views on the mechanism of heredity. Thus he has reversed his former
doctrine touching the absolute stability of germ-plasm, not so much
on account of any of the general considerations or particular facts
which I have adduced against it in Chapter IV, as because it would
not tally with the recent additions which he has made to other parts
of his system. Any one who cares to follow this matter will find the
reasons in question fully and lucidly stated in Chapter XIV of _The
Germ-plasm_[62].
It is of more importance to consider the means whereby Weismann
seeks to save his theory of evolution after he has thus removed its
foundation in his former postulate of the absolute stability of
germ-plasm. As far as I can understand, he seeks to do so as follows.
In the first place, it must be noted that after his changes of
view with regard to polar bodies, unicellular organisms, and the
significance of sexual reproduction, nothing remains of his original
theory of evolution save what he can manage to retain of his original
theory of variation as due to amphimixis. But, as we have just seen,
he has surrendered this latter theory also. Therefore, at first sight
it appears that no part of the former can possibly remain. Beginning
at the apex, he has removed, stone by stone, his doctrine of descent,
and, on arriving at its fundamental postulate—the absolute stability
of germ-plasm—simply turns it upside down. Surely, therefore, it may
be thought, there is here as complete a destruction as well could be
of all this side of Weismann’s system. Such, however, he endeavours to
show is not the case. He regards it as still possible to retain so much
of his theory of descent as is presented by what he can save of his
theory of variation, thus:—
Although he now represents that the _instability_ of germ-plasm
is such that in _no_ case can amphimixis have had anything to do
with the _origin_ of congenital variations, he continues to regard
the _stability_ of germ-plasm sufficiently great to necessitate, in
_all_ cases, the occurrence of amphimixis in order to promote the
_development_ of congenital variations. In other words, notwithstanding
that he now thinks all congenital variations must be begun by
external conditions acting directly on an unstable germ-plasm, he
also thinks that the _amount_ of variation thus produced is likely to
be exceedingly minute, and must therefore be increased by subsequent
amphimixis in order to fall within the range of natural selection. So
that, although powerless to initiate congenital variation, amphimixis
must still play an indispensable part in the process of evolution,
as in all cases a necessary condition to the occurrence of natural
selection. External conditions first cause slight changes in the
determinants of a species; but these are so slight that they have to
be augmented by amphimixis before they constitute material on which
natural selection can act, and hence before they can become of any
significance either in ontogeny or phylogeny.
Such, I take it, is what Professor Weismann would now have us to
understand; for otherwise I should have expected from him as frank a
surrender of his theory of evolution (or the remnant thereof in his
theory of variation) as he has made of its fundamental postulate. But,
if such is his meaning, I may mention the reasons which appear to me to
render it nugatory.
In the first place, it is evident that in thus minimizing the possible
range of congenital variation due to the action of external conditions
on a non-absolutely stable substance of heredity, Weismann is making
a wholly gratuitous assumption, for the sole purpose of saving what
remains of his theory of evolution—i. e., the doctrine of the immense
importance of amphimixis.
We have already seen in the foregoing chapter that his original
assumption of the _absolute_ stability of germ-plasm was a gratuitous
one, made for the purpose of supplying a foundation for constructing
his theory of evolution. But still more gratuitous is the assumption
which he has now substituted, for the purpose of saving as much of this
theory as is left—the assumption, namely, that germ-plasm, although
universally unstable, nevertheless everywhere presents only a certain
low degree of instability, which serves to accommodate his modified
theory of heredity on the one hand, and all that is possible of his
previous theory of evolution on the other. His original assumption,
untenable though it was, furnished at least a logical basis for the
necessary conclusion that amphimixis was the only possible cause of
congenital variations. But there is not so much as any logical sequence
in the now substituted assumption, that (A) all congenital variations
are ultimately due to the _universal instability_ of germ-plasm, and
(B) that nevertheless they are all more proximately due to such _a
high degree of stability_ of germ-plasm as necessitates amphimixis as
the only means whereby variations can be made “perceptible.” These
statements are as independent of one another as any two statements can
well be; and, therefore, if the second of them is to be substantiated,
it can only be so by some totally distinct line of reasoning. The first
statement does not even tend to suggest the second; in fact it tends
to suggest the precise contrary. For, obviously, there is nothing in
the logic of the matter to show why, if all congenital variations
depend for their origin on the instability of germ-plasm, such
instability must nevertheless be always so slight that the variations
due to it must afterwards depend on amphimixis for their development
to the point where they become “perceptible.” As above indicated, it
is surely little short of absurd thus to assume that a universally
unstable germ-plasm universally presents only that particular degree
of instability which will serve to accommodate Professor Weismann’s
newer theory of heredity, and at the same time to save thus much of his
previous theory of evolution.
But now, in the second place, not only is this assumption wholly
gratuitous, but there are many considerations which render it in the
highest degree improbable, while there are not wanting facts which
appear to demonstrate that it is false. For, unquestionably, most of
the considerations which have already been advanced in the preceding
chapter against the assumption of an absolute stability of germ-plasm,
are here equally available against the assumption of an imperceptibly
small amount of instability[63]. Similarly, all the facts there given
with regard to the a-sexual origin of species—and even genera—of
parthenogenetic organisms, bud-variation[64], &c., amply demonstrate
that congenital variations due to the instability of germ-plasm alone,
or apart from amphimixis, are sometimes enormous. Hence, we cannot
accept the gratuitous suggestion that in all other cases they are too
insignificant to count for anything till they have been augmented by
amphimixis, even although we may be prepared to agree that amphimixis
is probably one important factor in the production of congenital
variations. What degree of importance it presents in this connexion,
however, we have not at present any means of determining; all we can
conclude with certainty is, that in some cases it is demonstrably very
much less than Weismann supposes, while it is extremely improbable
that it is ever in any case the sole and necessary antecedent to the
operation of natural selection.
This extreme improbability is shown, not only by what I have already
said in the previous chapter, and need not here repeat; but likewise
by the “several considerations” which Darwin has adduced with regard
to this very point, and which, as he says, “alone render it probable
that variability of every kind is directly or indirectly caused by
changed conditions of life,” with the consequence that “those authors
who attribute all variability to the mere act of sexual union are in
error.” I have already quoted these words further back in the present
chapter, in order to show that by now attributing the _origin_ of all
congenital variations to the direct action of external conditions,
Weismann has brought himself into line with Darwin so far as this
fundamental point of doctrine is concerned. But I here re-quote the
words in order to show that by further attributing the _development_ of
congenital variations “to the mere act of sexual union,” Weismann is
again falling out of line with Darwin. So to speak, he first performs a
right-about-face movement as regards his original position towards the
“stability of germ-plasm,” and immediately afterwards makes a half-turn
back again. Now, it is this half-turn to which I object as unwarranted
in logic and opposed to fact.
In a previous chapter (pp. 66-7) I presented to him the dilemma,
that germ-plasm must be either absolutely stable or else but highly
stable, and that in the former case his theory of amphimixis as the
sole cause of congenital variations would be valid, while in the
latter case the theory would collapse. But it did not then occur to
me that Weismann might seek a narrow seat between the horns of this
dilemma, by representing that germ-plasm is universally unstable up to
a certain very low degree of instability—viz., exactly that degree
which is required for starting a congenital variation by means of
external causes, without its being possible for the variation to become
perceptible unless afterwards increased by means of amphimixis. And now
that this extremely sophistical position has been adopted, I cannot
see any imaginable reason for adopting it other than a last endeavour
to save as much as possible of his former theory of evolution. There
can be nothing in the nature of things thus to limit, within the
narrowest possible range, the instability of a universally unstable
germ-plasm—distributed, as this most complex of known substances
is, throughout all species of plants and animals, and exposed to
inconceivably varied conditions of life in all quarters of the globe.
And these considerations are surely of themselves enough to dispose
of the assumption as absurd, without again rehearsing the facts of
congenital variation which definitely prove it to be false.
_Conclusion._
For reasons stated at the commencement of this chapter, I have
restricted its subject-matter almost exclusively to a consideration
of the more fundamental changes which Professor Weismann has wrought
in his general system of theories by the publication of his most
recent works. In other words, I have purposely avoided considering
those immensely elaborate additions to his theory of heredity which
constitute by far the largest portion of his essays on _Amphimixis_ and
_The Germ-plasm_, and which have for their object an ideal construction
of “the architecture of germ-plasm.”
The fundamental changes to which allusion has just been made are as
follows.
Professor Weismann has to a large extent abandoned his theory of polar
bodies, and in my opinion would have done well had he taken a further
step and surrendered the theory _in toto_.
Similarly, he has withdrawn his previous distinctions between the
unicellular and multicellular organisms. The Protozoa and Protophyta
are now included by him in the same category as the Metazoa
and Metaphyta, as regards all matters of individual variation,
reproduction, subjection to the law of natural selection, and so forth.
The only difference which he continues to allege is the somewhat
metaphysical one touching mortality and immortality. But I have
given what appears to me sufficiently good reasons for ignoring this
distinction; and therefore, as it seems to me, every one of Weismann’s
previous doctrines respecting unicellular organisms have vanished—very
much to the benefit of his system as a whole.
By far the greatest change, however, which he has made in this general
system is that which he has effected by surrendering the postulate
of the absolute stability of germ-plasm. The rift in his lute which
has been noticed with regard to this matter has now been widened to
an extent which _does_ prevent any further harping on the theme of
evolution. It is true that Weismann endeavours to retain as far as
possible the general character of his former postulate of the universal
stability of germ-plasm, with the consequent “significance of sexual
reproduction” as the sole cause of congenital variation. For although
he now reverses both these doctrines by saying that germ-plasm is
universally unstable, and that sexual reproduction is in no case
the sole cause of congenital variation, he seeks at the same time
to minimize the logical consequences of such reversal by making an
ingenious assumption, the possibility of which I had not foreseen
when writing the previous chapters. The assumption is, that although
germ-plasm is universally unstable, the degree of its instability is
everywhere restricted within the narrowest possible limits; so that
sexual propagation is still necessary for the purpose of _developing_
congenital variations to the point where they can fall within the
range of natural selection, notwithstanding that they must all have
been _originated_ by external causes acting directly on a germ-plasm
universally unstable within the narrow limits assumed. But clearly
this assumption is arbitrary to the last degree, and, no less clearly,
it is made by Weismann for the sole purpose of saving as much as he
can of his previous theory of variation. His more recent speculations
touching the mechanism of heredity are incompatible with his former
view of amphimixis as the _sole_ cause of congenital variations,
and therefore he makes this arbitrary assumption for the purpose of
representing that amphimixis may nevertheless still be regarded as a
_necessary con-cause_. I need not here repeat what has so recently
been said touching the sophistry of this assumption in theory, or
the demonstrable falsity of it in fact. It is enough to remark, in
conclusion, that the game is not worth the candle. It was originally
well worth Weismann’s while to sustain his fundamental postulate of
the _absolute_ stability of germ-plasm, because he was able to rear
upon it his whole theory of evolution. But the only part of this theory
which he has now left standing, or which he can now save by his newer
postulate of a germ-plasm both stable and unstable at the same time, is
his doctrine of variation. So to speak, it is his desire to reserve
as much as is speculatively possible from the general ruin of his
theory of descent, that causes him to go so far to attempt so little.
For I cannot suppose that he himself will expect any of his readers to
entertain so arbitrary, fanciful, and demonstrably false an assumption
as the one in question. Surely it would have been better to have
surrendered _in toto_ this “Weismannian theory of variation,” rather
than to have attempted its rescue by means so plainly nugatory. It
might still have been held that amphimixis plays a large and important
part as one of the causes of variation, and therefore also as one of
the factors of organic evolution. After having reversed his postulate
of amphimixis being the sole cause of variability, and therefore having
agreed with Darwin that “those writers are in error who attribute
all variability to the mere act of sexual union,” he might well have
questioned Darwin’s further statement as to its being “probable that
variability of every kind is directly or indirectly caused by changed
conditions of life.” But by now assuming that variations due to any
causes other than amphimixis must be “imperceptible” until they
have been augmented by amphimixis, Weismann is shutting out, with a
futile hypothesis, the important question as to whether, or how far,
amphimixis really is a cause of variation. Observe, the case is not as
it might have been were there no reasons assignable for the occurrence
of sexual propagation, other than that of assisting in the production
of congenital variations. The theory of “rejuvenescence,” for example,
is _prima facie_ a more probable one than that which ascribes to sexual
propagation the function of causing variability[65]; while Galton’s
hypothesis, which supposes the object of this form of propagation to be
that of conserving the “germs” (= “determinants”) of the phyla, has a
good deal to say for itself[66]. Of course such alternative hypotheses
touching “the significance of sexual reproduction” are not necessarily
exclusive of one another: the process may subserve two or more adaptive
purposes[67]. But he would be a bold man who, in the present state of
our knowledge, could accept unreservedly the particular view of this
process which Darwin so emphatically rejected; and I think he must be a
biased man who could entertain for an instant the modification of this
view which Weismann has now substituted.
Thus, the Weismannian theory of evolution has entirely fallen to pieces
with the removal of its fundamental postulate—the absolute stability
of germ-plasm. It only remains to mention once more the effects of
this removal upon the other side of his system—viz., the companion
postulate of the uninterrupted continuity of germ-plasm, with its
superstructure in his theory of heredity.
Briefly, these effects are as follows:—
1. Germ-plasm ceases to be continuous in the sense of having borne a
perpetual record of congenital variations from the first origin of
sexual propagation.
2. On the contrary, as all such variations have been originated by the
direct action of external conditions, the continuity of germ-plasm in
this sense has been interrupted at the commencement of every inherited
change during the phylogeny of all plants and animals, unicellular as
well as multicellular.
3. But germ-plasm remains continuous in the restricted, though still
highly important sense, of being the sole repository of hereditary
characters of each successive generation, so that acquired characters
can never have been transmitted to progeny “representatively,” even
although they have frequently caused those “specialized” changes in the
structure of germ-plasm which, as we have seen, must certainly have
been of considerable importance in the history of organic evolution.
4. By surrendering his doctrine of the _absolute_ stability of
germ-plasm on the one hand, and of its _perpetual_[68] continuity on
the other, Weismann has greatly improved his theory of heredity. For,
whatever may be thought of his recent additions to this theory in
the way of elaborate speculation touching the ultimate mechanism of
heredity, it is a great gain to have freed his fundamental postulate
of the continuity of germ-plasm from the two further postulates which
have just been mentioned, and the sole purpose of which was to provide
a basis for his untenable theory of evolution.
5. In my opinion it only remains for him to withdraw the last remnant
of his theory of evolution by cancelling his modified and even less
tenable views on amphimixis, in order to give us a theory of heredity
which is at once logically intact and biologically probable.
6. The theory of germ-plasm would then resemble that of stirp in all
points of fundamental importance, save that while the latter leaves
the question open as to whether acquired characters are ever inherited
in any degree, the former would dogmatically close it, chiefly on the
grounds which I have considered in Appendix II. It seems to me that in
the present state of our knowledge it is more prudent to follow Galton
in suspending our judgement with regard to this question, until time
shall have been allowed for answering it by the inductive methods of
observation and experiment.
7. Hence, in conclusion, we have for the present only to repeat what
Weismann himself has said in one of the wisest of his utterances,—“The
question as to the inheritance of acquired characters remains, whether
the theory of germ-plasm be accepted or rejected.”
It is now close upon twenty years that I accepted the substance of this
theory under the name of stirp; and since that time the question as to
the inheritance of acquired characters remains exactly where it was.
No new facts, and no new considerations of much importance, have been
forthcoming to assist us in answering it. Therefore, as already stated
in the Preface, I intend to deal with this question hereafter as a
question _per se_, or one which is not specially associated with the
labours of Professor Weismann.
APPENDIX I:
ON GERM-PLASM.
AS already stated in the text (p. 71), Weismann’s general reasoning in
support of his own theory of germ-plasm, as against Darwin’s theory of
gemmules in any form, admits of being reduced to arguments in favour
of three propositions—viz., first, that there is no evidence of the
transmission of somatogenetic characters; secondly, that the theory
of pangenesis, which seeks to explain their supposed transmission,
is “inconceivable”; and, thirdly, that its logical antithesis—the
theory of germ-plasm—is so much less beset with difficulties, that
by comparison it is simple, self-coherent, and offers a real, as
distinguished from a “formal,” explanation of the facts of heredity.
The first of these propositions will be discussed at considerable
length in my next volume. The second and third propositions, however,
may be dealt with here.
The following paragraph, which I shall quote sentence by sentence,
sets forth the grounds on which Weismann bases the second proposition,
namely, that any theory belonging to the order of pangenesis—i.
e., which supposes the carriers of heredity ever to travel
centripetally—is, from its very nature, inconceivable.
At first sight this hypothesis seems to be quite reasonable. It is
not only conceivable that particles might proceed from the somatic to
the reproductive cells, but the very nutrition of the latter at the
expense of the former is a demonstration that such a passage actually
takes place. But a closer examination reveals immense difficulties.
In the first place, the molecules of the body devoured are never
simply added to those of the feeding individual without undergoing
any change, but, as far as we know, they are really assimilated, that
is, converted into the molecules of the latter. We cannot therefore
gain much by assuming that a number of molecules can pass from the
growing somatic cells into the growing reproductive cells, and can be
deposited unchanged in the latter, so that, at their next division,
the molecules are separated to become the somatic cells of the
following generation[69].
The obvious answer to this is, that no one has ever supposed “gemmules”
to be merely “_molecules_,” in the chemical sense of this word; nor
has any one ever imagined that they are “_devoured_” by the germ-cells
into which they pass. Of course, if this were the case—i.e., if
gemmules serve merely as _food_ to the germ-cells—they would become
disintegrated down even to their chemically molecular structure, and
there would be an end of them as organized “carriers of heredity.”
In the second place, it is asked:—
How can such a process [i.e. the passage of gemmules into growing
germ-cells] be conceivable, when the colony becomes more complex, when
the number of somatic cells becomes so large that they surround the
reproductive cells with many layers, and when at the same time, by an
increasing division of labour, a great number of different tissues
and cells are produced, all of which must originate _de novo_ from a
single reproductive cell?
Here, again, the obvious answer is, that no one has ever propounded
such a statement. Far from supposing that “all the different cells
and tissues of a complex organism must originate _de novo_ from a
_single_ reproductive cell,” the theory of pangenesis supposes the very
contrary—viz., that somatic changes in the past history of the phyla
have _not_ thus originated in _any_ reproductive cell. The idea of
somatic changes originating in reproductive cells belongs to the theory
of _germ-plasm_; but even this theory does not suppose all the great
number of different cells and tissues which compose a complex organism
to have ever originated _de novo_ from a _single_ reproductive cell.
The difficulty touching germ-cells becoming isolated, or buried, by the
phylogenetic increase of somatic cells, is enforced in the immediately
succeeding sentences, thus:—
Each of these various elements [somatic cells] must, _ex hypothesi_,
give up certain molecules to the reproductive cells; hence those which
are in immediate contact with the latter would obviously possess an
advantage over those which are more remote. If, then, any somatic cell
must send the same number of molecules to each reproductive cell[70],
we are compelled to suspend all known physical and physiological
conceptions, and must make the entirely gratuitous assumption of an
affinity on the part of the molecules for the reproductive cells.
Even if we admit the existence of this affinity, its origin and means
of control remain perfectly unintelligible if we suppose that it
has arisen from differentiation of the complete colony. An unknown
controlling force must be added to this mysterious arrangement, in
order to marshal the molecules which enter the reproductive cell in
such a manner that their arrangement corresponds with the order in
which they must emerge as cells at a later period.
Now I do not see much force in the suggestion that those somatic
cells which happen to be in immediate contact with germ-cells, “must
obviously possess an advantage over those which are more remote.”
On the contrary, I do not see that mere proximity of one species of
cell to another species within the same organism need have anything
to do with the matter—still less that “we must suspend all physical
and physiological conceptions,” if we demur to the statement that it
“obviously must.” As for “physical conceptions,” how many thousands of
cases might not be pointed to among chemical and mechanical processes
where contact or proximity are conditions of comparatively little
importance? And as for “physiological conceptions,” do we find that
any part of the organism is affected by its distance, say, from the
liver and kidneys, for getting rid of its effete products? Is it not
rather the case that every gland in the body is wholly unaffected by
its distance from any part of the body, in regard to its function of
draining off the particular substances with which it is concerned?
Why then should the reproductive gland constitute a conspicuous
exception? Or how do we suspend all physiological conceptions, if
we suppose that this gland resembles every other gland in being
specialized to _secrete_ a particular kind of “molecule,” which,
because thus specially _selected_, may be said to have for that gland
a special “affinity”? If there are such things as gemmules, I do not
see any violation of physiological analogies—still less an “entirely
gratuitous assumption”—in supposing that they can be filtered out from
all parts of the body by the sexual glands, and there aggregated as a
special product to be discharged in the form of sexual elements[71].
But, it is further represented, “even if we admit the existence of
this affinity, an unknown controlling force must be added to this
mysterious arrangement, in order to marshal the molecules which enter
the [growing] reproductive cell in such a manner that their arrangement
corresponds with the order in which they emerge as cells at a later
period.” Surely, however, for Weismann of all naturalists it ought
not to be difficult to find this “unknown controlling force.” For of
all naturalists he is perhaps the most ready to invoke the agency
of natural selection as sufficient to explain every case—actual
or imaginable—of _adaptation_. Now, here is a case where natural
selection, one would think, is positively bound to act—supposing that
there be such things as gemmules. For, if “the carriers of heredity”
are gemmules, it is evident that their mutual “affinities” must be
adaptively “marshalled” at each step of phylogenetic evolution, before
any further advance of such evolution can be possible. And I do not
see anything more “inconceivable” in supposing the establishment of
such mutual affinities step by step through natural selection, than in
supposing any other course of adaptive development by similar means.
For, as Darwin has well shown, while anticipating this particular
objection to his theory,—“The assumed elective affinity of each
gemmule for that particular cell which precedes it in due order of
development is supported by many analogies.” The analogies which he
then gives are so numerous that I must here refer to his own discussion
of the subject[72]—a discussion which is entirely ignored by Weismann.
Lastly, the principal ground, as far as I can see, which Weismann has
for regarding Darwin’s theory in any shape “inconceivable,” is his
own supposition that there is as complete an anatomical separation
between the soma and its germ-cells as there is, for example, between
the mammalian soma and these same cells when afterwards detached from
the ovary and developing as foetuses _in utero_. In other words, the
only connexion is supposed to be that of deriving nourishment by way
of imbibition. But, as regards the germ-cell while still forming in
the ovary or testicle, there is for this supposition no basis in
fact. There is nothing in the histology of spermatogenesis that lends
countenance to the supposition, while in the case of the ovum such
histological evidence as we possess makes altogether against it. As
Professor Vines has remarked:—
It cannot be seriously maintained that the whole body of the embryo
is developed solely from the germ-plasm of the ovum. On the contrary,
since the embryo is developed from the whole of the nucleus and
more or less of the cytoplasm of the ovum, it must be admitted
that the non-germ-plasm of the ovum provides a large part of the
material in embryogeny. It is an obvious inference that, under these
circumstances, hereditary characters may be transmitted from the
parent to the offspring, not only by the germ-plasm, but also by the
somato-plasm, of the ovum[73].
Again, and apart from this consideration, it is now known that a very
intimate network of protoplasmic fibres connects the cell-contents of
cellular tissues, both in plants and animals. So that here we have
another very possible means of communication between the germ-cells and
the somatic-cells which together constitute a multicellular organism.
Therefore, in so far as histology can be trusted to constitute a basis
for generalizations of this kind at all, it does not sustain the
supposition that there can be no medium of communication between the
general cellular tissues of an organism and its specially reproductive
elements. On the contrary, the microscope is able to demonstrate
possible roads of connexion—and this even upon Weismann’s own view as
to a specialized germinal substance which is restricted to the nucleus
of an ovum. In short, the supposition as to an absolute anatomical
separation between germ-plasm and somato-plasm is a deduction from
Weismann’s theory itself: it is not supported—it is discredited—by
histological observation. Hence, it cannot be accepted as valid
evidence in favour of the theory from which alone it is derived, or as
a valid objection to the rival theory of pangenesis.
Once more, even if it were true that histology proves an absolute
anatomical isolation on the part of germ-cells, it would still have
remained unquestionable that there is no absolute _physiological_
isolation. For, at least, the germ-plasm derives its nourishment from
the soma in which it resides; and who shall say that the process of
mere imbibition is not amply sufficient to admit of the passage of
“gemmules”? Call them what we choose, the “carriers of heredity” must
be so unimaginably small, that in relation to histological cells they
must be as gnats to camels. Yet we know that even camels in the form
of “migrating cells” of various kinds are able to pass through living
membranes; and we also know that the microbes of syphilis can penetrate
both ova and spermatozoa. Why then should it be deemed inconceivable
that, where all such things can pass, gemmules can do so likewise?
Lastly, I have recently spoken of the detached condition of a ripe ovum
_in utero_. Now it seems to me more “inconceivable” that such an ovum
should be capable of announcing, as it were, to the walls of the uterus
whether or not it is in a fertilized condition, than it is that, before
quitting the ovary, it should have had some kind of physiological
converse with its environing soma. Yet it is certain that, without any
visible medium of communication, the impregnated ovum is able to inform
the uterus that it is impregnated; and thereupon the uterus behaves
towards that ovum in an altogether astonishing manner, such as it never
displays towards an unimpregnated ovum. Of course various hypotheses
may now be formed to account for this fact, seeing that no one can
question it as a fact. But supposing that the fact could be questioned,
with how much greater effect might it be argued that any communication
between the ovum and its soma is even more antecedently incredible when
the ovum is entirely free than when it is still contained within its
ovary.
Now these, as far as I can find, are the only grounds for Weismann’s
repeated assertion that the theory of pangenesis in any form is
“inconceivable.” I have therefore endeavoured to show that this is
too strong a statement. All the facts and considerations whereby he
seeks to support it were present to the mind of Darwin; and, quite
apart from any question of relative authority, I cannot avoid agreeing
with Darwin that, whether or not the theory is true, at all events the
“difficulties” attaching to it on these merely _a priori_ grounds are
not insuperable, or such as to render his “pet child” an unconceived
monstrosity in logic, or a proved absurdity in science.
Be it understood, however, that I am not here defending the theory
of pangenesis. I am investigating the theory of germ-plasm; and it
is because Weismann seeks to sustain the latter by excluding the
former as preposterous, that I have been obliged thus to consider the
validity of his criticism. For the point to which I am leading is,
that Weismann gains nothing in the way of support to his own theory by
this disparagement of Darwin’s, _unless he can show that the former
supplies some more “conceivable” explanation touching the mechanism
of heredity_. Now I am unable to see that he has shown this. What I
do see is that his _a priori_ argument from “inconceivability” cuts
both ways, and that it makes at least as much against germ-plasm
as it does against gemmules. Therefore, having now considered what
Weismann has said against the conceivability of gemmules on grounds of
general reasoning, I shall proceed to show that quite as much—or even
more—may be said in the way of a _tu quoque_. In other words, we have
now finished with the second of the three propositions which we are
examining (see p. 71), and proceed to our consideration of the third.
First of all, I do not see any greater difficulty in supposing that
the “carriers of heredity” proceed centripetally from somatic-cells to
germ-cells, than in supposing that they proceed centrifugally from the
germ-cells to the somatic-cells which they are engaged in constructing.
Nor do I see any more difficulty in imagining these “carriers of
heredity” to be capable of constructing a new organism if they have
first proceeded centripetally, and are thus severally representative
of all parts of the parent organism _after its construction has been
completed_, than I do if they have proceeded centrifugally, and are
thus similarly representative of all parts of that organism _before its
construction has been commenced_[74].
Similarly, it seems to me, whatever cogency there may be in Weismann’s
objection to Darwin’s theory on the score that it must assume “an
unknown controlling force in order to marshal the molecules,” is
equally great as regards his own. True, Weismann has a lot to say about
the control which nucleo-plasm can exercise on cell-formation, and
germ-plasm on marshalling successive stages of ontogeny; but all that
this amounts to is a re-statement of the facts. Such a controlling
force must be equally assumed by both theories; but in each alike there
is an absence of any ghost of an explanation.
Again, whatever difficulty there may be in conceiving the transition
of somatic substance, _mutatis mutandis_ there must be an equal
difficulty in conceiving the transition of germinal substance into
somatic substance. Indeed, as far as I can see, the difficulty is
even greater in the latter case than it is in the former. For the
very essence of Weismann’s view is that germ-plasm differs from all
or any other “plasm” in origin or kind: germ-plasm, and germ-plasm
alone, has been immortal, perpetually continuous, capable of indefinite
self-multiplication, and so of differentiating itself into an endless
number and variety of somatic tissues. But, according to Darwin’s view,
there is not, and never has been, any such fundamental difference
between the essential nature of somatic elements, and the essential
nature of sexual elements. On the contrary, it is supposed that
both formative and formed material are one in kind—that all the
cellular tissues of a multicellular organism, like the single cell of
a unicellular organism, are _per se_ endowed with the vital property
of self-multiplication; and that whether this property finds its
expression in normal growth, in abnormal increments of growth (such
as tumours), in processes of repair, in the various forms of a-sexual
reproduction, or in the more specialized form of sexual fertilization,
there is everywhere an exhibition of one and the same capacity. Now,
without going further than this contrast between the fundamental
principles of the two theories, does it not become evident that the
difficulty of conceiving a transition of A into A´ is at any rate no
greater than that of conceiving a transition of A into B, where A is in
both cases the formative substance, A´ this same substance in another
stage of evolution (i.e., elaborated for the performance of some
special function, but never so as to lose its original function A),
while B is a substance which differs from A almost as much as a woven
texture differs from the hands that weave it?
Once more, in all his arguments which are directed to prove the
continuity of germ-plasm, Weismann nowhere seems to perceive the
necessity of arguing the correlative hypothesis—viz., that of the
discontinuity of somato-plasm. Yet, as Professor Vines has remarked,
it is as incumbent on him to disprove any possible continuity on the
part of somato-plasm, as it is to prove a perpetual continuity on the
part of germ-plasm. And here I am disposed to go further than Professor
Vines has gone; for it appears to me even _more_ incumbent on Weismann
to argue a discontinuity on the part of somato-plasm, than it is on him
to argue a continuity on the part of germ-plasm.
This must be immediately apparent if we remember that, unless the
discontinuity of somato-plasm be assumed, the theory of the continuity
of germ-plasm in telluric time (as distinguished from eternity) becomes
identical in form with all those theories of heredity to the family
of which pangenesis belongs. All these theories go upon the assumption
that living material has been continuous in telluric time—i.e.,
always derived from pre-existing material of the same kind; but they
embody the further assumption that _all_ living material _is_ material
of the same kind—i.e., everywhere presents the same fundamental
properties. Weismann’s theory on the other hand, while adopting the
first assumption, rejects the second; and assumes in its stead that
living material exists in “two kinds,” only one of which has been
continuous, while the other is discontinuous—being, in fact, formed
anew at each ontogeny. Therefore, to my mind, it seems more needful to
argue the point wherein his theory differs from these other theories of
heredity, than it is to argue the point wherein it agrees with them. We
look to him for a proof of the discontinuity of somato-plasm much more
than we do for a proof of the continuity of germ-plasm. Now the only
proof that he has to give of the discontinuity of somato-plasm—or, in
other words, that the self-multiplication of somatic cells cannot take
place unless the nucleus of each contains a self-multiplying idio-plasm
derived from the nucleus of a germ-cell—is the non-transmissibility of
somatogenetic characters. Here, however, there is an obvious equivoque.
For his only test of characters as somatogenetic and blastogenetic
consists in observing whether or not they are inherited: if they are
inherited, he says they are blastogenetic: if they are not inherited,
he says they are somatogenetic. But this is manifestly circular
reasoning, so long as the question in debate is as to the truth of his
theory. What we require in proof of the distinguishing feature of that
theory—i.e., the discontinuity of the hypothetical somato-plasm—is
not merely the obvious fact that some characters are inherited while
others are not, but independent proof that inherited and non-inherited
characters correspond to a continuity of germ-plasm on the one hand,
and a discontinuity of somato-plasm on the other. He shows us,
indeed, what was well known before, that characters developed during
the lifetime of the individual are seldom (if ever) inherited, while
characters developed during the lifetime of the species are always
inherited. Obviously, however, this fact is no proof of the assumed
correlation just mentioned, because, as Darwin has clearly pointed out,
it may very well be due to the much shorter time which has been allowed
for what may be termed the impress of heredity. Therefore, supposing
(with Darwin and others) that living material is all of one kind, and
continuous, the fact on which Weismann relies admits of being explained
without resorting to his more complex supposition of living material in
two kinds, the one perpetually continuous, and the other interrupted at
each ontogeny.
For these reasons it appears to me that, so far as the argument from
“inconceivability” is concerned, it makes at least as much against
the theory of germ-plasm as it does against the theory of pangenesis;
and, therefore, that no argumentative advantage is gained from its
use by Weismann. The truth probably is that, _whatever_ the mechanism
of heredity may actually be, it is at once so minute and so complex
that its action is “inconceivable,” or, more correctly, unimaginable.
Be it again understood, therefore, that I am not arguing in favour
of pangenesis. I am merely criticising what appears to me an unsound
argument in favour of germ-plasm. All this general or merely _a priori_
reasoning with regard to inconceivability is, as I have attempted to
show, as available on the one side as on the other, and so fails to
yield any observable advantage to either.
* * * * *
In conclusion it must be noticed, that Weismann now appears to have
himself perceived the grave difficulties which lie against his
antithesis between a hypothetical “germ-plasm” and a hypothetical
“somato-plasm,” notwithstanding that the former becomes converted
into the latter at each ontogeny. At any rate, he allows that Vines’
criticism upon this head is sound. But he is strongly of the opinion
that, by means of a later emendation of his theory as originally
published, he has succeeded in obviating these difficulties _in toto_.
For my own part, as already several times observed in the text, I
cannot in the least perceive that such is the case; and therefore I
will quote _in extenso_ what he has said in answer to Professor Vines.
It will be seen that his newer emendation of the theory consists in
substituting for his original “somato-plasm” two substances, which
are called respectively “somatic idio-plasm” and “cytoplasm.” And it
is by means of this substitution that he thinks he has, in some way
or another, overcome the contradiction involved in the doctrine (and,
as it still seems to me, the essential doctrine of his whole theory
of heredity) that “germ-plasm” becomes converted into “somato-plasm”
during the course of every ontogeny. The following, at any rate, is his
latest utterance upon the subject:—
I believe that the objections which Professor Vines makes to my theory
of the continuity of germ-plasma rest solely on an unintentional
confusion of my ideas, as he compares the opinions expressed in
the second essay with those of the later ones, with which they do
not tally. I will endeavour to make this clear. In this second
essay (1883) I contrasted the body (soma) with the germ-cells, and
explained heredity by the hypothesis of a “Vererbungs-substanz”
in the germ-cells (in fact the germ-plasma), which is transmitted
without breach of continuity from one generation to the next. I was
not then aware that this lay only in the nucleus of the ovum, and
could therefore contrast the entire substance of the ovum with the
substance of the body-cells, and term the latter “somato-plasm.” In
Essay IV (1885) I had arrived, like Strasburger and O. Hertwig, at the
conviction that the nuclear substance, the chromatin of the nuclear
loops, was the carrier of heredity, and that the body of the cell was
nutritive but not formative. Like the investigators just named, I
transferred the conception of idio-plasm, which Nägeli had enunciated
in essentially different terms, to the “Vererbungs-substanz” of
the ovum-nucleus, and laid down that the nuclear chromatin was the
idio-plasm not only of the ovum but of every cell, that it was the
dominant cell-element which impressed its specific character upon
the originally indifferent cell-mass. From then onwards, I no longer
designated the cells of the body simply as “somato-plasm,” but
distinguished, on the one hand, the idio-plasm or “Anlagen-plasma” of
the nucleus from the cell-body or “Cytoplasma,” and, on the other, the
idio-plasm of the ovum-nucleus from that of the somatic cell-nucleus;
I also for the future applied “germ-plasm “to the nuclear idio-plasm
of ovum and spermatozoon, and “somatic idio-plasm” to that of the body
cells (e.g., p. 184). The embryogenesis rests, according to my idea,
on alterations in the nuclear idio-plasm of the ovum, or “germ-plasm”;
on p. 186, et seq., is pictured the way in which the nuclear
idio-plasm is halved in the first cell-division, undergoing regular
alterations of its substance in such a way that neither half contains
all the hereditary tendencies, but the one daughter-nucleus has
those of the ectoblast, the other those of the entoblast; the whole
remaining embryogenesis rests on a continuation of this process of
regular alterations of the idio-plasm. Each fresh cell-division sorts
out tendencies which were mixed in the nucleus of the mother-cell,
until the complete mass of embryonic cells is formed, each with a
nuclear idio-plasm which stamps its specific histological character on
the cell.
I really do not understand how Professor Vines can find such
remarkable difficulties in this idea. The appearance of the sexual
cells generally occurs late in the embryogeny; in order, then, to
preserve the continuity of germ-plasm from one generation to the next,
I propound the hypothesis that in segmentation it is not _all_ the
germ-plasm (i. e., idio-plasm of the first ontogenetic grade) which is
transformed into the second grade, but that a minute portion remains
unaltered in one of the daughter-cells, mingled with its nuclear
idio-plasm, but in an inactive state; and that it traverses in this
manner a longer or shorter series of cells, till, reaching those cells
on which it stamps the character of germinal cells, it at last assumes
the active state. This hypothesis is not purely gratuitous, but is
supported by observations, notably by the remarkable wanderings of the
germinal cells of Hydroids from their original positions.
But let us neglect the probability of my hypothesis, and consider
merely its logical accuracy. Professor Vines says:—“The fate of the
germ-plasm of the fertilized ovum is, according to Professor Weismann,
to be converted in part into the somato-plasm (!) of the embryo, and
in part to be stored up in the germ-cells of the embryo. This being
so, how are we to conceive that the germ-plasm of the ovum can impress
upon the somato-plasm (!) of the developing embryo the hereditary
character of which it (the germ-plasm) is the bearer? This function
cannot be discharged by that portion of the germ-plasm of the ovum
which has become converted into the somato-plasm (!) of the embryo,
_for the simple reason that it has ceased to be germ-plasm_, and must
therefore have lost the properties characteristic of that substance.
Neither can it be discharged by that portion of the germ-plasm of the
ovum which is aggregated in the germ-cells of the embryo, for under
these circumstances it is withdrawn from all direct relation with the
developing somatic-cells. The question remains without an answer.” I
believe myself to have answered this above. I do not recognize the
somato-plasm of Professor Vines; my germ-plasm, or idio-plasm of the
first ontogenetic grade, is not modified into the somato-plasm of
Professor Vines, but into idio-plasm of the second, third, fourth,
hundredth, &c. grade, and every one impresses its character on the
cell containing it.
It may be dullness, but I confess that this does not appear to me an
“answer” to Professor Vines’ criticism. Even though “idio-plasm of
the first ontogenetic grade” has to become “idio-plasm of the second,
third, fourth, hundredth, &c. grade,” before in each of the grades
concerned it can give origin to the somatic-cells which are distinctive
of that grade, I cannot see that it makes any difference (in relation
to Vines’ criticism) whether we speak of those cells as containing
“somato-plasm,” or as containing “somatic idio-plasm” of such and such
a grade, _plus_ “cytoplasm.” For whether we thus follow Weismann’s
earlier terminology or his later, we are so far speaking about exactly
the same thing, namely, the transformation of “germ-plasm” into all
the constituent cells of the “soma.” The difficulty is, in Vines’
words above cited, “to conceive that the germ-plasm of the ovum can
impress upon the somato-plasm of the developing embryo the hereditary
characters of which it (the germ-plasm) is the bearer”; and Weismann
says that this difficulty, which he acknowledges, can now be answered
by substituting for his original statement that “germ-plasm” becomes
changed into “somato-plasm,” the statement that it is “idio-plasm”
_derived_ from “germ-plasm” which thus “impresses its character on the
cell containing it.” But, “as a matter of logical accuracy,” there
is surely here a distinction without a difference. For what is the
difference between saying that germ-plasm “impresses” its character
on the contents of _all_ somatic cells considered collectively under
the term “somato-plasm,” and saying that every “ontogenetic grade”
of germ-plasm “impresses” _its_ character on _each_ successive group
of somatic cells considered severally under the term “idio-plasm” of
such and such a grade? At best this newer terminology has reference
merely to a superadded hypothesis touching the _mode_—or rather the
_history_—of the transition in question: it does not affect the
original and essential doctrine of the transition itself.
APPENDIX II:
ON TELEGONY.
A WIDELY different view, however, is taken by Mr. Herbert Spencer with
regard to the theoretical interpretation of telegony. This, indeed,
is precisely the opposite view to the one which is given in the text.
For while I agree with Professor Weismann in holding that the facts
of telegony (supposing them to be facts) are as compatible with the
theory of germ-plasm as with that of gemmules, “physiological units,”
or any other theory which postulates a centripetal flow of the carriers
of heredity from somatic-cells to germ-cells, Mr. Spencer is of the
opinion that these facts are destructive of any theory which postulates
a continuity in the substance of heredity—i.e., a centrifugal flow
of the carriers of heredity. And, unquestionably, Mr. Spencer’s view
is the prevalent one. Therefore, seeing that his opinion is not only
of weight _per se_, but is shared by the scientific world in general,
I will here transcribe a somewhat lengthy discussion which I have
recently held with him upon the subject.
In the _Contemporary Review_ for March, Mr. Spencer wrote as follows:—
We pass now to evidence not much known in the world at large, but
widely known in the biological world, though known in so incomplete
a manner as to be undervalued in it. Indeed, when I name it probably
many will vent a mental pooh-pooh. The fact to which I refer is
one of which record is preserved in the museum of the College of
Surgeons, in the shape of paintings of a foal borne by a mare not
quite thoroughbred, to a sire which was thoroughbred—a foal which
bears the markings of the quagga. The history of this remarkable foal
is given by the Earl of Morton, F.R.S., in a letter to the President
of the Royal Society (read November 23, 1820). In it he states that
wishing to domesticate the quagga, and having obtained a male, but not
a female, he made an experiment.