WEISMANN’S THEORY OF HEREDITY (1891).
WE now proceed to examine Weismann’s theory of germ-plasm, and as this
in its various developments has now become a highly complex theory, we
had best begin by marking out the lines on which the examination will
be conducted.
As I have already pointed out, the Weismannian system is not concerned
only with the physiology of reproduction: it is concerned also—and
in an even larger measure—with the doctrine of descent. The theory
of germ-plasm as a whole is very much more than a theory of heredity;
it is a new theory of evolution. The latter, indeed, is deduced from
the former; but although the two are thus intimately related, they
are nevertheless not mutually dependent. For the relationship is such
that the new theory of evolution stands upon the basis supplied by
the new theory of heredity, and although it follows from this that
if the latter were disproved the former would collapse, it does not
follow that if the former were to be found untenable the latter must
necessarily be negatived. Hence, for the sake of clearness, and also
for the sake of doing justice to both theories, we had best deal
with them separately. The present chapter, then, will be devoted to
examining Weismann’s theory of heredity, while the ensuing chapter will
be concerned with his sequent theory of evolution.
Again, Weismann’s theory of heredity stands on his fundamental
postulate—the continuity of germ-plasm; and also on a fact well
recognized by all other theories of heredity, which he calls the
stability of germ-plasm. But his sequent theory of evolution stands
not only on this fundamental postulate, and on this well-recognized
fact; it requires for its logical basis two further postulates—viz.,
that germ-plasm has been _perpetually_ continuous “since the first
origin of life,” and _unalterably_ stable “since the first origin of
sexual propagation.” That these things are so, a very few words will be
sufficient to prove.
Any theory of heredity which supposes the material of heredity to
occupy a more or less separate “sphere” of its own, is not obliged
further to suppose that this material has _always_ been thus isolated,
or even that it is now _invariably_ so. There have been one or two
such theories prior to Weismann’s, and they were founded on the
well-known fact of congenital characters being at any rate _much more_
heritable than are acquired characters. But it has not been needful
for these theories to assume that the “continuity” thus postulated
has been _perpetually_ unbroken. Even if it has been frequently to
some extent interrupted, all the facts of _heredity_ could be equally
well comprised under such theories—and this even if it be supposed
that acquired characters are but rarely, or never, transmitted to
progeny. For, in as far as the continuity may have been interrupted,
it does not follow that the acquired characters (body-changes),
which by hypothesis caused the interruption, must be inherited
by progeny exactly as they occurred in the parents. Or, in other
words and adopting Weismann’s terminology, _so far as the facts of
heredity are concerned_, there is no reason why germ-plasm should not
frequently have had its hereditary qualities modified by some greater
or less degree of commerce with somatic-tissues, and yet never have
reproduced in progeny the identical acquired characters which caused
the modification of germ-plasm in the parents: some other and totally
different characters might with equal—or even more—likelihood have
been the result, as we shall see more clearly a few pages further on.
Why, then, does Weismann so insist upon this continuity of germ-plasm
as _perpetual_ “since the origin of life”? It appears to me that his
only reason for doing so is to provide a basis, not for his theory
of heredity, but for his additional theory of evolution. It is of
no consequence to the former that germ-plasm should be regarded as
thus perpetual, while it is of high importance to the latter that the
fundamental postulate of continuity should be supplemented by this
further postulate of the continuity as thus perpetual.
Similarly as regards the postulate of the stability of germ-plasm as
absolute. It is enough for all the requirements of Weismann’s theory of
heredity that the material basis of heredity should present a merely
_high degree_ of stability, such as the facts of atavism, degeneration,
&c. abundantly prove that it possesses. For his sequent theory of
evolution, however, it is necessary to postulate this stability as
_absolute_ “since the first origin of sexual reproduction.” Otherwise
there would be no foundation for any of the distinctive doctrines
which go to constitute this theory.
It may not be immediately apparent that Weismann’s theory of heredity
is not _per se_ concerned with either of these two additional
postulates of the continuity of germ-plasm as _perpetual_, and the
stability of germ-plasm as _absolute_; while both are logically
necessary to his further theory of evolution. On this account, and
also for the sake of clearness in all that is to follow, we had best
begin by comparing his theory of heredity with those of his principal
predecessors—Darwin and Galton.
* * * * *
For the purposes of this comparison we may start by again alluding to
the fact, that even in the multicellular organisms reproduction is not
confined to the sexual methods. Many kinds of invertebrated animals
will reproduce entire organisms from the fragments into which a single
organism has been chopped: plants of various kinds can be propagated
indefinitely by cuttings, grafts, and buds, or even by leaves, as we
have already observed in Chapter I. Now, when the whole organism is
thus reproduced from a severed portion of somatic-tissue, it reproduces
its sexual elements. Whence, then, in such cases are these elements
derived? Obviously they are not derived immediately from the sexual
organs—or even from the sexual cells—of their parents: they are
derived from the somatic-cells of a single parent, if we choose to
retain this term; and therefore, as Strasburger pointed out soon after
Weismann’s theory was published, it seems as if such facts are in
themselves destructive of the theory. How, then, does Weismann meet
them? As we have already seen in Chapter II, he meets them in the
only way they can be met on the lines of his theory—viz., by those
newer amendments of his theory which suppose that in all these cases
the germ-plasm is _not_ confined to the specially sexual cells, but
occurs also in the nuclear substance of those somatic-cells which thus
prove themselves capable of developing into entire organisms. In other
words, the sexual elements which develop during what I have previously
called this “somatic reproduction” of multicellular organism, are
supposed to be derived from the sexual cells of ancestors, not indeed
immediately (for this they plainly are not), but mediately through
the somatic-tissues of their a-sexual parent. Now, in view of this
extension, the theory of germ-plasm becomes somewhat closely allied
to that of pangenesis. For example, when the fragment of a leaf of
_Begonia_ is laid upon moist soil, there strikes root, and grows a new
_Begonia_ plant capable of sexual reproduction, Darwin supposes the
explanation to be that what he calls “formative material” occurs in all
cells of the leaf, while Weismann supposes the explanation to be that
what he calls “germ-plasm “ occurs in all—or at any rate in most—of
the cells of the leaf. So that, except as regards the terms employed,
the two theories are identical in their mode of viewing this particular
class of phenomena.
Moreover by thus allowing, in his second essay on Heredity, that
germ-plasm need not be restricted to the specially sexual cells, but
in some cases, at any rate[12], may occur distributed in full measure
of reproductive efficiency throughout the general tissues of the
organism, Weismann cannot refrain from taking the further step of
supposing that the germ-plasm, like the gemmules of Darwin, is capable
of any amount of multiplication _in the general cellular tissues of
plants_—seeing that plants can be propagated by cuttings, buds, &c.,
indefinitely. And this, as we have seen, Professor Weismann, in his
second essay, does not shrink from doing. Moreover, although I cannot
remember that he has anywhere expressly said so, it is obvious that the
allied phenomena of regeneration and repair admit of explanation by his
hypothesis of “ontogenetic grades,” after the manner already stated
in Chapter II. Indeed, it is evident that in no other way can these
phenomena be brought within the range of his theory. But from this
it follows that not only in the case of organisms which are capable
of somatic reproduction is the formative nucleo-plasm (idio-plasm-B)
diffused throughout the somatic-tissues: on the contrary, it must be
_universally_ diffused throughout _all_ the somatic-cells of _all_
living organisms; and whether as it there occurs it is capable of
reproducing entire organisms, single organs, single tissues, or a mere
cicatrix, depends only on the “ontogenetic grade” of differentiation
which this diffused nucleo-plasm has (or has not) previously undergone.
Moreover, as we have already seen, at whatever ontogenetic grade of
differentiation it may be present in a given somatic-tissue, it must
there be capable of indefinite self-multiplication. Therefore, in all
these respects this “formative nucleo-plasm” (or idio-plasm-B) of
Weismann precisely resembles the “formative material” (or gemmules) of
Darwin.
Lastly, as De Vries has pointed out[13], there must be at least as
many divisions and subdivisions in the substance of germ-plasm, as
there are differences between the somatic organs, tissues, and even
cells, to which germ-plasm eventually gives rise—no matter through
how many ontogenetic grades of idio-plasm it may first have to pass.
Or, in other words, we must accept, as the material basis of heredity,
ultimate particles[14] of germ-plasm, which are already differentiated
into as many diverse categories as there are differences between all
the constituent parts of the resulting soma; for, as shown in the
Appendix, no change in the facts of the case has been shown by simply
changing the original term “germ-plasm” into “idio-plasm,” wherever
the phenomena of ontogeny are concerned. It may be convenient, for
the sake of presenting newer additions to the theory, to restrict the
term “germ-plasm” to “idio-plasm of the first ontogenetic stage”; but
as idio-plasms of all subsequent ontogenetic stages are supposed to
be ultimately derived from this idio-plasm of the first stage, it is
evident that the particulate differences in question must already have
been present in the so-called “undifferentiated idio-plasm of the first
ontogenetic stage.” Unless we are to have a mere juggling with words,
we cannot put into our successive idio-plasms any particles of kinds
differing from those which are contained in the original germ-plasm.
Therefore I say that, notwithstanding this change of terminology,
Weismann must continue to assume, as the material basis of heredity,
ultimate particles of germ-plasm which are already differentiated into
as many diverse categories as there are differences between the parts
of the resulting soma—although, of course, these ultimate particles
need not be nearly so numerous _in each of their categories_ as they
afterwards become by self-multiplication while forming each of the
resulting tissues.
But this is precisely what the theory of pangenesis supposes; so that
I see no reason why these ultimate particles of germ-plasm should
not be regarded as “gemmules,” so far as their _size_, _number_,
and _function_ are concerned. In point of fact, they differ from
gemmules only in respect to their _origin_: they are not particles
derived from somatic-cells of the preceding generation, but particles
derived from germ-plasm of the preceding generation. Or, to state
the difference in another form, if we regard the sexual elements as
constituting the physiological centre of the organism, then the theory
of germ-plasm supposes these ultimate carriers of heredity to originate
at this centre, and then to travel centrifugally; while the theory of
pangenesis supposes them to originate at the periphery, and then to
travel centripetally.
This point of difference, however, arises from the deeper ones,
which—having now exhausted the points of agreement—we must next
proceed to state.
If, as we have seen, “formative material” and “germ-plasm” agree in
being particulate; in constituting the material basis of heredity; in
being mainly lodged in highly specialized, or germinal, cells; in being
nevertheless also distributed throughout the general cellular tissues,
where they are alike concerned in all processes of regeneration,
repair, and a-sexual reproduction; in having an enormously complex
structure, so that every constituent part of the future organism is
already represented in them by corresponding particles; in being
everywhere capable of a virtually unlimited multiplication, without
ever losing their hereditary endowments; in often carrying these
endowments in a dormant state through a number of generations, until
at last they reappear again in what we recognize as reversions to
ancestral characters;—if in all these most important respects the
two substances are supposed to be alike, it may well appear at first
sight that there is not much room left for any difference between them.
And, in point of fact, the only difference that does obtain between
them admits of being stated in two words,—Continuity, and Stability.
Nevertheless, although thus so few in number, these two points of
difference are points of great importance, as I will now proceed
briefly to show.
If the substance which constitutes the material basis of heredity
has been _perpetually continuous_, in the sense of never having
had any of its hereditary endowments in any way affected by the
general body-tissues in which it resides, the following important
consequences, it will be remembered, arise. The process of organic
evolution must have been exclusively due to a natural selection of
favourable variations occurring within the limits of this substance
itself; and therefore the so-called Lamarckian factors can never have
played any part at all in the evolution of any but the unicellular
organisms. On the other hand, if this substance has not been thus
perpetually continuous, but more or less formed anew at each ontogeny
by the general body-tissues in which it resides, natural selection has
probably been in some corresponding degree assisted in its work of
organic evolution by the Lamarckian factors, with the result that the
experiences of parents count for something in the congenital endowments
of their offspring. So much for the first of the two differences
between germ-plasm and gemmules, or the difference which arises from
the perpetual continuity of germ-plasm.
Touching the second difference, or that which arises from the
_absolute stability_ of germ-plasm, it will be remembered how from
this character there arises another important chain of consequences.
Namely, individual variations of the congenital kind can only be due
to admixtures of different masses of germ-plasm in every act of sexual
fertilization; natural selection is therefore dependent, for the
possibility of its working, upon the sexual methods of propagation;
hence, natural selection is without any jurisdiction among the
unicellular organisms, where the Lamarckian factors hold exclusive
sway; and hence, also, the multicellular organisms are ultimately
dependent upon this absolute stability of their germ-plasm for all the
progress which they have made in the past, as well as for any progress
which they may be destined to make in the future.
Thus we see that the two points of difference between germ-plasm and
gemmules are not merely of great importance as regards the particular
problem which is presented by the phenomena of heredity: they are of
still greater importance as regards the general theory of evolution.
For if these two qualities of perpetual continuity and absolute
stability can be proved to belong to the material basis of heredity,
the entire theory of evolution will have to be reconstructed from its
very foundation—and this quite apart from the more special question
as to the transmission of acquired characters. Therefore we shall
presently have to consider these two alleged qualities with the care
that they demand, as having been seriously suggested by so eminent a
naturalist as Professor Weismann. But, before proceeding to do so, I
must briefly compare his theory with that of Mr. Galton.
“Stirp” resembles both “germ-plasm” and “gemmules” in all the respects
which have above been named as common to the two latter. But it differs
from gemmules and further resembles germ-plasm in all the following
particulars. It is derived from the stirp of proceeding generations,
and constitutes the sole basis of heredity. Only a part of it, however,
is consumed in each ontogeny—the residue being handed over to
“contribute to form the stirps of the offspring,” where it undergoes
self-multiplication at the expense of the nutriment supplied to it
from the somatic system of the offspring, and so on through successive
generations. Again, stirp is concerned in all processes of regeneration
and repair, in the same centrifugal manner as germ-plasm is so
concerned. Furthermore, the influence of sexual propagation in the
blending of hereditary qualities of the stirp is recognized, while the
principle of panmixia, or the cessation of selection, is entertained,
and shown to invalidate the evidence of pangenesis which Darwin
derived from the apparently transmitted effects of use and disuse in
our domesticated animals[15]. Lastly, it is clearly stated that on
the basis supplied by this “theory of heredity,” it becomes logically
possible to dispense with the Lamarckian principles _in toto_, leaving
natural selection as the sole known cause of organic evolution through
a perpetual continuity of stirp, together with individual variations of
the same, whether by sexual admixture or otherwise.
So far, then, there is not merely resemblance, but virtual identity,
between the theories of stirp and germ-plasm. Disregarding certain
speculative details, the coincidence is as complete as that between a
die and its impress. But although the two theories are thus similar
in _logical construction_, they differ in their interpretations
of _biological fact_. That is to say, although Galton anticipated
by some ten years all the main features of Weismann’s theory of
heredity[16], and showed that, as a matter of form, it was logically
intact, he refrained from concluding on this account that it must be
the true theory of heredity. He argued, indeed, that in the main it
was probably the true theory; but he guarded his presentation of it
by not undertaking to deny that there might still be some degree of
intercommunication between the material basis of heredity in stirp,
and the somatic tissues of successive organisms. The construction
of a theory which, as a matter of theory, could dispense with the
Lamarckian principles _in toto_, was seen to be a very different
thing from proving, as a matter of fact, that these principles are
non-existent—and this, even though it was seen that a recognition of
the principle of panmixia must be taken to have considerably attenuated
the _degree_ of their operation as previously estimated by Darwin
in the theory of pangenesis. In short, after pointing out that the
doctrine of stirp might very well adopt the position which about a
decade later was adopted by the doctrine of germ-plasm—namely, that
of altogether _supplanting_ the doctrine of gemmules,—Galton allowed
that this could be done only as a matter of formal speculation; and
that, as a matter of real interpretation of the facts of nature, it
seemed more judicious to stop at _modifying_ the doctrine of gemmules,
by provisionally retaining the hypothesis of gemmules, but assigning to
their agency a greatly subordinate _rôle_. Or to quote his own words:—
The conclusion to be drawn from the foregoing arguments is, that we
might almost reserve our belief that the structural [i. e., “somatic”]
cells can react on the sexual elements at all, and we may be confident
that at the most they do so in a very faint degree; in other words,
that acquired modifications are barely, if at all, inherited, in
the correct sense of that word. If they were not heritable, then
the second group of cases [i.e., those of acquired as distinguished
from congenital characters] would vanish, and we should be absolved
from all further trouble; if they exist, in however faint a degree,
a complete theory of heredity must account for them. I propose, as
already stated, to accept the supposition of their being faintly
heritable, and to account for them by a modification of Pangenesis[17].
Seeing, then, that Galton did not undertake to deny a possibly slight
influence of somatic-tissues on the hereditary qualities of stirp, it
follows that he did not have to proceed to those drastic modifications
of the general theory of descent which Weismann has attempted. Stirp,
like germ-plasm, is _continuous_; but, unlike germ-plasm, it is not
_necessarily_ or _absolutely_ so. Again, stirp, like germ-plasm,
is _stable_; yet, unlike germ-plasm, it is not _perpetually_ or
_unalterably_ so. Hence we hear nothing from Galton about our having to
explain the unlikeness of our children to ourselves by variations in
our protozoan ancestors; nor do we meet with any of those other immense
reaches of deductive speculation which, in my opinion, merely disfigure
the republication of stirp under the name of germ-plasm.
Now, I allude to these, the only important points of difference between
stirp and germ-plasm, for the sake of drawing prominent attention to
the fact that it makes a literally immeasurable difference whether we
suppose the material basis of heredity to be _perpetually_ continuous
and _unalterably_ stable, or whether we suppose that it is but
_largely_ continuous and _highly_ stable. In the former case, all the
far-reaching deductions which Weismann draws with reference to the
general theory of descent—or apart from the more special problem of
heredity—follow by way of logical consequence. In the latter case,
there is no justification for any such deductions. For, no matter how
faintly or how fitfully the hereditary qualities of the material in
question may be modified by the somatic-tissues in which it resides,
or by the external conditions of life to which it is exposed, these
disturbances of its absolute stability, and these interruptions of
its perpetual continuity, must cause more or less frequent changes on
the part of its hereditary qualities—with the result that specific
or other modifications of organic types need not have been solely due
to the varying admixture of such material in sexual unions on the one
hand, or to the unassisted power of natural selection on the other.
Numberless additional causes of individual variation are admitted,
while the Lamarckian principles are still allowed some degree of
play. And although this is a lower degree than Darwin supposed, their
influence in determining the course of organic evolution may still have
been enormous; seeing that their action, in whatever measure it may be
supposed to obtain, must always have been _cumulative_ on the one hand,
and _directive_ of variations in adaptive lines on the other. Or, as
Galton himself observes, in the passage already quoted, “if they exist,
_in however faint a degree_, a complete theory of heredity must account
for them.” He saw, indeed, that a most inviting _logical_ system could
be framed by denying that they can ever exist in any degree—or, in
other words, by supposing that stirp was _exactly_ the same as what was
afterwards called germ-plasm, in that it always occupied a separate
“sphere” of its own, where its continuity has been uninterrupted “since
the first origin of life.” But Galton was not seduced by the temptation
to construct an ideally logical system; and he had what I regard as the
sound judgement to abstain from carrying his theory of stirp into any
such transcendental “sphere” as that which is occupied by Weismann’s
theory of germ-plasm, in relation to the general doctrine of descent.
* * * * *
There is, then, a vast distinction between any theory of heredity
which postulates the material of heredity as highly stable and largely
continuous, and Weismann’s theory, which postulates this material as
absolutely stable and perpetually continuous. But we must next take
notice that Weismann himself has not kept this distinction in view
with the constancy which we should have expected from so forcible a
thinker. On the contrary, although in the construction of his theory
of evolution he never fails to press the postulates of _absolute_
stability and _perpetual_ continuity to their logical conclusions in
the various doctrines above enumerated (pp. 57-58), when engaged on
his more special theory of heredity he every now and then appears to
lose sight of the distinction. Indeed, he occasionally makes such large
concessions with regard to both these postulates, that, were they to be
entertained, the occupation of his critics would be gone: his theory
of heredity would become converted into Galton’s, while his theory of
evolution would vanish altogether. It is therefore necessary to quote
some of these concessions, if only to justify ourselves in subsequently
ignoring them. I will give one instance of each; but it is necessary to
preface the illustrations with a few words to mark emphatically three
very distinct cases of congenital variation—leaving aside for the
present the question whether or not they all occur in fact, as they are
held to do by one or other of the theories of heredity.
1. The case where impoverished nutrition of the body has the effect
of simply _starving_ its germinal material. This is not a case
where either the continuity or the stability of such material is
affected. Its full efficiency as “formative material” may indeed be
thus deteriorated to any extent, so that the progeny may be to any
extent puny or malformed; but this will not necessarily cause any
such re-shuffling of its “molecules” as will thereafter result in
a permanent phylogenetic change. At most it will affect only the
immediate offspring of poorly nourished parents; and natural selection
will always be ready to eliminate such inefficient individuals. This
case I will always hereafter call the case of _nutritive_ congenital
changes.
2. The case where germinal material is influenced by causes which
do effect a re-shuffling of its “molecules,” so that a permanent
phylogenetic change does result. Observe, in this case, it does not
signify whether the causes arise from external conditions of life, from
any action of the soma on its own germinal material, or from so-called
“spontaneous” changes on the part of such material itself. But the
one cause which has not been concerned in producing an hereditary
modification of this class is the mixture of “germ-plasms” in an act
of sexual union. In hereafter speaking of this case I will follow
Weismann’s terminology, and call congenital changes thus produced
_specialized_ congenital changes.
3. Lastly, we have the case of the Lamarckian factors. This precisely
resembles case 2, save that the congenital changes produced are still
more “specialized.” For while in the preceding case the re-shuffling
before mentioned may have produced a congenital change of any kind,
in the present case the congenital change produced must be of one
particular kind—viz., a reproduction by heredity of the very same
modification which occurred in the parents. “The fathers have eaten
sour grapes, and the children’s teeth are set on edge.” This would
be an extreme example of “use-inheritance,” and so of case 3. But
if the fathers had eaten sour grapes, and the children, instead of
having their teeth set on edge, were to be born with a wryneck or a
squint, then we should have a good example of case 2. In order, then,
to mark the important distinction between these two cases, I will
hereafter call the highly specialized changes due to the Lamarckian
factors—supposing such changes to be possible—_representative_
congenital changes.
These several distinctions being understood, I will proceed to furnish
the two quotations from Weismann, which are respectively illustrative
of his concessions touching his two fundamental postulates, as
previously explained.
We may fairly attribute to the adult organism influences which
determine the phyletic development of its descendants. For the
germ-cells are contained in the organism, and the external
influences which affect them are intimately connected with the state
of the organism in which they lie hid. If it be well nourished, the
germ-cells will have abundant nutriment; and, conversely, if it be
weak and sickly, the germ-cells will be arrested in their growth. It
is even possible that the effects of these influences may be more
specialized; that is to say, they may act only upon certain parts
of the germ-cells. But this is indeed very different from believing
that the changes of the organism which result from external stimuli
can be transmitted to the germ-cells, and will re-develop in the next
generation at the same time as that at which they arose in the parent,
and in the same part of the organism[18].
It will be perceived that Weismann himself here very clearly draws
all the distinctions between cases 1, 2, and 3, as above explained.
Therefore it becomes the more remarkable that he should not have
perceived how radically inconsistent it is in him thus to entertain
as “possible” congenital variations belonging to the case 2. For, as
we have now so fully seen, the theory of germ-plasm (as distinguished
from that of stirp) cannot entertain the possibility of an hereditary
and specialized change of _any_ kind as thus produced by external
conditions of life: should such a possibility be entertained, there
must obviously be an end to the _absolute stability_ of germ-plasm,
and a consequent collapse of Weismann’s theory of evolution. Either
germ-plasm is absolutely stable, or else it is but highly stable. If
it is absolutely stable, individual variations of an hereditary kind
can occur only as results of sexual admixtures of germ-plasm, and
Weismann’s theory of evolution is established. But if germ-plasm is
not absolutely stable (no matter in how high a degree it may be so)
hereditary individual variations may be produced by other causes, and
Weismann’s theory of evolution collapses. Therefore, if we are to
examine his theory of _evolution_, we can do so only by ignoring such a
passage as the one just quoted, which surrenders the postulate of the
_absolute stability_ of germ-plasm.
Again, if we are to examine Weismann’s theory of _heredity_, we must
similarly ignore such a passage as the following, where he represents
that he is similarly prepared to surrender his still more fundamental
postulate of the _perpetual continuity_ of germ-plasm.
After remarking that some of his own experiments on the climatic
varieties of certain butterflies raise such difficulties against his
whole theory of heredity that even now “he cannot explain the facts
otherwise than by supposing the passive acquisition of characters
produced by the direct influence of climate,” he goes on to remark more
generally—“We cannot exclude the possibility of such a transmission
occasionally occurring, for, even if the greater part of the effects
must be attributed to natural selection, there might be a smaller part
in certain cases which depends on this exceptional factor[19]”—i.e.,
the Lamarckian factor!
Now, it must be particularly noted that in this passage Weismann is
speaking, not as in the previous passage, of _specialized_ congenital
characters, but of _representative_ congenital characters. In other
words, he here entertains the possibility which in the passage
previously quoted he very properly rejects—namely, “that changes of
the organism which result from external stimuli can be transmitted to
the germ-cells, and will re-develop in the next generation _at the same
time as that at which they arose in the parent, and in the same part
of the organism_.” But it is evident that if the theory of germ-plasm
is undermined by the concession made in the passage thus previously
quoted, in the passage last quoted a match is put to the fuse. It
does not signify whether the particular case of the butterflies in
question will ever admit of any other explanation more in accordance
with the theory of germ-plasm: the point is that in _no_ case can this
theory entertain the possibility of causes other than admixtures of
germ-plasm in sexual unions producing hereditary changes, (A) of _any_
kind, (B) still less of a _specialized_ kind, and (C) least of all of
a _representative_ kind. For the distinguishing essence of this theory
is, that germ-plasm must always have moved, so to speak, in a closed
orbit of its own: its “sphere” must have been perpetually distinct from
those of whatever other “plasms” there may be in the constellations of
living things. So that, in such passages as those just quoted, Weismann
is not only destroying the very foundations of his general theory of
evolution, but at the same time he is identifying his more special
theory of heredity with those which had been already published by his
predecessors, and more particularly by Galton. Now, it is not Galton’s
theory that we are considering; and therefore we must hereafter
ignore those fundamental admissions, whereby Weismann every now and
again appears ready to relinquish all that is most distinctive of, or
original in, his own elaborate system of theories.
It is, indeed, impossible not to admire the candour of these
admissions, or to avoid recognizing the truly scientific spirit which
they betoken. But, at the same time, one is led to doubt whether in
making them Professor Weismann has sufficiently considered their full
import. He appears to deem it of comparatively little importance
whether or not acquired characters can sometimes and in some degrees
influence the hereditary qualities of germ-plasm, provided he can
show that _much the larger_ part of the phenomena of heredity must be
ascribed to the continuity of germ-plasm. In other words, he seems
to think that it matters but little whether in the course of organic
evolution the Lamarckian factors have played but a very subordinate
part, or whether they have not played any part at all. Moreover, I have
heard one or two prominent followers of Weismann give public expression
to the same opinion. Therefore I must repeat that it makes a literally
immeasurable difference whether we suppose, with Galton, that the
Lamarckian factors may sometimes and in some degrees assert themselves,
or whether we suppose, with the great bulk of Weismann’s writings and
in accordance with the logical requirements of _his_ theory, that they
can never possibly occur in any degree. The distinctive postulate
of his theory of heredity, and one of the two fundamental doctrines
on which he founds his further theory of evolution, is, that the
physiology of sexual reproduction cannot admit of any inversion of the
relations between “germ-plasm” and “somatic idio-plasm[20].” This is a
perfectly intelligible postulate, but it is not one with which we may
play fast and loose. Either there is such a physiological mechanism
as it announces, in which case the relations in question can never
be inverted “occasionally,” any more than rags may “occasionally”
help to construct the mill which is to form them into paper;—or else
there is no such mechanism, in which case we may have to do with
gemmules, physiological units, stirp, micellae, pangenes, plastidules,
or any of the other hypothetical “carriers of heredity” to which our
predilections may happen to incline; but the one substance with which
we certainly have not to do is germ-plasm[21].
* * * * *
After these tedious but necessary preambles, we may now proceed
to examine Professor Weismann’s postulate as to the perpetual
continuity of germ-plasm, with its superstructure in his theory of
heredity—reserving for the next chapter our examination of his further
postulate touching the absolute stability of germ-plasm, with its
superstructure in his theory of evolution.
The evidence which Weismann has presented in favour of his fundamental
postulate of the perpetual continuity of germ-plasm may be conveniently
dealt with under two heads—namely, indirect evidence as derived from
general reasoning, and direct evidence derived from particular facts.
The general reasoning is directed to show, (1) that there is no
evidence of the transmission of acquired characters; (2) that the
theory of pangenesis is “inconceivable”; and, (3) that the alternative
theory of germ-plasm is amply conceivable. Now, to the best of my
judgement, not one of these propositions is borne out by the general
reasoning in question. But as the latter is almost entirely of an _a
priori_ character, and also of a somewhat abstruse construction, I
think the patience of any ordinary reader will be saved by relegating
this part of our subject to an Appendix. Therefore, remarking only
that any one who cares to look at Appendix I ought, in my opinion, to
perceive that there is no real evidence against the transmission of
acquired characters to be derived from Weismann’s general reasoning in
this connexion, I will at once proceed to consider the evidence which
he has adduced in the way of particular facts.
* * * * *
In the first place, as one result of his brilliant researches on the
_Hydromedusae_, he has found that the generative cells occur only in
certain localized situations, which, however, vary greatly in different
species, though they are always constant for the same species. He
has also found that the varying situations in different species of
the localized or generative areas correspond, place for place, with
successive stages in a process of gradual transposition which has
occurred in the phylogeny of the _Hydromedusae_. Lastly, he has found
that in each ontogeny these successive stages of transposition are
repeated, with the result that during the individual lifetime of
one of these animals the germ-cells migrate through the body, from
what used to be their ancestral situation to what is now the normal
situation for that particular species. Such being the facts, Weismann
argues from them that the germ-cells of the _Hydromedusae_ are thus
proved to present properties of a peculiar kind, which cannot be
supplied by any of the other cells of the organism; for, if they could,
whence the necessity for this migration of these particular cells? Of
course it follows that these peculiar properties must depend on the
presence of some peculiar substance, and that this is none other than
the “germ-plasm,” which here exhibits a demonstrable “continuity”
throughout the entire phylogeny of these unquestionably very ancient
Metazoa.
The second line of direct evidence in favour of the continuity of
germ-plasm which Weismann has adduced is, that in the case of some
invertebrated animals the sexual apparatus is demonstrably separated
as reproductive cells (or cells which afterwards give rise to the
reproductive glands) at a very early period of ontogeny—so early
indeed, in certain cases, that this separation constitutes actually
the first stage in the process of ontogeny. Therefore, it is argued,
we may regard it as antecedently improbable that the after-life of the
individual can in any way affect the congenital endowments of its ova,
seeing that the ova have been thus from the first anatomically isolated
from all the other tissues of the organism.
The third and only other line of direct evidence is, that organisms
which have been produced parthenogenetically, or without admixture of
germ-plasms in any previous act of sexual fertilization, do not exhibit
congenital variations.
Taking, then, these three lines of verification separately, none of
them need detain us long. For although the fact of the migration of
germ-cells becomes one of great interest in relation to Weismann’s
theory _after the theory has been accepted_, the fact in itself does
not furnish any evidence in support of the theory. In the first
place, it tends equally well to support Galton’s theory of stirp;
and therefore does not lend any special countenance to the theory of
germ-plasm—or the theory that there cannot now be, and never can
have been, any communication at all between the plasm of the germ and
that of the soma. In the second place, the fact of such migration is
not incompatible even with the theory of pangenesis, or the theory
which supposes such a communication to be extremely intimate. There
may be many other reasons for this migration of germ-cells besides
the one which Weismann’s theory supposes. For example, the principle
of physiological economy may very well have determined that it is
better to continue for reproductive purposes the use of cells which
have already been specialized and set apart for the execution of those
purposes, than to discard these cells and transform others into a kind
fitted to replace them. Even the theory of pangenesis requires to
assume a very high degree of specialization on the part of germ-cells;
and as it is the fact of such specialization alone which is proved
by Weismann’s observations, I do not see that it constitutes any
criterion between his theory of heredity and that of Darwin—still
less, of course, between his theory and that of Galton. Lastly, in this
connexion we ought to remember that the _Hydromedusae_ are organisms
in which the specialization in question happens to be least, as is
shown by the fact that entire individuals admit of being reproduced
from fragments of somatic-tissues; so that these are organisms where we
would least expect to meet with the migration of germ-cells, were the
purpose of such migration that which Weismann suggests. This line of
evidence therefore seems valueless.
Nor does it appear to me that the second line of evidence is of any
more value. In the first place, there is no shadow of a reason for
supposing that an apparently anatomical isolation of germ-cells
necessarily entails a physiological isolation as regards their special
function—all “physiological analogy,” indeed, being opposed to such
a view, as is shown in Appendix I. In the second place, there is no
proof of any anatomical isolation, as we may likewise see in that
Appendix. In the third place, the fact relied upon to indicate such
an isolation—viz., the early formation of germ-cells—is not a fact
of any general occurrence. On the contrary, it obtains only in a
comparatively small number of animals, while it does not obtain in any
plants. In the Vertebrates, for example, the reproductive cells are
not differentiated from the somatic cells till after the embryo has
been fully formed; while in plants their development constitutes the
very last stage of ontogeny. In the fourth place, the argument, even
for what it is worth, is purely deductive; and deductive reasoning in
such a case as this—where the phenomena are enormously complex and
our ignorance unusually profound—is always precarious. Lastly, in the
fifth place, Weismann has now himself abandoned this argument. For in
one of his later essays he says:—
Those instances of early separation of sexual from somatic cells,
upon which I have often insisted as indicating the continuity of the
germ-plasm, do not now appear to be of such conclusive importance as
at the time when we were not sure about the localization of the plasm
in the nuclei. In the great majority of cases the germ-cells are not
separated at the beginning of embryonic development, but only in some
of the later stages.... It therefore follows that cases of early
separation of the germ-cells afford no proof of a direct persistence
of the parent germ-cells in those of the offspring.
The last line of direct evidence, or that derived from the alleged
non-variability of parthenogenetic organisms, is, as Professor Vines
has shown, opposed to fact. Therefore, in his later writings, Weismann
has abandoned this line of evidence also.
Upon the whole,then, we must conclude with regard to the fundamental
postulate of perpetual continuity, that there is actually no evidence
of a direct kind in its favour. And, as Weismann’s arguments of an
indirect kind are dealt with in Appendix I, it remains only to state
such evidence _per contra_ as, to the best of my judgement, appears
valid.
* * * * *
The fundamental proposition which we have been considering, and to
the further consideration of which we have now to proceed, is, in
effect, that germ-plasm differs from stirp in having been _perpetually_
restricted to a “sphere” of its own, “_since the first origin of
life_.” Criticism, therefore, must be directed to show that the
“sphere” in question has not been proved so entirely independent as
this fundamental proposition sets forth; but that, on the contrary,
there appears to be a certain amount of reciprocal action between this
sphere and that of the somatic-tissues—even though we may agree (as I
myself agree) with Galton in holding that the degree of such reciprocal
action is neither so intimate nor so constant as it was held to be
by Darwin. This, indeed, is the direction which the course of our
criticism has taken already. For it has just been shown that Weismann
has failed to adduce any facts (preceding text) or considerations
(Appendix I) in support of his fundamental proposition as above stated,
save such as proceed on a prior acceptance of the proposition itself.
The facts and considerations which he has adduced are therefore
useless as evidence in support of this proposition, although they
would admit of being explained by it supposing it to have been already
substantiated by any facts or considerations of an independent kind.
Which is merely another way of saying, as already said, that there is
no evidence in favour of the proposition.
But I am now about to argue that there _is_ evidence _against_ the
proposition. For I am about to argue, not only as heretofore that for
anything Weismann has shown to the contrary there _may_ be a certain
amount of reciprocal action between the sphere of germinal-substance
and the sphere of body-substance; but that, as a matter of fact, there
_is_ a certain amount of such reciprocal action.
Without laying undue stress on the intimate “correlation” that subsists
between the reproductive organs and all other parts of the organism,
I nevertheless think that the fact ought here to be noted. For the
changes which occur at puberty and after the reproductive functions
have ceased, as well as those which may be artificially produced by
castration, &c., prove at any rate some extremely important association
between the soma as a whole and its reproductive apparatus as a whole.
No doubt it may properly enough be answered that this proof does not
extend to the vital point of showing the association to be between
the soma as a whole, and that particular part of the reproductive
apparatus in which the “carriers of heredity” reside—namely, the ova
and spermatozoa; and, therefore, that the facts in question may be
due only to some changed conditions of nutrition on the part of the
somatic-tissues which these alterations on the part of the reproductive
glands entail. On this account we must fully allow that the facts in
question are not in themselves of any conclusive weight; but I think
they are worth mentioning, because they certainly seem to countenance
the theory which supposes some reciprocal influence as exercised by
the germinal elements on the somatic-tissues and _vice versa_, rather
than they do the theory which supposes the germinal elements and the
somatic-tissues to have always occupied totally different “spheres.”
Here, however, is a stronger class of facts. It has not unfrequently
been observed, at any rate in mammals, that when a female has borne
progeny to a male of one variety, and subsequently bears progeny to a
male of another variety, the younger progeny presents a more or less
unmistakable resemblance to the father of the older one. Now, this
is a fact to which Weismann has nowhere alluded; and therefore I do
not know how he would meet it. But, as far as I can see, it can be
explained only in one or other of two ways. Either there must be some
action of the spermatic element on the hitherto unripe ovum, or else
this element must exercise some influence on the somatic-tissues of
the female, which in their turn act upon the ovum[22]. Now, I do not
deny that the first of these possibilities might be reconcilable with
the hypothesis of an absolute continuity of germ-plasm; for it is
conceivable that the life of germ-plasm is not coterminous with that
of the spermatozoa which convey it, and hence that, if the carriers of
heredity, after the disintegration of their containing spermatozoa,
should ever penetrate an unripe ovum, the germ-plasm thus introduced
might remain dormant in the ovum until the latter becomes mature, and
is then fertilized by another sire. In this way it is conceivable that
the hitherto dormant germ-plasm of the previous sire might exercise
some influence on the progeny of a subsequent one. But it seems clear
that the second of the two possibilities above named could not be thus
brought within the hypothesis of an absolute continuity of germ-plasm.
Therefore it seems that the school of Weismann must adopt the first,
to the exclusion of the second. Unfortunately for them, however, there
is another (and clearly analogous) fact, which goes to exclude the
first possibility, and most definitely to substantiate the second. For,
in the case of plants, where there can be no second progeny borne by
the same “ovary,” but where we happen to be able to see that a marked
effect is sometimes produced on the somatic-tissues of the mother by
the pollen of the father, there can be no question as to the male
element being able to exercise a direct influence on the soma of the
female. Consequently, whatever we may think with regard to the case of
animals, the facts with regard to plants are in themselves enough to
sustain the only position with which we are concerned—viz., that the
male element is capable of directly modifying the female soma.
The facts with regard to plants are these. When one variety fertilizes
the ovules of another, not unfrequently the influence extends beyond
the ovules to the ovarium, and even to the calyx and flower-stalk, of
the mother plant. This influence, which may affect the shape, size,
colour, and texture of the somatic-tissues of the mother, has been
observed in a large number of plants belonging to many different
orders. The details of the matter have already been dealt with by
Darwin, in the eleventh chapter of his work on _Variation_, &c.; and
this is what he says. The italics are mine.
The proofs of the action of foreign pollen on the mother-plant
have been given in considerable detail, because this action is of
the highest theoretical importance, and because it is in itself
a remarkable and apparently anomalous circumstance. That it is
remarkable under a physiological point of view is clear, for the male
element not only affects, in accordance with its proper function, the
germ, but at the same time various parts of the mother-plant, _in the
same manner as it affects the same parts in the seminal offspring
from the same two parents_. We thus learn that an ovule is not
indispensable for the reception of the influence of the male element.
Darwin then proceeds to show that this direct action of the male
element on the somatic tissues of another organism is not so rare or
anomalous as it at first sight appears; for in the case of not a few
flowers it comes into play as a needful preliminary to fertilization.
Thus, for instance:—
Gärtner gradually increased the number of pollen grains until he
succeeded in fertilizing a Malva, and has proved that many grains
are first expended in the development, or, as he expresses it, in
the satiation, of the pistil and ovarium. Again, when one plant
is fertilized by a widely distinct species, it often happens that
the ovarium is fully and quickly developed without any seeds being
formed; or the coats of the seeds are formed without any embryo being
developed therein.
So much, then, in proof of the direct action of the male element on the
somatic-tissues of another organism. It remains to show that a similar
action may be exercised by this element on the somatic tissues of its
own organism. This has been proved by Hildebrand, who found “that
in the normal fertilization of several Orchideae, the action of the
plant’s own pollen is necessary for the development of the ovarium; and
that this development takes place not only long before the pollen tubes
have reached the ovules, but even before the placentae and ovules have
been formed”; so that with these orchids the pollen acts directly on
their own ovaria, as a preliminary to the formation of the ovules which
are subsequently to be fertilized.
It is to be regretted that Professor Weismann has not given us his
opinion upon this whole class of facts, for assuredly they appear
directly to contradict his theory. The theory is, “that the germ-plasm
and the somato-plasm have always occupied different spheres”: the fact
is, that the germ-plasm may directly act upon the somato-plasm, both
within and beyond the limits of the same organism.
* * * * *
Hitherto we have been considering certain very definite facts, which
seem to prove that the germinal elements are able directly to affect
the somatic-tissues. We have next to consider such facts as seem to
prove the opposite side of a reciprocal relationship—viz., that the
somatic-tissues are able directly to affect the germinal elements.
And here there are two distinct lines of evidence to be distinguished.
Firstly, in certain cases—exceptional it is true, but this does not
signify—somatic-tissues have been found capable of modifying the
hereditary endowments of germinal elements by means of simple grafting.
This line of evidence has also been disregarded both by Weismann and
his followers; but it is nevertheless an important one to consider.
For, if it be the case that the somatic-tissues of an organism A,
by being merely grafted on-those of organism B, can so affect the
germinal elements of B as to cause their offspring to resemble A—or,
contrariwise, if the somatic-tissues of A can thus act on B—then,
although it may not be properly said that any “acquired characters”
have been transmitted from A to the progeny of B, (or _vice versa_,)
such an a-sexual transmission of alien characters, in its relation to
the theory of germ-plasm, is scarcely less awkward than are certain
facts which they appear to prove.
Secondly, that acquired characters may be transmitted to progeny by
the more ordinary methods of sexual propagation (Lamarckian factors).
This second line of evidence will be fully and independently dealt with
in future chapters, specially devoted to the subject. Therefore we have
here to consider only the first.
Now, the force of this first line of evidence will become apparent,
if we reflect that the only way in which the facts can be met by
Weismann’s theory, would be by supposing that the somatic germ-plasms
which are respectively diffused through the cellular tissues of the
scion and the graft become mixed in some such way as they might have
been, had the hybrid been due to seminal propagation instead of to
simple grafting. But against this, the only interpretation of the facts
which is open to the theory, there lies the following objection, which
to me appears insuperable.
Where sexual cells are concerned there is always a definite arrangement
to secure penetration of the one by the other, and we can see the
necessity for such an arrangement in order to effect an admixture
of their nuclear contents, where alone germ-plasm is supposed by
Weismann’s theory to reside. But in tissue-cells, which have not
been thus specialized, it would be difficult to believe that nuclear
contents can admit of being intimately fused by a mere apposition of
cell-walls. For not only are the nuclear contents of any two such cells
thus separated from one another by two cell-walls and two masses of
“cytoplasm”; but it is not enough to suppose that in order to produce
a graft-hybrid only two of these somatic-cells need mix their nuclear
contents, as we know is all that is required in order to produce
a seminal hybrid by means of sexual cells. On the contrary, in the
former case most, if not all, the somatic-cells which are brought into
apposition by the graft must be supposed thus to mix their nuclear
contents at the plane of the graft; for otherwise the hybrid would
not afterwards present equally the characters of stock and scion.
Now, there may be hundreds of thousands of such cells, and therefore
it seems impossible that the facts of graft-hybridization can be
reconciled with the theory of germ-plasms[23].
The third line of evidence against this theory—i.e., the evidence in
favour of the transmission of acquired characters—is to constitute the
subject-matter of future chapters. Therefore it will here be sufficient
to adduce only one fact of this kind. And I select it because it is one
that has been dealt with by Weismann himself. In one of his more recent
statements he says:—
The distinguished botanist De Vries has proved that certain
constituents of the cell body—e. g., the chromatophores of
Algae—pass directly from the maternal ovum to the daughter organism,
while the male germ-cells generally contain no chromatophores. Here it
appears possible that a transmission of somatogenetic variation has
occurred[24].
Now although, as Weismann goes on to observe, “in these lower plants,
the separation between somatic and reproductive cells is slight,”
in the facts to which he alludes we appear to have good evidence of
an influence exercised by somatic cells upon the germinal contents
of reproductive cells. And if such an influence is capable of being
exercised in the case of “these lower plants,” it follows that
there is no such _absolute_ separation between somatic tissues and
germ-plasm as Weismann’s theory requires. Moreover it follows that,
if the essential distinction between germ-plasm and somato-plasm (or
“somatic idio-plasm”) is thus violated at the very foundation of the
multicellular organisms, there ceases to be any _a priori_ reason for
drawing arbitrary limits, either as to the level of organization at
which such “transmission of somatogenetic variation has occurred,” or
as to the degree of detail into which it may extend. Both these matters
then stand to be tested by observation; and the burden of proof lies
with the school of Weismann to show at what level of organization,
and at what degree of representation, somatogenetic changes cease to
reproduce themselves by heredity.
Passing on, then, to higher levels of organization, and therefore to
higher degrees of representation, I shall endeavour to show that this
burden of proof cannot be discharged. For I shall endeavour to show,
not merely, as just shown, that there ceases to be any _a priori_
reason for drawing arbitrary limits with respect either to levels of
organization or to degrees of representation, but that, as a matter of
fact, there are no such limits as the passage above quoted assigns.
On the contrary, I believe there is as good evidence to prove the not
unfrequent transmission of acquired (“somatogenetic”) characters
among the higher plants—and even among the higher animals—as there
is of the occurrence of this phenomenon in the case of the Alga just
mentioned. But in order to do this evidence justice, I shall have to
take a new point of departure and consider as a separate question
the transmissibility of acquired characters. Meanwhile, and as far
as Weismann’s theory of heredity is concerned, it is enough to have
shown,—if I have been successful in doing so,—that not only is
there no evidence to sustain his fundamental postulate touching the
material of heredity having always occupied a separate “sphere” of
its own “since the first origin of life”; but that there is good
evidence to prove the contrary. For whether or not the reciprocal
action of “somato-plasm” and “germ-plasm” can ever proceed to the
extent of causing acquired characters to be inherited (so as to produce
“representative congenital changes”), all that is distinctive in this
theory must be regarded as barren speculation, unless it can be shown
that the foregoing facts have failed to prove such a reciprocal action
as ever occurring in any lower degree (so as to produce “specialized
congenital changes”).