EXAMINATION OF WEISMANN’S THEORY OF EVOLUTION (1891).
HAVING now considered germ-plasm as perpetually continuous, we have
next to regard it as unalterably stable.
First, let it be noted that these two fundamental and distinctive
postulates of the whole Weismannian system are so intimately connected
as to be in large measure mutually dependent. For, on the one hand, if
germ-plasm has not been perpetually continuous since the first origin
of life, it cannot have been absolutely stable “since the first origin
of sexual propagation”: every time that its hereditary characters are
modified by its containing soma (whether or not representatively so),
its stability has been so far upset. On the other hand, if germ-plasm
has not been absolutely stable, it cannot have been perpetually
continuous “since the first origin of life.” As often as its stability
has been upset, its “molecular structure” has been modified by causes
_ab extra_, as distinguished from mixtures of germ-plasms in sexual
unions. Therefore, it can no longer have been continuous in the sense
of having borne an ineffaceable record of all congenital variations,
_due to sexual unions_, throughout the entire phylogeny of the
Metaphyta and Metazoa. At most it can have been continuous only in the
attenuated sense, that however much and however often its hereditary
characters may have been modified by somatic changes on the one hand or
by changes in the external conditions of life on the other, they can
never have been thus modified _representatively_, as supposed by the
theory of pangenesis.
From which it follows that, while examining in our last chapter
Weismann’s doctrine of the perpetual continuity of germ-plasm, we
have been indirectly examining also his companion doctrine of the
unalterable stability of germ-plasm. Nevertheless, for the sake of
doing justice to both these doctrines, I have thought it desirable to
examine each on its own merits, without prejudice arising from our
criticism of the other. To such a separate and independent examination
of the doctrine of unalterable stability we will, therefore, now
proceed.
* * * * *
As we have already and repeatedly seen, this doctrine of the
unalterable or absolute stability of germ-plasm “since the first origin
of sexual propagation” is a logically essential part of Weismann’s
theory of evolution, or of his system of hypotheses considered as a
whole. It is so because upon this doctrine depends his reference of
individual variations in the Metazoa to an ultimate origin in the
Protozoa, the significance of sexual reproduction in the theory of
natural selection, &c., &c. Therefore this doctrine of the absolute
stability of germ-plasm is enunciated by Weismann, not merely for the
purpose of meeting any one class of facts, such as those of atavism,
persistence of rudimentary organs, &c. The doctrine is enunciated for
the purpose of constituting one of the foundation-stones of his general
theory of evolution. We have now to consider how far the quality of
this stone renders it trustworthy as a basis to build upon.
In the first place, we can scarcely fail to perceive that this doctrine
of the absolute stability of germ-plasm is not only gratuitous, but
intrinsically improbable. That the most complex material in nature
should likewise be the most stable is opposed to all the analogies of
nature, and therefore to all the probabilities of the case.
Again, the germ-plasm, as it originally occurred (and still exists) in
unicellular organisms, is supposed to be exactly the same _kind_ of
material as now occurs in the germ-cells of multicellular organisms.
Yet the very same theory which supposes so absolute a stability on the
part of germ-plasm when located in germ-cells (or diffused through
somatic-cells), likewise supposes so high a degree of variability on
the part of germ-plasm when not thus located, as to represent that all
individual variations which have ever taken place in the unicellular
organisms—and all the innumerable species of such organisms which
have arisen therefrom—have been due to the direct action of external
conditions of life; or, in other words, to the _instability_ of
germ-plasm. The very same substance which at one time and in one place
is supposed to be so absolutely unchangeable, at another time and in
another place is supposed to be highly susceptible of change.
Lastly—and this is, perhaps, the most curious part of the whole
matter—the place where germ-plasm is supposed to be unchangeable is
not the place where it is most likely to be so, but the place where
it is least likely. For germ-plasm as it occurs in the germ-cells of
multicellular organisms must have a constitution greatly more complex
even than that which it has in unicellular organisms—seeing that in
the former case, and by hypothesis, it bears a living record of the
whole phylogeny of the Metaphyta and Metazoa in all their innumerable
branchings. And not only so, but when germ-plasm occurs in germ-cells
it becomes exposed to much greater vicissitudes: its environment has
become vastly more complex, as well as greatly more liable to change
with the changing conditions of life of the many mutable species in
which it resides, and on the individual somas of which it now depends
for its nourishment. So that, altogether, we have here on merely
_a priori_ grounds about as strong a case against this doctrine of
absolute stability as it is well conceivable that on merely _a priori_
grounds a case can be.
Turning next to arguments _a posteriori_, let us begin by considering
those which Weismann has adduced in support of the doctrine.
First, he alleges that there is a total absence of variability on the
part of all organisms which have been produced parthenogenetically, or
from unfertilized ova. We may look in vain, he says, for any individual
differences on the part of any multicellular organisms, which have been
brought into existence independently of the blending of germ-plasms in
a previous act of sexual union. Now, unquestionably, if this statement
could be corroborated by sufficiently extensive observation, the fact
would become one of immense significance—so much so, indeed, that of
itself it would go far to neutralize all antecedent objections, and
to verify his theory as to sexual propagation being the sole cause
of congenital variation. But seeing that the alleged fact stands so
entirely out of analogy with the phenomena of bud-variation (which will
be alluded to later on), it is highly improbable, even on antecedent
grounds; while Professor Vines has refuted the statement on grounds of
actual fact. Thus, speaking of the _Basidiomycetes_, he says—
These Fungi are not only entirely a-sexual, but it would appear that
they have been evolved in a purely a-sexual manner from a-sexual
ascomycetous or æcidiomycetous ancestors. The Basidiomycetes, in fact,
afford an example of a vast family of plants, of the most varied form
and habit, including hundreds of genera and species, in which, so far
as minute and long-continued investigation has shown, there is not,
and probably never has been, any trace of a sexual process[25].
Here, then, we have actual proof of “hereditary individual variations”
among a-sexually propagating organisms, sufficient in amount to
have given origin, not merely to “individual differences,” but to
innumerable species, and even genera. Consequently Weismann allows
that the criticism abolishes this line of evidence in favour of the
absolute stability of germ-plasm[26]. Consquently, also, we must now
add, in whatever measure the alleged fact would have corroborated
the theory had it been proved to be a fact, in that measure is the
theory discredited by proof that it is not a fact. For, if the theory
were sound, this particular fact would certainly have admitted of
demonstration: therefore the proof that it is not a fact—but the
reverse of a fact—amounts at the same time to a disproof of the
theory[27].
The only other line of evidence to be adduced in favour of the absolute
stability of germ-plasm is that which is furnished by the high
antiquity of some specific types, by the facts of atavism, and by the
persistency of vestigial organs. But this line of evidence is as futile
as the other. Nobody has ever questioned that hereditary characters
are persistently stable as long as they are persistently maintained by
natural selection; and this, according to Weismann himself, must have
been the case with all long-enduring species: these, therefore, fail to
furnish any evidence of the _inherent_ stability of germ-plasm, which
is the only point in question.
Again, as regards the facts of atavism, nobody is disputing these
facts. What we are disputing is whether the _degree_ of inherent
stability which they unquestionably prove can be rationally regarded
as such that it may endure, not merely for such a comparatively small
number of generations as these facts imply, but actually for any
number of generations, or through the practically infinite series of
generations that now intervene between the higher metazoa and their
primeval parentage in the protozoa. Clearly, the ratio between these
two things is such that no argument derived from the facts of atavism
can be of any avail for the purposes of this Weismannian doctrine.
Lastly, as regards vestigial organs, the consideration that,
surprisingly persistent as they unquestionably are, nevertheless they
do eventually disappear, seems to prove that the power of heredity
does in time become exhausted, even in cases most favourable to
its continuance. That it should thus become finally exhausted is
no more than Darwin’s theory of perishable gemmules, or Galton’s
theory of a not absolutely stable stirp, would expect. But the fact
is irreconcilable with Weismann’s theory of an absolutely stable
germ-plasm.
Hence, we can only conclude that there is no evidence in favour of the
hypothesis that germ-plasm has been unalterably stable “since the first
origin of sexual propagation”; while the suggestion that it may have
been so is on antecedent grounds improbable, and on inductive grounds
untenable. It only remains to add that the _degree_ of stability has
been proved in not a few cases to be less than even the theory of
gemmules might anticipate. Many facts in proof of this statement might
be given, but it will here suffice to quote one, which I select because
it has been dealt with by Professor Weismann himself.
Professor Hoffmann has published an abstract of a research, which
consisted in subjecting plants with normal flowers to changed
conditions of life through a series of generations. In course of
time, certain well-marked variations appeared. Now, in some cases
such directly-produced variations were transmitted by seed from the
affected plants; and therefore Weismann acknowledges,—“I have no
doubt that the results are, at any rate in part, due to the operation
of heredity.” Hence, whether these results be due to the transmission
of somatogenetic characters (“representative changes”), or to the
direct action of changed conditions of life on the germ-plasm itself
(“specialized changes”), it is equally certain that the hereditary
characters of the plants were congenitally modified to a large extent,
within (at most) a few generations. In other words, it is certain that,
if there be such a material as germ-plasm, it has been proved in this
case to have been highly unstable. Therefore, in dealing with these
and other similar facts, Weismann himself can only save his postulate
of continuity by surrendering for the time being his postulate of
stability[28].
If to this it be replied that Hoffmann’s facts are exceptional—that
Gärtner, Nägeli, De Candolle, Peter, Jordan, and others, did not find
individual variations produced in plants by changed conditions of life
to be inherited,—the reply would be irrelevant. It does not require
to be proved that all variations produced by changed conditions of
life are inherited. If only some—even though it be but an extremely
small percentage—of such variations are proved to be inherited, the
many millions of years that separate the germ-plasm of to-day from its
supposed origin in the protozoa, must have furnished opportunities
enough for the occurrence of such variations to have obliterated, and
re-obliterated numberless times, any aboriginal differences in the
germ-plasms of incipiently sexual organisms. Moreover, it is probable
that when further experiments shall have been made in this direction,
Hoffmann’s results will be found not so exceptional as they at present
appear. Mr. Mivart, for example, has mentioned several instances[29];
while there are not a few facts of general knowledge—such as the
modifications undergone by certain Crustacea as a direct result of
increased salinity of the water in which they live—that will probably
soon be proved to be facts of the same order. But here attention
must be directed to another large body of facts, which are of high
importance in the present connexion.
The phenomena of what is called bud-variation in plants are phenomena
of not infrequent occurrence, and they consist in the sudden appearance
of a peculiarity on the part of a shoot which develops from a single
bud. When such a peculiarity arises, it admits of being propagated, not
only by cuttings and by other buds from that shoot, but sometimes also
by seeds which the flowers of the shoot subsequently produce—in which
case all the laws of inheritance that apply to congenital variations
are found to apply also to bud-variation. Or, as Darwin puts it, “there
is not any particular in which new characters arising by bud-variation
can be distinguished from those due to seminal variation”; and,
therefore, any theory which deals with the latter is bound also to take
cognizance of the former. Now, as far as I can find, there is only one
paragraph in which Weismann alludes to bud-variation, and what he there
says I do not find very easy to understand. Therefore I will quote the
whole paragraph _verbatim_.
I have not hitherto considered budding in relation to my theories,
but it is obvious that it is to be explained, from my point of view,
by supposing that the germ-plasm which passes on into a budding
individual consists not only of the unchanged germ-plasm of the
first ontogenetic stage, but of this substance altered, so far as
to correspond with the altered structure of the individual which
arises from it—viz., the rootless shoot which springs from the stem
or branches. The alteration must be very slight, and perhaps quite
insignificant, for it is possible that the differences between the
secondary shoots and the primary plant may depend chiefly on the
changed conditions of development, which takes place beneath the earth
in the latter case, and in the tissues of the plant in the former.
Thus we may imagine that the idio-plasm [? of that particular bud],
when it develops into a flowering shoot, produces at the same time
the germ-cells which are found in the latter. We thus approach an
understanding of Fritz Müller’s observation; for if the whole shoot
which produces the flower arises from the same idio-plasm which also
forms its germ-cells, we can readily understand why the latter should
contain the same hereditary tendencies which were previously expressed
in the flower which produced them. The fact that variations may
occur in a single shoot depends on the changes explained above, which
occur in the idio-plasm during the course of its growth, as a result
of the varying proportions in which the ancestral idio-plasms may be
contained in it.[30]
The meaning here appears to be twofold. For there are only two ways
of explaining the phenomena of bud-variation. Either they are due
to the influence of external conditions acting on the particular
bud in question, or else they are due to so-called “spontaneous”
changes taking place within the bud itself. Possibly it may be
both, but at least it must be either. Well, in the above passage,
Weismann appears to assume that it is both. For at the beginning of
the passage he speaks of the “germ-plasm of the first ontogenetic
stage” becoming “altered so far as to correspond with the altered
structure of the individual which arises therefrom,” and he goes
on to say that the alteration “may depend chiefly on the changed
conditions of development”—that is, as I understand, the influence
of external conditions. But at the end of the paragraph he says that
“the changes which occur in the idio-plasm during the course of its
growth” in the sporting bud, are due to “the varying proportions in
which the ancestral idio-plasms may be contained in it.” Thus, I
take it, Weismann here entertains both explanations of the phenomena
in question: he appears to regard these phenomena as partly due to
peculiar admixtures of ancestral idio-plasms in the bud itself (or
“spontaneous” variation), but partly also to an alteration of the
germ-plasm by its changed condition of development (or variation caused
by external conditions).
However, it is but of little consequence whether or not this is the
meaning which Weismann intends to convey. For the point we are coming
to is, that, whatever he intends to convey, “from the point of view”
of the theory of germ-plasm, there is only _one_ interpretation
possible. It is not open to Weismann (as it was to Darwin, or even to
Galton,) to entertain _both_ the explanations, whether separately or
in conjunction. For germ-plasm (unlike gemmules, or even stirp) must
be held always and everywhere _unalterably_ stable: else the whole
superstructure of Weismann’s theory of evolution falls to the ground.
We cannot consent to his retaining this theory on the one hand, and,
on the other, explaining bud-variation by “germ-plasm of the first
ontogenetic stage” becoming altered “chiefly by changed conditions of
development.” Even if it were true that “the alteration must be very
slight, if not quite insignificant,” there would here be a rift in the
lute, which must finally stop any further harping on the subject of
Evolution.
From the point of view of this theory, then, there is only one
interpretation open,—viz., that a bud-variation is ultimately due to
a peculiar admixture of germ-plasms in the seed from which the bud was
ultimately derived. But the objections to entertaining this as even
a logically possible explanation of the phenomena in all cases, is
insuperable.
In the first place, such a variation, when it does arise, is usually
a variation of an extremely pronounced character; therefore it is
very far from supporting Weismann’s view, that the “alteration”
of germ-plasm which is needed to produce it “must be very slight,
and perhaps quite insignificant.” In most cases where it occurs
bud-variation presents so extreme a departure from the normal type,
that no other kind of variation can be fitly compared with it in this
respect. In particular, the degree of variation is usually very much
greater than that which customarily obtains in congenital variations
of the ordinary kind; and, therefore, if these be supposed due to
particular admixtures of germ-plasm in sexual propagation, much more
must those admixtures which give rise to sporting buds be characterized
by peculiarities of no “insignificant” order. And much more, therefore,
ought they to assert themselves in sister-buds developed from the
same individual seed (ovule), than we find to be the case with any
sister-organisms which are developed from different individual seeds.
Yet, in the second place, so far is this from being the case, that
the most remarkable feature connected with bud-variation—next to the
suddenness and extreme amount of the variation itself—is the usually
isolated nature of its occurrence. There may be thousands of other
buds on the same plant, and yet it is one bud alone that deviates so
suddenly and so widely from its ancestral characters. Nay, more, a
single bud-variation may—and usually does—occur in plants which are
habitually propagated by cuttings and graftings; so that there may not
only be thousands, but millions of buds all derived from one original
seed, and all for many years remaining perfectly true to their parent
type, with the single exception of the sporting bud, which, while it
departs so widely from that type, is usually capable of transmitting
its extraordinary characters indefinitely by a-sexual, and not
infrequently also by sexual, methods. So that, altogether, it seems
impossible to suppose that in millions and millions of sister-buds,
which through years and years exhibit no variation, a highly peculiar
admixture of germ-plasm (which was originally present in the
parent-seed) should have been latent; that it should then suddenly
become so patent in a single bud, after which it never occurs in any
other bud, save in the progeny of the sporting one.
On the whole, then, while it thus seems impossible to attribute
all cases of bud-variation to mixtures of germ-plasms in sexual
propagation, the theory of germ-plasm is unable to entertain any
other explanation, on pain of surrendering its postulate touching the
unalterable stability of germ-plasm, on which the Weismannian theory of
evolution is founded.
So much for Weismann’s evidence touching the extreme, or virtually
everlasting, stability of germ-plasm. We have seen that this evidence
is not merely of a very poor character _per se_, or on antecedent
grounds; but that it is directly negatived as evidence by the a-sexual
origin of species in the plants alluded to by Professor Vines; by
certain facts which prove so high a degree of instability on the
part of this hypothetical substance, that in some cases it admits of
being very considerably modified in the course of only two or three
generations by exposure to changed conditions of life; while in
other cases it may “sport,” so as to produce “hereditary individual
variations,” which are much more pronounced than any of those that
ordinarily result from a blending of hereditary qualities in an act of
sexual union.
* * * * *
It will be well to conclude our examination of Weismann’s system by
stating exactly the effect produced on his theory of evolution by the
foregoing disproof of its fundamental postulate—the absolute stability
of germ-plasm.
Clearly, in the first place, if germ-plasm has not been absolutely
stable “since the first origin of sexual propagation,” the hereditary
characters of germ-plasm may have been modified any number of times,
and in always accumulating degrees. It matters not whether the
modifications have been due mainly to external or to internal causes.
It is enough to have shown that modifications occur. For, it will
be remembered, the doctrine of the absolute stability of germ-plasm
is, that inasmuch as the “molecular” structure of germ-plasm cannot
be affected either from without or from within, the only source of
“hereditary individual variations” is to be found in admixtures of
germ-plasms taking place in sexual fertilization. Slight “molecular”
differences having been originally impressed upon different masses of
germ-plasm when these were severally derived from their unicellular
sources, so unalterable has been the stability of germ-plasm ever
since, that these slight “molecular” differences have never been
in any degree effaced; and although in sexual unions they have for
untold ages been obliged to mix in ever-varying proportions, they
still continue—and ever must continue—to assert themselves in each
ontogeny. Therefore, as Weismann himself formulates this astonishing
doctrine,—“The origin of hereditary individual variations cannot
indeed be found in the higher organisms, the Metazoa and Metaphyta;
but is to be sought for in the lowest—the unicellular organisms.” Or
again,—“The formation of new species, which among the lower Protozoa
could be achieved without amphigony, could only be attained by means of
this process in the Metazoa and Metaphyta. It was only in this way that
hereditary individual differences could arise and persist[31].”
Now this doctrine is the most distinctive, as it is the most original
feature in Weismann’s system of theories. That it is of interest as an
example of boldly carrying the premises of a theory to their logical
termination, no one will deny. But as little can it be denied that the
very stringency of this logical process brings the theory itself into
collision with such facts as those which have now been stated, and
which, as far as I can see, are destructive of the theory—or, at any
rate, of all that side of the theory which depends on the doctrine of
absolute stability.
Take, for instance, the sequent doctrine that natural selection
is inoperative among the unicellular organisms. Here, indeed, we
have another of those doctrines which are so improbable on merely
antecedent grounds, that their presence might well be deemed a source
of irremediable weakness to the whole theory of evolution of which
they form integral, or logically essential, parts. For seeing that
the rate of increase in most of the unicellular organisms is quite
as high as—and in most cases very much higher than—the rate that
obtains in any of the multicellular, it becomes on merely antecedent
grounds incredible that the struggle for existence should here _not_
lead to any survival of the fittest. When, for instance, we learn from
Maupas that a single Stylonichia is potentially capable of yielding a
billion descendants within a week, we should need some extraordinarily
good evidence to make us believe that as regards this organism natural
selection is inoperative. But the point at present is that, quite apart
from all general and _a priori_ considerations of this kind, Weismann’s
doctrine that unicellular organisms cannot be influenced by natural
selection must be abandoned. For this doctrine followed deductively
from the premiss that in the multicellular organisms congenital
variations can only be due to admixtures of germ-plasms in acts of
sexual fertilization; so that, in the absence of such admixtures,
there could be no material for natural selection to work upon. But
now we have found that this premiss must be given up; and, therefore,
the deduction with respect to the unicellular organisms falls to the
ground. Although it is true that the unicellular organisms propagate
by fission, and although we grant, for the sake of argument, that they
never propagate by way of sexual unions—even so this can no longer be
taken to argue that none of their innumerable species owe their origin
to natural selection. And, although it is probably true that the sexual
methods of propagation constitute one source of hereditary individual
variation among the multicellular organisms, there is no vestige of any
independent reason for supposing that this is the _only_ source of such
variation; while the sundry facts which have now been given amount to
nothing short of a demonstration to the contrary[32].
Lastly, and as regards the multicellular organisms, it is evident
that Weismann’s essay _On the Significance of Sexual Reproduction in
the Theory of Natural Selection_ must be cancelled. For, apart from
the contradictory manner in which this matter has been stated (pp.
70, 93, notes), and apart also from the consideration that other
and quite as probable reasons have been suggested for the origin of
sexual reproduction, there is the fact that Weismann’s theory is no
longer tenable after the above destruction of its logical postulate
in the absolute stability of germ-plasm. For, in the absence of
this postulate, there is no basis for the theory that admixtures of
germ-plasms in sexual reproduction furnish the sole means whereby
heritable variations can be supplied for the working of natural
selection.
_Summary._
THE theory of germ-plasm is not only a theory of heredity: it is
also, and more distinctively, a theory of evolution. As a theory of
heredity it is grounded on its author’s fundamental postulate—the
continuity of germ-plasm; and, further, on a fact well recognized
by all other theories of heredity, which he expresses by the term
stability of germ-plasm. But as a theory of evolution it requires two
additional postulates for its support—viz., that germ-plasm has
been _perpetually_ continuous “since the first origin of life,” and
_absolutely_ stable “since the first origin of sexual reproduction.”
It is clear that these two additional postulates are not needed
for his theory of heredity, but only for his additional theory of
evolution. There have been other theories of heredity, prior to this
one, which, like it, have been founded on the postulate of “continuity”
(in Weismann’s sense) of the substance of heredity; but it has not
been needful for any of these theories to postulate further that this
substance has been _always_ thus isolated, or even that it is now
_invariably_ so. For even though the isolation be frequently invaded
by influences of body-changes on the congenital characters of this
substance, it does not follow that the body-changes must be transmitted
to offspring exactly as they occurred in parents. They may produce in
offspring what we have agreed to call “specialized” hereditary changes,
even if they never produce “representative” hereditary changes,—i.e.,
the transmission of acquired characters. But it is essential to
Weismann’s theory of _evolution_ that body-changes should not exercise
a modifying influence of _any_ kind on the ancestral endowments of this
substance; hence, for the purposes of this further theory he has to
assume that germ-plasm presents, not only _continuity_, but continuity
_unbroken since the first origin of life_.
Similarly as regards his postulate of the stability of germ-plasm
as _absolute_. It is enough for all the requirements of his theory
of heredity, that the substance in question should present the high
degree of stability which the facts of atavism, persistence of
vestigial organs, &c., prove it to possess. But for his further
theory of evolution it is necessary to make this further postulate of
the stability of germ-plasm _as undisturbed since the first origin of
sexual propagation_: otherwise there would be no logical foundation for
any of the distinctive doctrines which go to constitute that theory.
Thus much understood, we proceeded to examine the theory of germ-plasm
in each of its departments separately—i.e., first as a theory of
heredity, and next as a theory of evolution. And we begun by comparing
it as a theory of heredity with the preceding theories of Darwin and
Galton. In the result we found that germ-plasm resembles gemmules in
all the following respects. It is particulate; constitutes the material
basis of heredity; is mainly lodged in highly specialized cells; is
nevertheless also distributed throughout the general cellular tissues,
where it is concerned in all processes of regeneration, repair, and
a-sexual reproduction; presents an enormously complex structure,
in that every constituent part of a potentially future organism is
represented in a fertilized ovum by corresponding particles; is
everywhere capable of virtually unlimited multiplication, without ever
losing its hereditary endowments; is often capable of carrying these
endowments in a dormant state through a long series of generations,
until at last they reappear again in what we recognize as reversions.
Such being the points of resemblance, the only points of difference
may be summed up in the two words—continuity, and stability. For,
as regards continuity, while Darwin’s theory supposes the substance
of heredity to be more or less formed anew in each generation by
the body-tissues of that generation, Weismann’s theory regards
this substance as owing nothing to the body-tissues, further than
lodgement and nutrition. Therefore, while the theory of gemmules can
freely entertain the doctrines of Lamarck, the theory of germ-plasm
excludes them as physiologically impossible, in all cases where
sexual reproduction is concerned. Again, as regards stability, while
Darwin’s theory simply accepts the fact of such a degree of stability
appertaining to the substance of heredity as the phenomena of atavism,
&c. prove, Weismann’s theory postulates the stability of this substance
as absolute. But, as we have now so often seen, he does so in order to
provide a hypothetical basis for his further theory of evolution. In as
far as his theory of heredity is concerned, there is no reason why it
should differ from Darwin’s in this respect.
Again, comparing Weismann’s theory of heredity with that of Galton, we
found that germ-plasm resembles stirp in all the points wherein we have
just seen that it resembles germ-plasm. Or, otherwise stated, all three
theories are thus far coincident. But germ-plasm resembles stirp much
more closely than it does gemmules, seeing that the theory of stirp is
founded on the postulate of “continuity” in exactly the same manner
as is the theory of germ-plasm. In point of fact, the only difference
between these two theories consists in the two further postulates
presented by the latter—viz., that the “continuity” in question has
been unbroken since the origin of life, while the “stability” in
question has been uninterrupted since the origin of sexual propagation.
But seeing that both these additional postulates have reference to
Weismann’s theory of evolution, we may say that his theory of heredity
is, as regards all essential points, indistinguishable from that of
Galton.
The truly scientific attitude of mind with regard to the problem of
heredity is to say, as Galton says, “that we might almost reserve our
belief that the structural [i.e., somatic] cells can react on the
sexual elements at all, and we may be confident that at most they do
so in a very faint degree; in other words, that acquired modifications
are barely, if at all, _inherited_, in the correct sense of that word.”
But for Weismann’s further theory of evolution, it is necessary to
postulate the two additional doctrines in question; and it makes a
literally immeasurable difference to the theory of evolution whether
or not we entertain these two additional postulates. For no matter how
faintly or how fitfully the substance of heredity may be modified by
somatic tissues, by external conditions of life, or even by so-called
spontaneous changes on the part of this substance itself, numberless
causes of congenital variation are thus admitted, while even the
Lamarckian principles are hypothetically allowed some degree of play.
And although this is a lower degree than Darwin supposed, their
influence in determining the course of organic evolution may still have
been enormous; seeing that their action in any degree must always have
been _directive_ on the one hand, and _cumulative_ on the other.
Having thus pointed out the great distinction between the theories
of stirp and of germ-plasm, it became needful to note that Weismann
himself is not consistent in observing it. On the contrary, in some
passages he apparently expresses himself as willing to resign both
his distinctive postulates—continuity as _perpetual_, and stability
as _absolute_. But it is evident that such passages must be ignored
by his critics, because, although as far as his theory of heredity is
concerned they betoken an approach to the less speculative views of
Galton, any such approach is proportionally destructive of his theory
of evolution. It must not be supposed that I am taking an ungenerous
advantage of these occasionally fundamental concessions. On the
contrary, one cannot but admire the candour which they display. But,
as I have said, it is necessary for us to ignore them, if only in
order to examine the Weismannian theory of germ-plasm as a distinctive
theory at all. And more than this. Seeing that his theory of heredity
differs from Galton’s chiefly in being further an elaborate theory
of evolution (founded on the two additional postulates in question),
my main object has been to show the enfeeblement of the former which
Weismann has caused by his addition of the latter. If he were to
express his willingness to abandon his theory of evolution for the
sake of strengthening his theory of heredity by identifying its
main features with those of Galton’s, personally I should have no
criticism to pass. Indeed, I was myself one of the first evolutionists
who called in question the Lamarckian factors; and ever since the
publication of Galton’s theory of heredity at about the same time, I
have felt that in regard to its main principles—or those in which
it agrees with Weismann’s—it is probably the true one. But I can
nowhere find that Weismann is thus prepared to surrender his theory of
evolution. Occasionally he plays fast and loose with the two additional
postulates on which this theory is founded; but he does so without
appearing to perceive the speculative impossibility of any longer
sustaining his temple of evolution if he were to remove its pillars of
germ-plasm.
Ignoring, then, these inconsistencies, we proceeded to examine
separately, and on their own respective merits, the two distinctive
postulates of the theory of germ-plasm—_perpetual_ continuity since
the first origin of life, and _absolute_ stability since the first
origin of sexual propagation.
It does not appear to me that very much has to be said, either for
or against the former postulate, on merely antecedent grounds, or
grounds of general reasoning. Therefore I relegated to an Appendix
my examination of what Weismann has argued on these grounds, while
in the text I considered only what he has advanced as evidence _a
posteriori_. Here, as we saw, he has developed three distinct lines
of verification—viz. (A) the migration of germ-cells in some of
the _Hydromedusae_,(B) the early separation of germ-cells in the
ontogeny of certain Invertebrata, and (C) the alleged invariability of
organisms which are produced parthenogenetically. But we have seen,
with respect to (A), that the specialized character of germinal cells
is a fact which every theory of heredity must more or less recognize;
and, therefore, that the migration of these cells, wherever it may be
found to occur, does not lend any peculiar countenance to Weismann’s
theory. There may be many reasons for such migration other than the one
which this theory assigns; while the reason which it does assign is
rendered improbable by the consideration that in the _Hydromedusae_
the material of heredity is already and richly diffused throughout the
general tissues. (B) and (C) are both contrary to fact; and, therefore,
in whatever measure they would have corroborated the theory had they
proved to be true, in that measure must they be held to discountenance
the theory now that they have been shown to be false.
It appears, then, that there is no evidence in support of the postulate
of the _perpetual_ continuity of germ-plasm. There is nothing to show
the necessary non-inheritance of acquired characters. The only evidence
which one can recognize as good, is that which makes equally in
favour of the theory of stirp—or rather, of the well-known fact that
congenital characters are at any rate much _more_ heritable than are
acquired: which, it is needless to repeat, is a widely different thing
from proving—or even rendering probable—the _absolute_ restriction of
germ-plasm to a separate “sphere” of its own “since the origin of life.”
But now, although there is no evidence in support of this postulate,
there is no small amount of evidence against it. For this evidence
goes to indicate that no small amount of reciprocal action habitually
takes place between body-tissues and germinal elements: indeed it seems
almost to prove that the orbits of germ-plasm and somato-plasm are not
mutually exclusive, but touch and cut each other to a considerable
extent. The evidence in question, it will be remembered, is derived
from the effects of puberty, senility, castration, &c.; the occasional
effect of pollenization on the somatic tissues of plants; the influence
which a stock occasionally exercises upon a scion, or _vice versa_,
which proves the possibility of a transmission of hereditary characters
by a mere grafting together of somatic tissues; the direct evidence
given by De Vries that in certain Algae constituents of cellular tissue
pass immediately from the maternal ovum to the daughter organism; and
the evidence, both direct and indirect, which remains to be given
on a larger scale in my subsequent volume, where we shall have to
challenge the validity of Weismann’s fundamental postulate touching
the non-occurrence of Lamarckian factors in any of the multicellular
organisms.
It must here again be noticed that in those passages where he concedes
the possibly “occasional” transmission of acquired characters
Weismann is annihilating his own theory, root and branch. Thus,
for example, in allusion to De Vries’ observation just mentioned,
he says that we cannot exclude the possibility of “changes being
induced by external conditions in the organism as a whole, and
then communicated to the germ-cells after the manner indicated in
Darwin’s hypothesis of pangenesis.” But it is obvious that the
theory of germ-plasm _must_ “exclude the possibility of such a
transmission occasionally occurring”; for the very essence of that
theory consists in its postulating a difference between germ-plasm
and the general body-substance _in kind_, such that there never _can_
be any “communication” from the one to the other “after the manner
indicated by Darwin’s hypothesis of pangenesis.” Any prevarication
over this point amounts simply to abandoning the theory of germ-plasm
altogether, and opening up a totally distinct issue—namely, the
_relative importance_ of natural selection and the Lamarckian factors
in the process of organic evolution. It may be perfectly true—and I
myself believe it is perfectly true—that Darwin attributed too large
a measure of importance to the Lamarckian factors; but whether or
not he did so is quite a different question from that which obtains
between his theory of pangenesis and Weismann’s theory of germ-plasm.
The former question is whether we are to “_modify_” the theory of
pangenesis, so as to constitute it the theory of stirp; the latter
question is whether we are to “_abolish_” the theory of pangenesis, in
favour of its logical antithesis, the theory of germ-plasm. And this
question remains to be dealt with in my next volume.
* * * * *
Coming then, lastly, to the companion postulate of germ-plasm as
absolutely stable since the first origin of sexual propagation, we had
to observe that, unlike the one we have just been considering, there
is an immensely strong presumption against it on merely antecedent
grounds. That the most complex substance in nature should likewise
be the most stable substance with regard to complexity of “molecular
structure”; that the greater its complexity becomes the greater becomes
its stability, so that while in the comparatively simple unicellular
organisms it is eminently susceptible of modification by external
conditions, it entirely ceases to be thus susceptible when it becomes
evolved into the incomparably more complex and immensely more varied
structures which form the bases of heredity in the multicellular
organisms—where, also, it must come into ever more and more
intricate as well as more and more diverse relations with the external
world;—all this is, I repeat, well nigh incredible. At any rate,
speaking for myself, I should require some enormous weight of evidence
to balance so enormous an antecedent improbability, or before I could
regard such a doctrine as meriting any serious attention.
What, then, is the evidence that has been adduced? We have found
that this evidence is _nil_. On the other hand, we have found that
the evidence against the doctrine is abundantly sufficient to
annihilate the doctrine—and this quite apart from all the antecedent
considerations just alluded to. For not only have we the sundry facts
of bud-variation, a-sexual origin of species, &c., which contradict
the doctrine; but we have also the results of direct experiment, which
prove that the alleged stability of germ-plasm may be conspicuously
upset by slight changes in the external conditions of life. So that
both from within and from without the stability which is alleged in
theory admits of being overturned by facts.
And here, in order to avoid all possible confusion, I must ask it once
more to be noted that there is not, and never has been, any question
touching the _high degree_ of stability which is exhibited by whatever
substance it is that constitutes the material basis of heredity.
But this is a widely different thing from supposing the stability
_absolute_, so that it can never have been affected in any degree since
the first origin of multicellular organisms, or in any of the millions
of species into which these organisms have ramified. And the fact that
in some cases we are actually able to observe a change of congenital
characters as resulting from some “spontaneous” change in the
hereditary material itself (as in bud-variation), or from some change
in the external conditions of life (as in Hoffmann’s experiments)—this
fact is more than is required in order finally to overthrow the
intrinsically untenable doctrine which is in question.
Now, with the collapse of this doctrine there collapses also the
important chain of deductions therefrom, which together constitute
Weismann’s new theory of evolution. In particular, that natural
selection is the exclusive means of modification among all the Metazoa
and Metaphyta, while it is as exclusively ruled out with respect to
all the Protozoa and Protophyta; that individual variations among the
former can only be determined by sexual unions, while among the latter
they can only be determined by the direct action of the environment;
that the origin of congenital variability in all the Metazoa and
Metaphyta is to be sought, and can only be found, in variations which
occurred millions of years ago in the Protozoa and Protophyta; that
the “significance of sexual propagation” is to be found in the view,
that by this means alone can congenital variations have been ever since
produced; &c., &c.
* * * * *
Upon the whole then, it appears to me that both the fundamental
postulates of the theory of germ-plasm are unsound. That the substance
of heredity is largely continuous and highly stable I see many and
cogent reasons for believing. But that this substance has been
uninterruptedly continuous since the origin of life, and absolutely
stable since the origin of sexual propagation, I see even more and
better reasons for disbelieving. And inasmuch as these two latter,
or distinctive, postulates are not needed for Weismann’s theory of
heredity, while they are both essential to his theory of evolution,
I cannot but regret that he should thus have crippled the former by
burdening it with the latter. Hence my object throughout has been
to display, as sharply as possible, the contrast that is presented
between the brass and the clay in the colossal figure which Weismann
has constructed. Hence, also, my emphatic dissent from his theory of
evolution does not prevent me from sincerely appreciating the great
value which attaches to his theory of heredity. And although I have
not hesitated to say that this theory is, in my opinion, incomplete;
that it presents not a few manifest inconsistencies, and even logical
contradictions; that the facts on which it is founded have always
been facts of general knowledge; that in all its main features it was
present to the mind of Darwin, and distinctly formulated by Galton;
that in so far as it has been constituted the basis of a more general
theory of organic evolution, it has clearly proved a failure:—such
considerations in no wise diminish my cordial recognition of the
services which its distinguished author has rendered to science by his
speculations upon these topics. For not only has he been successful
in drawing renewed and much more general attention to the important
questions touching the transmissibility of acquired characters, the
causes of variation, and so on; but even those parts of his system
which have proved untenable are not without such value as temporary
scaffoldings present in relation to permanent buildings. Therefore, if
I have appeared to play the _rôle_ of a hostile critic, this has only
been an expression of my desire to separate what seems to me the grain
of good science from the chaff of bad speculation. And the candour
which Professor Weismann has always displayed towards criticism of this
character enables me to hope with assurance, that I have said nothing
which he himself will regard as inconsistent with high admiration of
his work as a naturalist, or of his originality as a philosopher.