LATER ADDITIONS TO WEISMANN’S SYSTEM UP TO THE YEAR 1892.
I HAVE now furnished as complete a _résumé_ as seems desirable for
present purposes of Weismann’s theory of germ-plasm, considered both
as a theory of heredity and as a sequent theory of organic evolution.
But before proceeding to examine this elaborate system as a whole, I
must devote another chapter to a further statement of certain later
additions to—and also emendations of—the system as it was originally
propounded. These additions and alterations have reference only to
the theory of heredity: they do not affect the theory of organic
evolution as originally deduced therefrom. Moreover they have all been
due to our more recently acquired knowledge touching the morphology
and physiology of cell-nuclei: it is for the purpose of bringing his
theory of germ-plasm into accord with these results of later researches
that Weismann has thus modified the theory as it originally stood.
For my own part, I do not see that very much is gained by these
newer additions and modifications; but, be this as it may, they are
certainly very complicated, and on this account I have thought it best
to devote a separate chapter to their consideration. Furthermore,
not only in the opinion of Weismann himself, but also in that both of
his friends and foes, the main question with which his later essays
are concerned—viz., as to whether the nucleus of a cell is the only
part of a cell which is concerned in the phenomena of heredity—is
regarded as of fundamental importance to his entire edifice. Hence,
although I cannot myself perceive that the indisputable importance of
this question to any speculations on the subject of heredity is of
such special or vital significance to Weismann’s theory, it becomes
necessary for me to supply this further chapter for the purpose of
presenting the further developments of his theory.
First of all, Weismann has of late years considerably modified his
original view touching the relation of germ-cells to body-cells.
For while he originally supposed the fundamental distinction in
kind to obtain as between the whole contents of a germ-cell and the
whole contents of a somatic-cell, he now regards this distinction as
obtaining only between the nucleus of a germ-cell and the nucleus of
a somatic-cell. In other words, he regards the whole of a germ-cell,
with the exception of its nucleus, as resembling the whole of any other
cell, with the exception of _its_ nucleus. It is the nucleus of a
germ-cell alone that contains germ-plasm: all the rest of such a cell
being “nutritive, but not formative.”
This transference of the peculiar or hereditary powers of a germ-cell
from the cell as a whole to the nucleus, necessitates certain
emendations of the original theory of germ-plasm. In particular,
the broad distinction between the whole contents of a germ-cell
as “germ-plasm,” and the whole contents of a somatic-cell as
“somato-plasm,” is now discarded; and in its stead we have all nuclear
matter (whether of germ-cells or somatic-cells) comprised under the one
denomination of “nucleo-plasm,” in contradistinction to all the other
protoplasm of a cell, which is called “cytoplasm.” Hence Weismann now
regards the cytoplasm of a germ-cell as identical with the cytoplasm
of all other cells. Its function is merely that of “nourishing” the
nucleus, while, on the other hand, it is “controlled” by the nucleus as
to its own growth, shape, size, and eventual division.
But it is evident that the nucleo-plasm of a germ-cell must differ from
the nucleo-plasm of a somatic-cell, in that it not only “controls” the
growth, &c. of its own cell, but likewise presents all the additional
characters peculiar to a germ-cell. That is to say, the nucleo-plasm
of a germ-cell resembles the nucleo-plasm of a somatic-cell in that
it is nourished by, and exercises control over, the cytoplasm of
its own particular cell; but it differs from the nucleo-plasm of a
somatic-cell in admitting of fertilization, in the capability of
reproducing an entire organism, in the endowing of that organism with
all its hereditary characters, and, lastly, in providing for its own
reproduction in the next generation.
Thus it is evident, as Weismann puts it, that the nucleo-plasm of
a germ-cell must be of _two kinds_—one being concerned with the
formation and control of the germ-cell only, while the other has
to do with the construction of an entire future organism, and the
subsequent reproduction thereof. But not only so; for at each stage
in the construction of this future organism, all the somatic-cells,
as successively constructed, must likewise contain nucleo-plasm in
two kinds—one having to do only with the formation and control of its
own individual cell, and the other having to do with the formation
of the future somatic-cells, which will have to follow in the course
of ontogeny. Therefore, in order to designate this second kind of
nucleo-plasm (whether in a germ-cell or a somatic-cell) Weismann
borrows from Nägeli the term “idio-plasm[6],” or rather, I should say,
he uses the term “nucleo-plasm” when he is speaking of all the contents
of a nucleus indiscriminately, while he uses the term “idio-plasm” when
he has occasion to speak specially of the two kinds of nucleo-plasm now
before us.
Hence, the nuclear contents (nucleo-plasm) of every cell, whether
germinal or somatic, present two substances, which we may, in the
absence of any better terms supplied by Weismann himself, respectively
designate “idio-plasm-A” and “idio-plasm-B.” Idio-plasm-A is the
substance which has to do only with the formation and control of the
individual cell in which it resides, like a mollusc in its shell.
Idio-plasm-B is the substance out of which future cells are to be
formed and controlled, when in due course either of ontogeny or
phylogeny this idio-plasm-B becomes converted into idio-plasm-A,—i.e.,
into each subsequently developing tissue or organism, as the case may
be. I say ontogeny or phylogeny, and tissue or organism, because, where
a _germ-cell_ is concerned, idio-plasm-B is capable of reproducing
entire organisms of its own and of subsequent generations; whereas,
in the case of all _somatic_-cells, idio-plasm-B is capable only
of reproducing, stage by stage, some greater or less number of the
cells which are to construct the single organism of which they form a
part. Or, otherwise expressed, in the particular case of a germ-cell
idio-plasm-B is germ-plasm, and therefore is alone capable of producing
an entire organism of somatic-cells, while it is likewise alone capable
of reproducing successive organisms; for it alone contains the carriers
of heredity[7].
Thus, idio-plasm-B of an unsegmented germ-nucleus is germ-plasm. But
as soon as the germ-nucleus has undergone its first nuclear division,
its nucleo-plasm is no longer germ-plasm, inasmuch as each of the
half-portions is now no longer capable of reproducing an entire
organism—unless it be in the case of identical twins. Similarly in
the second nuclear division, each of the four resulting idio-plasms-B
is still further removed from the pristine character of germ-plasm;
and so on through all successive stages of segmentation. Hence
these successive nuclear divisions must indicate a partitioning and
re-partitioning of the original idio-plasm-B (germ-plasm) into the
idio-plasms-B severally distinctive of all the various cells of the
soma.
Now, it is evident that not _all_ the idio-plasm-B of a germ-cell which
thus passes over into the nuclei of somatic-cells can be represented
by the idio-plasm-B of those cells. At every stage of successive
cell-formation a certain part of the original idio-plasm-B of the
germ-cell must become the idio-plasm-A of somatic-cells distinctive
of that stage. For, supposing that at its differentiation stage 99
the original germ-plasm (now somatic-idio-plasm-B of 99th stage) has
reached a phase of ontogeny where the formation of tissue _m_ has
next to be followed by the formation of tissue _n_, then there still
remain the further differentiation stages 101, 102, 103, &c., to be
provided for, which, when their time arrives, will go to form the
still later tissues _o_, _p_, _q_, &c. Consequently the idio-plasm-B
of stage 100 cannot be _all_ consumed in making the tissue _n_. There
must be a residual portion which will afterwards be called upon to
form successively the idio-plasm-A of _o_, _p_, _q_, &c. Where, then,
is this residual portion of idio-plasm posited? Clearly it must be
posited in the nuclei of _n_. Thus it is that, as we began by stating,
all the nuclei of any given tissue _n_ really contain two kinds of
substance,—(1) their own idio-plasm-A, which was part of idio-plasm-B
of the preceding tissue, _m_; and (2) the idio-plasm-B, which is
destined to become idio-plasms-A of succeeding tissues _o_, _p_, _q_,
&c. Thus it follows also that the more the original idio-plasm-B is
differentiated into these successive formations of idio-plasms-A the
less of it remains for further differentiation, till, at the last
stage of ontogeny, all the original idio-plasm-B (germ-plasm) has
been thus changed into idio-plasms-A severally distinctive of all the
somatic-tissues _a_, _b_, _c_—_x_, _y_, _z_,—save only the portion
of it which has been carried through all these ontogenetic stages in a
wholly _un_differentiated condition, for the purpose of securing the
_phylogenetic_ production of the next generation. And this, of course,
is secured by the portion of undifferentiated germ-plasm in question
being deposited in the nuclei of germ-cells, at whatever stage of the
ontogeny these may be formed.
Finally, it is evident that, _at each stage_ of the differentiation of
idio-plasm-B into idio-plasms-A, the portion concerned must be capable
of self-multiplication to an almost incalculable extent,—yet this only
as idio-plasm-B of the particular kind required for constructing the
idio-plasm-A which is appropriate to the particular stage. Such is a
necessary deduction from the terms of Weismann’s theory, inasmuch as we
know that at each of the ontogenetic stages there is an incalculable
multiplication of cells belonging to that stage—cells, the “cytoplasm”
of which necessarily presupposes for its formation its own appropriate
idio-plasm in both kinds, and this in similarly increased quantities.
* * * * *
From the above theory it follows that an explanation can be given of
the healing of wounds (as in ourselves), of the regeneration of lost
parts (as the limb of a newt), or even of the reproduction of an entire
organism from a mere fragment of somatic-tissue (as in the cases
already alluded to at the commencement of this chapter—viz. the leaf
of Begonia, portions of sea-anemones, jelly-fish, &c.). For in all
these cases of repair, regeneration, and what may be called _somatic
reproduction_, we have only to suppose that not all the idio-plasm-B of
any given ontogenetic stage is consumed in the formation of that stage,
and therefore that the residue is passed on to the later stages _in a
latent condition_. It will then be available at any time to re-develop
tissue corresponding to that particular stage, should that particular
tissue happen to be lost by accident or disease. For example, if some
of the idio-plasm-B of the very first ontogenetic stage, or true
germ-plasm, should thus be passed on in an undifferentiated condition
through the somatic-tissues subsequently formed at later ontogenetic
stages, then we can understand why an _entire_ organism is reproduced
from a fragment of these tissues—or of those among which particles of
such residual and undifferentiated germ-plasm happen to be scattered.
Similarly, if idio-plasm-B of the ontogenetic stage at which a limb is
formed be not all consumed in constructing the limb, then the limb, if
afterwards lost, will be reconstructed, although an entire organism
will not be reproduced from a fragment of somatic-tissue. And similarly
also with the mere repair of injuries, where the only overplus of
idio-plasm-B is that of idio-plasm-B belonging to the very last stages
of ontogeny.
But, it is almost needless to observe, this kind of transmission of
idio-plasm-B from one stage of ontogeny in an unaltered condition
to subsequent stages, is not to be confused with the other kind of
transmission previously referred to, whereby idio-plasm-B of one
stage becomes successively transformed into the idio-plasms-A of
successive stages. In the former case, at whatever stage of ontogeny
the transmission may start from, the idio-plasm-B from that stage lies
dormant, and is never destined to undergo further differentiation,
unless the results of accident or disease should call upon it to do
so. In the latter case, on the other hand, the idio-plasm-B of any
given stage is passed on to the next stage for the express purpose of
transforming itself into the idio-plasms-A of that and, in due order,
of all subsequent stages.
* * * * *
It will be observed that all this elaboration of the original theory
of germ-plasm—an elaboration which is largely derived from the
speculative writings of Nägeli—serves no other purpose than that of
indicating what Professor Weismann now regards as the most probable
_mode_ in which germ-plasm undergoes its modification into the various
kinds of somatic-cells. For, inasmuch as the idio-plasms-B of all
somatic-cells are originally derived from that of the germ-cell,
and inasmuch as each expends its formative energies exclusively in
constructing and controlling the cells which, as idio-plasms-A, they
respectively inhabit, it is still the germ-plasm of the original
germ-cell that is finally converted into the various tissues which
together constitute the soma—notwithstanding that, in order thus to
become transmuted into body-substance, or somato-plasm,it must pass
through the sundry intermediate stages of idio-plasm-B, idio-plasm-A,
and cytoplasm, of any given ontogenetic stage. Hence I do not see that
it makes any substantial difference to Weismann’s theory of heredity,
whether we speak of germ-plasm being converted into “somato-plasm,”
or into “idio-plasm” _plus_ “somatic-idio-plasm,” _plus_ “cytoplasm.”
But as Weismann himself thinks that it does make some great difference
whether we adhere to his original generic term “somato-plasm,” or adopt
his newer and more specific terms as just enumerated, I append _in
extenso_ the most recent exposition of his views upon this subject[8].
Before quitting this somewhat complicated addition to the original
theory of germ-plasm, I must briefly allude to the descriptions and
illustrations of karyokinesis which were given in Part I of _Darwin and
after Darwin_, for the prospective benefit of any general readers who
might afterwards be sufficiently interested in Weismann’s speculations
to desire a statement of the main facts on which this further
development of his theory rests. It seemed undesirable to burden the
present volume with an account of recent investigations so well known
to naturalists, while, on the other hand, it was clearly desirable
that such an account should be given somewhere, if the speculations in
question were to be rendered intelligible to anybody else. Therefore I
must here request those of my readers who are not already acquainted
with the matter to consult pp. 128-134 of Part I. It will there be
seen how enormously complex are the visible processes which take
place in the nucleus of a germ-cell (and likewise of a somatic-cell),
preparatory to its division; and therefore, supposing that the nucleus
alone contains the material concerned in the phenomena of heredity,
it appears that no small corroboration is lent to Weismann’s views by
these histological observations. And, more particularly, if we suppose
with him that the material in question is restricted to that portion of
the segregating nuclear matter which is called the “nuclear thread[9],”
in the formation of the “loops” or “rods” of this substance we seem to
have presented a visible expression of the marshalling of “the carriers
of heredity,” and the successive passage of the originally generalized
“germ-plasm” of the germ-cell into the ever more and more specialized
“nucleo-plasms” of the somatic-cells. Indeed, the new theory of
heredity, when thus brought into relation with the new results of
histological observation, appears so well to fit the latter, that one
would be sorry to find the coincidence unmeaning, or the theory false.
But, without passing any criticism, it is sufficient to note that the
question whether or not the theory is true—and therefore correctly
interprets the phenomena of karyokinesis,—must depend chiefly on
whether it be eventually proved that the “nuclear thread” is indeed
the only part of a germ-cell, or even the only part of a tissue-cell,
which is concerned in controlling the phenomena of heredity on the one
hand, and of ontogeny on the other. Into this question, however, I do
not propose to enter. It will be enough to assume, for the sake of
argument, that Weismann’s view of the matter will eventually prove to
be true. At the same time, we must remember that at present this view
as to the nuclear thread being the sole repository of the material of
heredity is merely hypothetical.
* * * * *
We now arrive at the last of those features in Weismann’s theory of
heredity, the importance of which necessitates mention in such a mere
statement of the theory as the present chapter is concerned with.
According to Weismann’s own view of his theory, two objections have to
be met. In the first place, there is the objection that all individuals
_which are born of the same parents_ are not exactly alike, as the
theory might have expected they would be, seeing that the admixture of
identical germ-plasms has been concerned in the formation of the whole
progeny. In the second place, and quite apart from this objection,
there is the difficulty that, if every act of fertilization essentially
consists in a fusion of one mass of germ-plasm belonging to a male
germ-cell with another mass belonging to a female germ-cell, at each
generation the mass of germ-plasm contained in an egg-cell must be
doubled—with the result that ova must progressively increase in size
during the course of phylogeny. But ova do not thus progressively
increase in size. Therefore, if the imperishable nature of germ-plasm
is to be theoretically sustained, it is necessary to show some means
whereby ova and spermatozoa are able to get rid of at least one half of
their respective germ-plasms in each generation—i.e., before each act
of impregnation. Weismann meets both these difficulties by an appeal to
the following facts.
It is well known that the ripe ovum extrudes two minute particles
of protoplasmic substance, which are called polar bodies[10]. These
both proceed from the nucleus of the ovum, but are not formed
simultaneously. For the first polar body is really one half of the
original nucleus of the cell, and therefore is formed by the first
segmentation of this nucleus. The second polar body, on the other
hand, is one half of the remaining nucleus, and is similarly formed
by the second segmentation. Hence, when both polar bodies have been
extruded from the ovum, only one quarter of the original nuclear matter
remains. So far, of course, the facts prove too much for Weismann’s
theory, because the theory wants to get rid of only one half of the
original nuclear matter before impregnation, _if all the nuclear matter
be germ-plasm_. Therefore Weismann concludes that all the original
nuclear matter of the ripe ovum is not germ-plasm, but that only one
half of it is so, while the other half—or that half which goes to
constitute the first polar body—is idio-plasm-A, which, as we have
already seen, the egg-cell shares in common with all other cells. It
is merely “ovogenetic”: its function is that of constructing the ovum,
_quâ_ cell: it has nothing whatever to do with the germ-plasm which the
particular cell contains. Therefore, having discharged its function of
constructing this cell, it is itself discharged from the cell as the
first polar body.
The nucleus of the fully-formed ovum having thus got rid of all
its superfluous idio-plasm-A by throwing off the first polar body,
is supposed henceforth to consist of pure germ-plasm (i.e., of
idio-plasm-B belonging to the first ontogenetic stage), and one half
of this is next got rid of by the second segmentation in the form of
the second polar body. Therefore, according to the theory and so far as
the problems of heredity are concerned, we need not any further trouble
ourselves about the first polar body. But it will at once be seen that
by the interpretation which Weismann puts upon the second polar body,
and also, of course, upon the extrusion of some of its nuclear matter
by the male cell, he meets both the difficulties against his theory of
germ-plasm which we are now engaged in considering.
That he thus meets the second of those difficulties—i.e., concerning
the otherwise perpetual accumulation of germ-plasm—is evident without
explanation. That he likewise meets the first—i.e., concerning the
non-resemblance of individuals born of the same parents—is scarcely
less evident. For it is hardly conceivable that such a complex mass
of germ-plasms as the nucleus of a fertilized ovum must be could ever
present in any two eggs precisely the same proportional representation
of the “carriers of heredity,” after one half of each set had been thus
discharged from each egg. Therefore, if the second polar body removes
from each egg one half of the ancestral germ-plasms, “every egg will
contain a somewhat different combination of hereditary tendencies, and
thus the offspring which arise from the different germ-cells of the
same mother can never be identical[11].
Such, then, is Weismann’s theory of the physiological meaning of
polar bodies. And as the bearing of this particular theory on his
more general theory of heredity does not appear to me a vitally
intimate one, I think my subsequent examination of the main theory
will be simplified if I now proceed at once to an examination of the
subordinate one. For by doing this I shall hope to show that the
bearings just mentioned are of much less importance than he represents
them to be; and, therefore, that we may hereafter proceed to consider
his theory of heredity without any special reference to his theory of
polar bodies.
To begin with, as regards the first polar body, one would like to
know more clearly why it is necessary that this residuum of merely
“ovogenetic idio-plasm” (or idio-plasm-A of the egg-cell) has to be got
rid of before the germ-plasm can proceed to discharge its physiological
functions. Seeing that both these (hypothetically) very different
materials occur in the self-same nucleus, some very delicate mechanism
must be needed for their separation; and it is not apparent why such a
mechanism should have been evolved, rather than what would have been
the simpler plan of adapting the germ-plasm to hold its own against the
idio-plasm-A, even if one could see that any interference between these
very different substances is in any way probable. For my own part, at
all events, I cannot see why this microscopical atom of “ovogenetic
idio-plasm” should not simply be left to be absorbed among the millions
of cells that afterwards go to form the foetus.
Again, as regards the second polar body, Weismann’s theory of it is
framed to explain, (_a_) how the excess of germ-plasm is got rid of
in each ontogeny, and (_b_) why the offspring of the same parents
do not all precisely resemble one another. These, be it observed,
are the only two functions which Weismann’s theory of polar bodies
subserves in relation to his theory of germ-plasm. But, it appears
to me, neither of these functions is necessary, in so far as any
requirements of the latter theory are concerned. For surely, polar
bodies or no polar bodies, there is already a mechanism at work in
each ontogeny which is of itself sufficient to discharge both these
functions, and so to anticipate both the supposed difficulties which
the subsidiary theory is adduced to meet. The very essence of ontogeny,
as a process, itself consists in a continuous succession of nuclear
divisions—and this not only as regards somatic-cells, but also as
regards germ-cells. Now, in the great majority of organisms, there is
an infinitely greater number of germ-cells (both male and female) than
can possibly be required either for the purpose of getting rid of any
excess of germ-plasms in the nucleus of each cell, or of preventing
the germ-plasms of any one germ-cell precisely resembling those of any
other. If every plant or animal produced only a single female-cell or a
single male-cell, then indeed we might require from Professor Weismann
a demonstration of some special mechanism to secure the expulsion of
half its ancestral germ-plasms; since otherwise the single female-cell
or male-cell would have to increase its dimensions in each successive
generation. But, as matters actually stand, nature seems to have made
much more than ample provision for preventing the undue accumulation
of ancestral germ-plasms in any individual germ-cell, by enormously
multiplying; through continuous division and subdivision, the _number_
of germ-cells in each ontogeny. And similarly, of course, as regards
the different aggregations of ancestral germ-plasms which are left
for distribution among these innumerable germ-cells. “If one group
of ancestral germ-plasms is expelled from one egg, and a different
group from another egg, it follows that no two eggs can be exactly
alike as regards their contained hereditary tendencies.” Granted; but
this consideration applies equally to the original segregation of
germ-plasms in the multiplying eggs of each ontogeny—for it follows
from the theory of germ-plasm that the most primitive egg-cell in each
ontogeny must have contained all the ancestral germ-plasms which are
afterwards distributed among its innumerable progeny of egg-cells.
And, as far as the facts of “individual variation” are concerned, I do
not see why the differential partitioning of “ancestral idio-plasms”
should be any better secured by nuclear division of a mature germ-cell
than by that of an immature. Less so, indeed; for the wonder is that
during the many-thousand-fold division of an immature ovum so precise
a distribution of these “ancestral idio-plasms” is maintained, as is
proved to be maintained (on the theory of germ-plasm) by the facts of
heredity.
However, Weismann takes a widely different view of the matter. For
while he allows that “such an early reducing division would offer
advantages in that nothing would be lost, for both the daughter
nuclei would (? might) become eggs, instead of one of them being lost
as a polar body”—while he allows this, he nevertheless rejects the
possibility of “such an early reducing division.” But I do not see that
the reasons which he assigns for this rejection of it are adequate.
First, he says that if this were the way in which the superfluous
germ-plasm of each generation were got rid of, _far too much_ provision
has been made for the purpose,—seeing that the practically indefinite
number of nuclear divisions which the immature germ-cells undergo
would cause a much “greater decrease of the ancestral idio-plasms
of each than could afterwards be compensated by the increase due
to fertilization.” But this rejoinder is of cogency only if it be
supposed that at each nuclear division of an immature ovum, “the
ancestral idio-plasms” (germ-plasm) are incapable of the power of
self-multiplication which soon afterwards becomes one of its most
essential characters. Why, then, should we suppose this substance to be
totally incapable of increase in the multiplying ova of ontogeny, when
at the same time we are to suppose the same substance capable of any
amount of increase in the multiplying ova of phylogeny? To this obvious
question no answer is supplied: in fact the question is not put.
Secondly, Weismann says that in parthenogenetic ova only one polar body
is extruded. This he regards as equivalent to the first polar body of a
fertilizable ovum (i.e., as composed of ovogenetic nuclear substance);
and hence he argues that the second polar body of a fertilizable ovum
must be regarded as composed of germ-plasm. But even supposing that he
is right as to the fact that parthenogenetic ova invariably extrude but
one polar body, his argument from this fact would only be available
after we had already accepted his view touching the character of the
second polar body. So long as this view is itself the subject of
debate, he cannot prove it by the fact in question. In other words,
unless we have already agreed that the second polar body has the
function which Weismann assigns to it, we cannot accept the fact which
he adduces as furnishing any evidence of his view touching the function
of the second polar body.
For these reasons I cannot see that the subordinate theory of
polar bodies is required in the interests of the general theory of
germ-plasm. The difficulties which it is adduced to meet do not appear
to me to be any difficulties at all. Therefore, in now proceeding
to consider what in my opinion are the real difficulties which lie
against the major theory of germ-plasm, I shall not again allude to the
minor—and, in this connexion, superfluous—theory of polar bodies.
Such, then, is Professor Weismann’s theory of heredity in its original
and strictly logical form. In the course of our examination of it
which is to follow in Chapter III and IV, we shall find that in almost
every one of its essential features, as above stated, the theory has
had to undergo—or is demonstrably destined to undergo—some radical
modification. But I have thought it best to begin by presenting the
whole theory in its completely connected state, as it is in this
way alone that we shall be able to disconnect what I regard as the
untenable parts from the parts which still remain for investigation at
the hands of biological science. Such, indeed, is the only object of
my “Examination of Weismannism.” For, rightly or wrongly, it appears
to me that the unquestionable value of his elaborate speculations is
seriously discounted by certain oversights with regard to matters of
fact, and not a few inconsistencies touching matters of theory. In
displaying both these defects, I am not without hope that the result
may be that of inducing Professor Weismann so to modify his system of
theories as to strengthen the whole by removing its weaker parts.