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 undifferentiated 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, qu 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.