Volume Ii Part 8 (1/2)

CHAPTER V.

CHARACTERS AS HEREDITARY AND ACQUIRED (_continued_).

(A. and B.)

_Direct and Indirect Evidence in favour of the Non-inheritance of Acquired Characters_[81].

[81] [_See_ note appended to Preface. C. LI. M.]

The strongest argument in favour of ”continuity” is that based upon the immense difference between congenital and acquired characters in respect of heritability. For that there is a great difference in this respect is a matter of undeniable fact. And it is obvious that this difference, the importance of which must be allowed its full weight, is just what we should expect on the theory of the continuity of the germ-plasm, as opposed to that of pangenesis. Indeed it may be said that the difference in question, while it const.i.tutes important _evidence_ in favour of the former theory, is a _difficulty_ in the way of the latter. But here two or three considerations must be borne in mind.

In the first place, this fact has long been one which has met with wide recognition and now const.i.tutes the main ground on which the theory of continuity stands. That is to say, it was the previous knowledge of this contrast between congenital and acquired characters which led to the formulation of a theory of continuity by Mr. Galton, and to its subsequent development by Prof Weismann.

But, in the second place, there is a wide difference between the certainty of this fact and that of the theory based upon it. The certain fact is, that a great distinction in respect of heritability is observable between congenital and acquired characters. The theory, as formulated by Weismann, is that the distinction is not only great but absolute, or, in other words, that in no case and in no degree can any acquired character be ever inherited. This hypothesis, it will be observed, goes far beyond the observed fact, for it is obviously possible that, notwithstanding this great difference in regard to heritability between congenital and acquired characters, the latter may nevertheless, sometimes and in some degree, be inherited, however much difficulty we may experience in observing these lesser phenomena in presence of the greater. The Weismannian hypothesis of _absolute_ continuity is one thing, while the observed fact of at least a _high relative degree_ of continuity is quite another thing. And it is necessary to be emphatic on this point, since some of the reviewers of my _Examination of Weismannism_ confound these two things. Being apparently under the impression that it was reserved for Weismann to perceive the fact of there being a great difference between the heritability of congenital and acquired characters, they deem it inconsistent in me to acknowledge this fact while at the same time questioning the hypothetical basis of his fundamental postulate touching the absolute continuity of germ-plasm. It is one merit of Galton's theory, as against Weismann's, that it does not dogmatically exclude the possible interruption of continuity on some occasions and in some degree. Herein, indeed, would seem to lie the central core of the whole question in dispute. For it is certain and has long been known that individually acquired characters are at all events much less heritable than are long-inherited or congenital ones. But Lamarckian theory supposes that congenital characters were in some cases originally acquired, and that what are now blastogenetic characters were in some cases at first somatogenetic and have become blastogenetic only in virtue of sufficiently long inheritance. Since Darwin's time, however, evolutionists (even of the so-called Lamarckian type) have supposed that natural selection greatly a.s.sists this process of determining which somatogenetic characters shall become congenital or blastogenetic. Hence all schools of evolutionists are, and have long been, agreed in regarding the continuity principle as true in the main. No evolutionist would at any time have propounded the view that one generation depends for _all_ its characters on those acquired by its _immediate_ ancestors, for this would merely be to unsay the theory of Evolution itself, as well as to deny the patent facts of heredity as shown, for example, in atavism. At most only some fraction of a _per cent._ could be supposed to do so. But Weismann's contention is that this principle is not only true in the main, but _absolutely_ true; so that natural selection becomes all in all or not at all. Unless Weismannism be regarded as this doctrine of absolutism it permits no basis for his attempted theory of evolution.

And, whatever may be said to the contrary by the more enthusiastic followers of Prof. Weismann, I must insist that there is the widest possible difference between the truly scientific question of fact which is a.s.sumed by Weismann as answered (the base-line of the diagram on p.

43), and the elaborate structure of deductive reasoning which he has reared on this a.s.sumption (the Y-like structure). Even if the a.s.sumption should ever admit of inductive proof, the almost bewildering edifice of deductive reasoning which he has built upon it would still appear to me to present extremely little value of a scientific kind. Interesting though it may be as a monument of ingenious speculation hitherto unique in the history of science, the mere flimsiness of its material must always prevent its far-reaching conclusions from being worthy of serious attention from a biological point of view. But having already attempted to show fully in my _Examination_ this great distinction between the scientific importance of the question which lies at the base of ”Weismannism,” and that of the system which he has constructed on his a.s.sumed answer thereto, I need not now say anything further with regard to it.

Again, on the present occasion and in this connexion I should like to dissipate a misunderstanding into which some of the reviewers of the work just mentioned have fallen. They appear to have concluded that because I have criticized unfavourably a considerable number of Weismann's theories, I have shown myself hostile to his entire system.

Such, however, is by no means the case; and the misunderstanding can only be accounted for by supposing that the strongly partisan spirit which these critics display on the side of neo-Darwinism has rendered them incapable of appreciating any attempt at impartial--or even so much as independent--criticism. At all events, it is a matter of fact that throughout the work in question I have been particularly careful to avoid this misunderstanding as to my own position. Over and over again it is there stated that, far from having any objection to the principle of ”Continuity” as represented in the base-line of the above diagram, I have been convinced of its truth ever since reading Mr. Galton's _Theory of Heredity_ in 1875. All the ”hard words” which I have written against Weismann's system of theories have reference to those parts of it which go to const.i.tute the Y-like structure of the diagram.

It is, however, desirable to recur to another point, and one which I hope will be borne in mind throughout the following discussion. It has already been stated, a few pages back, that the doctrine of continuity admits of being held in two very different significations. It may be held as absolute, or as relative. In the former case we have the Weismannian doctrine of germ-plasm: the substance of heredity is taken to be a substance _per se_, which has always occupied a separate ”sphere” of its own, without any contact with that of somatoplasm further than is required for its lodgement and nutrition; hence it can never have been in any degree modified as to its hereditary qualities by use-inheritance or any other kind of somatogenetic change; it has been _absolutely_ continuous ”since the first origin of life.” On the other hand, the doctrine of continuity may be held in the widely different sense in which it has been presented by Galton's theory of Stirp. Here the doctrine is, that while for the most part the phenomena of heredity are due to the continuity of the substance of heredity through numberless generations, this substance (”Stirp”) is nevertheless not absolutely continuous, but may admit, in small though c.u.mulative degrees, of modification by use-inheritance and other factors of the Lamarckian kind. Now this all-important distinction between these two theories of continuity has been fully explained and thoroughly discussed in my _Examination_; therefore I will not here repeat myself further than to make the following remarks.

The Weismannian doctrine of continuity as absolute (base-line of the diagram) is necessary for the vast edifice of theories which he has raised upon it (the Y), first as to the minute nature and exact composition of the substance of heredity itself (”Germ-plasm”), next as to the precise mechanism of its action in producing the visible phenomena of heredity, variation, and all allied phenomena, and, lastly, the elaborate and ever-changing theory of organic evolution which is either founded on or interwoven with this vast system of hypothetic speculation. Galton's doctrine of continuity, on the other hand, is a ”Theory of Heredity,” and a theory of heredity alone. It does not meddle with any other matters whatsoever, and rigidly avoids all speculation further than is necessary for the bare statement and inductive support of the doctrine in question. Hence, it would appear that this, the only important respect wherein the doctrine of continuity as held by Galton differs from the doctrine as held by Weismann, arisen from the necessity under which the latter finds himself of postulating _absolute_ continuity as a logical basis for his deductive theory of the precise mechanism of heredity on the one hand, and of his similarly deductive theory of evolution on the other. So far as the doctrine of continuity is itself concerned (i.e. the question of the inheritance of acquired characters), there is certainly no more inductive reason for supposing the continuity absolute ”since the first origin of life,” than there is for supposing it to be more or less susceptible of interruption by the Lamarckian factors. In other words, but for the sake of constructing a speculative foundation for the support of his further theories as to ”the architecture of germ-plasm” and the factors of organic evolution, there is no reason why Weismann should maintain the absolute separation of the ”sphere” of germ-plasm from that of somatoplasm. On the contrary, he has no reason for concluding against even a considerable and a frequent amount of cutting, or overlapping, on the part of these two spheres.

But although this seems to me sufficiently obvious, as I have shown at greater length in the _Examination of Weismannism_, it must not be understood that I hold that there is room for any large amount of such overlapping. On the contrary, it appears to me as certain as anything can well be that the amount of such overlapping from one generation to another, if it ever occur at all, must be exceedingly small, so that, if we have regard to only a few sequent generations, the effects of use-inheritance, and Lamarckian factors are, at all events as a rule, demonstrably imperceptible. But this fact does not const.i.tute any evidence--as Weismann and his followers seem to suppose--against a possibly important influence being exercised by the Lamarckian factors, in the way of gradual increments through a long series of generations.

It has long been well known that acquired characters are at best far less fully and far less certainly inherited than are congenital ones.

And this fact is of itself sufficient to prove the doctrine of continuity to the extent that even the Lamarckian is rationally bound to concede. But the fact yields no proof--scarcely indeed so much as a presumption--in favour of the doctrine of continuity as absolute. For it is sufficiently obvious that the adaptive work of heredity could not be carried on at all if there had to be a discontinuity in the substance of heredity at every generation, or even after any very large number of generations.

Little more need be said concerning the arguments which fall under the headings A and B. The Indirect evidence is considered in Appendix I of the _Examination of Weismannism_; while the Direct evidence is considered in the text of that work in treating of Professor Weismann's researches on the _Hydromedusae_ (pp. 71-76).

The facts of karyokinesis are generally claimed by the school of Weismann as making exclusively in favour of continuity as absolute. But this is a partisan view to take. In any impartial survey it should be seen that while the facts are fairly interpretable on Weismann's theory, they are by no means proof thereof. For any other theory of Heredity must suppose the material of heredity to be of a kind more or less specialized, and the mechanism of heredity extremely precise and well ordered. And this is all that the facts of karyokinesis prove. Granting that they prove continuity, they cannot be held to prove that continuity to be absolute. In other words, the facts are by no means incompatible with even a large amount of commerce between germ-plasm and somato-plasm, or a frequent transmission of acquired characters.

Again, Weismann's theory, that the somatic and the germ-plasm determinants may be similarly and simultaneously modified by external conditions may be extended much further than he has used it himself, so as to exclude, or at any rate invalidate, _all_ evidence in favour of Lamarckianism, other than the inheritance of the effects of use and disuse. All evidence from apparently inherited effects produced by change of external conditions is thus virtually put out of court, leaving only evidence from the apparently inherited effects of functionally produced modifications. And this line of evidence is invalidated by Panmixia. Hence there remain only the arguments from selective value and co-adaptation. Weismann meets these by adducing the case of neuter insects, which have been already considered at sufficient length.

(C.) _Experimental Evidence as to the Non-inheritance of Acquired Characters._

Let us now proceed to the experimental evidence which has been adduced on the side of Weismannism.

Taking this evidence in order of date, we have first to mention that on which the school of Weismann has. .h.i.therto been satisfied almost exclusively to rely. This is the line of negative evidence, or the seeming absence of any experimental demonstration of the inheritance of acquired characters. This kind of evidence, however, presents much less cogency than is usually supposed. And it has been shown in the last chapter that the amount of experimental evidence in favour of the transmission of acquired characters is more considerable than the school of Weismann seems to be aware--especially in the vegetable kingdom. I do not think that this negative line of evidence presents much weight; and, to show that I am not bia.s.sed in forming this judgement, I may here state that few have more reason than myself for appreciating the weight of such evidence. For, as already stated, when first led to doubt the Lamarckian factors, now more than twenty years ago, I undertook a research upon the whole question--only a part of which was devoted to testing the particular case of Brown-Sequard's statements, with the result recorded in the preceding chapter. As this research yielded negative results in all its divisions--and, not only in the matter of Brown-Sequard's statements--I have not hitherto published a word upon the subject. But it now seems worth while to do so, and for the following reasons.

First, as just observed, a brief account of my old experiences in this field will serve to show what good reason I have for feeling the weight of such negative evidence in favour of Continuity as arises from failure to produce any good experimental evidence to the contrary. In the second place, now that the question has become one of world-wide interest, it would seem that even negative results deserve to be published for whatever they may be worth on the side of Neo-Darwinism. Lastly, in the third place, although the research yielded negative results in my hands, it is perhaps not undesirable to state the nature of it, if only to furnish suggestions to other physiologists, in whose hands the experiments--especially in these days of antiseptics--may lead to a different termination. Altogether I made thousands of experiments in graft-hydridization (comprising bines, bulbs of various kinds, buds, and tubers); but with uniformly negative results. With animals I tried a number of experiments in grafting characteristic congenital tissues from one variety on another--such as the combs of Spanish c.o.c.ks upon the heads of Hamburgs; also, in mice and rats, the grafting together of different varieties; and, in rabbits and b.i.t.c.hes, the transplantation of ovaries of newly-born individuals belonging to different well-marked breeds. This latter experiment seems to be one which, if successfully performed (so that the transplanted ovaries would form their attachment in a young b.i.t.c.h puppy and subsequently yield progeny to a dog of the same breed as herself) would furnish a crucial test as to the inheritance or non-inheritance of acquired characters. Therefore I devoted to it a large share of my attention, and tried the experiment in several different ways. But I was never able to get the foreign ovary--or even any portion thereof--to graft. Eventually the pa.s.sing of the Vivisection Act caused me to abandon the whole research as far as animals were concerned--a research, indeed, of which I had become heartily tired, since in no one instance did I obtain any adhesion.

During the last few years, however, I have returned to these experiments under a licence, and with antiseptic precautions, but with a similar want of success. Perhaps this prolonged and uniformly fruitless experience may now have the effect of saving the time of other physiologists, by warning them off the roads where there seems to be no thoroughfare. On the other hand, it may possibly lead some one else to try some variation in the method, or in the material, which has not occurred to me. In particular, I am not without hope that the transplantation of ovaries in very young animals may eventually prove to be physiologically possible; and, if so, that the whole issue as between the rival theories of heredity will be settled by the result of a single experiment. Possibly some of the invertebrata will be found to furnish the suitable material, although I have been unable to think of any of these which present sufficiently well-marked varieties for the purpose.

But, pending the successful accomplishment of this particular experiment in the grafting of any animal tissue, I think it would be clearly unjustifiable to conclude against the Lamarckian factors on the ground of any other experiments yielding negative results in but one generation or even in a large number of sequent generations.

For instance, the latter consideration applies to the negative results of Mr. Francis Galton's celebrated _Experiments in Pangenesis_.[82].