Volume Ii Part 3 (1/2)

somatic] cells can react on the s.e.xual 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[27].”

[27] _Theory of Heredity_ (Journ. Anthrop. Inst. 1875, p. 346).

So far Mr. Galton; but for Weismann's further theory of evolution, &c., it is necessary to postulate the additional doctrine in question; and it makes a literally immeasurable difference to any theory of evolution whether or not we entertain this additional postulate. For no matter how faintly or how fitfully the substance of heredity may be modified by somatic tissues, 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_ of variation on the one hand, and _c.u.mulative_ on the other.

Thus, by merely laying this theory side by side with Weismann's we can perceive at a glance how a _pure_ theory of _heredity_ admits of being based on the postulate of Continuity alone, without c.u.mbering itself by any further postulate as to this Continuity being _absolute_. And this, in my opinion is the truly scientific att.i.tude of mind for us to adopt as preliminary to the following investigation. For the whole investigation will be concerned--and concerned only--with this question of Continuity as absolute, or as admitting of degrees. There is, without any question, abundant evidence to prove that the substance of heredity is at least partly continuous (Gemmules). It may be that there is also abundant evidence to prove this substance much more _largely_ continuous than Darwin supposed (Stirp); but be this as it may, it is certain that any such question as to the _degree_ of continuity differs, _toto caelo_, from that as to whether there can ever be any continuity at all.

How, then, we may well ask, is it that so able a naturalist and so clear a thinker as Weismann can have so far departed from the inductive methods as to have not merely propounded the question touching Continuity and its degrees, or even of Continuity as absolute; but to have straightway a.s.sumed the latter possibility as a basis on which to run a system of branching and ever-changing speculations concerning evolution, variation, the ultimate structure of living material, the intimate mechanism of heredity, or, in short, such a system of deductive conjectures as has never been approached in the history of science? The answer to this question is surely not far to seek. Must it not be the answer already given? Must it not have been for the sake of rearing this enormous structure of speculation that Weismann has adopted the a.s.sumption of Continuity as absolute? As we have just seen, Galton had well shown how a theory of heredity could be founded on the general doctrine of Continuity, without anywhere departing from the inductive methods--even while fully recognizing the possibility of such continuity as absolute. But Galton's theory was a ”_Theory of Heredity_,” and nothing more. Therefore, while clearly perceiving that the Continuity in question _may_ be absolute, he saw no reason, either in fact or in theory, for concluding that it _must_ be. On the contrary, he saw that this question is, for the present, necessarily unripe for profitable discussion--and, _a fortiori_, for the shedding of clouds of seed in all the directions of ”Weismannism.”

Hence, what I desire to be borne in mind throughout the following discussion is, that it will have exclusive reference to the question of fact already stated, without regard to any superjacent theories; and, still more, that there is a vast distinction between any question touching the degrees in which acquired characters are transmitted to progeny, and the question as to whether they are ever transmitted in any degree at all. Now, the latter question, being of much greater importance than the former, is the one which will mainly occupy our attention throughout the rest of this Section.

We have already seen that before the subject was taken up by Weismann the difference between acquired and congenital characters in respect to transmissibility was generally taken to be one of degree; not one of kind. It was usually supposed that acquired characters, although not so fully and not so certainly inherited as congenital characters, nevertheless were inherited in some lesser degree; so that if the same acquired character continued to be successively acquired in a number of sequent generations, what was at first only a slight tendency to be inherited would become by summation a more and more p.r.o.nounced tendency, till eventually the acquired character might become as strongly inherited as a congenital one. Or, more precisely, it was supposed that an acquired character, in virtue of such a summation of hereditary influence, would in time become congenital. Now, if this supposition be true, it is evident that more or less a.s.sistance must be lent to natural selection in its work of evolving adaptive modifications[28].

And inasmuch as we know to what a wonderful extent adaptive modifications are secured during individual life-times--by the direct action of the environment on the one hand, and by increased or diminished use of special organs and mental faculties on the other--it becomes obvious of what importance even a small measure of transmissibility on their part would be in furnis.h.i.+ng to natural selection ready-made variations in required directions, as distinguished from promiscuous variations in all directions. Contrariwise, if functionally-produced adaptations and adaptations produced by the direct action of the environment are never transmitted in any degree, not only would there be an incalculable waste, so to speak, of adaptive modifications--these being all laboriously and often most delicately built up during life-times of individuals only to be thrown down again as regards the interest of species--but so large an additional burden would be thrown upon the shoulders of natural selection that it becomes difficult to conceive how even this gigantic principle could sustain it, as I shall endeavour to show more fully in future chapters. On the other hand, however, Weismann and his followers not only feel no difficulty in throwing overboard all this ready-made machinery for turning out adaptive modifications when and as required; but they even represent that by so doing they are following the logical maxim, _Entia non sunt multiplicanda praeter necessitatem_--which means, in its relation to causality, that we must not needlessly multiply hypothetical principles to explain given results. But when appeal is here made to this logical principle--the so-called Law of Parsimony--two things are forgotten.

[28] Mr. Platt Ball has, indeed, argued that ”use-inheritance would often be an evil,” since, for example, ”the condyle of the human jaw would become larger than the body of the jaw, because as the fulcrum of the lever it receives more pressure”; and similarly as regards many other hypothetical cases which he mentions. (_The Effects of Use and Disuse_, pp. 128-9 _et seq._) But it is evident that this argument proves too much.

For if the effects of use and disuse as transmitted to progeny would be an evil, it could only be because these effects as they occur in the parents are an evil--and this they most certainly are not, being, on the contrary and as a general rule, of a high order of adaptive value. Moreover, in the race, there is a superadded agency always at work, which must effectually prevent any undue acc.u.mulation of these effects--namely, natural selection, which every Darwinist accepts as a controlling principle of all or any other principles of change. Therefore, if, as first produced in the life-time of individuals, the effects of use and disuse are not injurious, much less can they become so if transmitted through the life-time of species. Again, Mr. Wallace argues that, even supposing use-inheritance to occur, its adapting work in the individual can never extend to the race, seeing that the natural selection of fortuitous variations in the directions required must always produce the adaptations _more quickly_ than would be possible by use-inheritance. This argument, being one of more weight, will be dealt with in a future chapter.

In the first place, it is forgotten that the very question in debate is whether causes of the Lamarckian order _are_ unnecessary to explain all the phenomena of organic nature. Of course if it could be proved that the theory of natural selection alone is competent to explain all these phenomena, appeal to the logical principle in question would be justifiable. But this is precisely the point which the followers of Darwin refuse to accept; and so long as it remains the very point at issue, it is a mere begging the question to represent that a cla.s.s of causes which have hitherto been regarded as necessary are, in fact, unnecessary. Or, in other words, when Darwin himself so decidedly held that these causes are necessary as supplements to natural selection, the burden of proof is quite as much on the side of Weismann and his followers to show that Darwin's opinion was wrong, as it is on the side of Darwin's followers to show that it was right. Yet, notwithstanding the elaborate structure of theory which Weismann has raised, there is nowhere one single fact or one single consideration of much importance to the question in debate which was not perfectly well known to Darwin.

Therefore I say that all this challenging of Darwinists to justify their ”Lamarckian a.s.sumptions” really amounts to nothing more than a pitting of opinion against opinion, where there is at least as much call for justification on the one side as on the other.

Again, when these challenges are thrown down by Weismann and his followers, it appears to be forgotten that the conditions of their own theory are such as to render acceptance of the gauge a matter of great difficulty. The case is very much like that of a doughty knight pitching his glove into the sea, and then defying any antagonist to take it up.

That this is the case a very little explanation will suffice to show.

The question to be settled is whether acquired characters are ever transmitted by heredity. Now suppose, for the sake of argument, that acquired characters are transmitted by heredity--though not so fully and not so certainly as congenital characters--how is this fact to be proved to the satisfaction of Weismann and his followers? First of all they answer,--a.s.suredly by adducing experimental proof of the inheritance of injuries, or mutilations. But in making this answer they appear to forget that Darwin has already shown its inefficiency. That the self-styled Neo-Lamarckians have been much more unguarded in this respect, I fully admit; but it is obviously unfair to identify Darwin's views with those of a small section of evolutionists, who are really as much opposed to Darwin's teaching on one side as is the school of Weismann on the other. Yet, on reading the essays of Weismann himself--and still more those of his followers--one would almost be led to gather that it is claimed by him to have enunciated the distinction between congenital and acquired characters in respect of transmissibility; and therefore also to have first raised the objection which lies against the theory of Pangenesis in respect of the non-transmissibility of mutilations. In point of fact, however, Darwin is as clear and decided on these points as Weismann. And his answer to the obvious difficulty touching the non-transmissibility of mutilations is, to quote his own words, ”the long-continued inheritance of a part which has been removed during many generations is no real anomaly, for gemmules formerly derived from the part are multiplied and transmitted from generation to generation[29].” Therefore, so far as Darwin's theory is concerned, the challenge to produce evidence of the transmission of injuries is irrelevant: it is no more a part of Darwin's theory than it is of Weismann's to maintain that injuries _are_ transmitted.

[29] _Variation under Domestication_, ii. 392.

There is, however, one point in this connexion to which allusion must here be made. Although Darwin did not believe in the transmissibility of mutilations when these consist merely in the amputation of parts of an organism, he did believe in a probable tendency to transmission when removal of the part is followed by gangrene. For, as he says, in that case, all the gemmules of the mutilated or amputated part, as they are gradually attracted to that part (in accordance with the law of affinity which the theory a.s.sumes), will be successively destroyed by the morbid process. Now it is of importance to note that Darwin made this exception to the general rule of the non-transmissibility of mutilations, not because his theory of pangenesis required it, but because there appeared to be certain very definite observations and experiments--which will be mentioned later on--proving that when mutilations are followed by gangrene they are apt to be inherited: his object, therefore, was to reconcile these alleged facts with his theory, quite as much as to sustain his theory by such facts.

So much, then, for the challenge to produce direct evidence of the transmissibility of acquired characters, so far as mutilations are concerned: believers in Darwin's theory, as distinguished from Weismann's, are under no obligation to take up such a challenge. But the challenge does not end here. Show us, say the school of Weismann, a single instance where an acquired character _of any kind_ (be it a mutilation or otherwise) has been inherited: this is all that we require: this is all that we wait for: and surely, unless it be acknowledged that the Lamarckian doctrine reposes on mere a.s.sumption, at least one such case ought to be forthcoming. Well, nothing can sound more reasonable than this in the first instance; but as soon as we begin to cast about for cases which will satisfy the Neo-Darwinians, we find that the structure of their theory is such as to preclude, in almost every conceivable instance, the possibility of meeting their demand. For their theory begins by a.s.suming that natural selection is the one and only cause of organic evolution. Consequently, what their demand amounts to is throwing upon the other side the burden of disproving this a.s.sumption--or, in other words, of proving the negative that in any given case of transmitted adaptation natural selection has _not_ been the sole agent at work. Now, it must obviously be in almost all cases impossible to prove this negative among species in a state of nature. For, even supposing that among such species Lamarckian principles have had a large share in the formation of hereditary and adaptive characters, how would Weismann himself propose that we should set about the proof of such a fact, where the proof demanded by his a.s.sumption is, that the _abstract possibility_ of natural selection having had anything to do with the matter must be excluded? Obviously this is impossible in the case of inherited characters which are also _adaptive_ characters. How then does it fare with the case of inherited characters which are not also adaptive? Merely that this case is met by another and sequent a.s.sumption, which const.i.tutes an integral part of the Neo-Darwinian creed--namely, that in nature there _can be no such characters_. Seeing that natural selection is taken to be the only possible cause of change in species, it follows that all changes occurring in species must necessarily be adaptive, whether or not we are able to perceive the adaptations. In this way apparently useless characters, as well as obviously useful ones, are ruled out of the question: that is to say, _all_ hereditary characters of species in a state of nature are _a.s.sumed_ to be due to natural selection, and then it is demanded that the validity of this a.s.sumption should be disproved by anybody who doubts it. Yet Weismann himself would be unable to suggest any conceivable method by which it can be disproved among species in a state of nature--and this even supposing that the a.s.sumption is entirely false[30].

[30] In subsequent chapters, especially devoted to the question (i.e. Section II), the validity of this a.s.sumption will be considered on its own merits.

Consequently, the only way in which these speciously-sounding challenges can be adequately met is by removing some individuals of a species from a state of nature, and so from all known influences of natural selection; then, while carefully avoiding artificial selection, causing these individuals and their progeny through many generations unduly to exercise some parts of their bodies, or unduly to fail in the exercise of others. But, clearly, such an experiment is one that must take years to perform, and therefore it is now too early in the day to reproach the followers of Darwin with not having met the challenges which are thrown down by the followers of Weismann[31].

[31] I say ”the followers of Weismann,” because Weismann himself, with his clear perception of the requirements of experimental research, expressly states the above considerations, with the conclusions to which they lead. Nevertheless, he is not consistent in his utterances upon this matter; for he frequently expresses himself to the effect, ”that the _onus probandi_ rests with my opponents, and therefore they ought to bring forward actual proofs” (_Essays_, i. p. 390). But, as above shown, the _onus_ rests as much with him as with his opponents; while, even if his opponents are right, he elsewhere recognizes that they can bring ”actual proofs” of the fact only as a result of experiments which must take many years to perform.

Probably enough has now been said to show that the Neo-Darwinian a.s.sumption precludes the possibility of its own disproof from any of the facts of nature (as distinguished from domestication)--and this even supposing that the a.s.sumption be false. On the other hand, of course, it equally precludes the possibility of its own proof; and therefore it is as idle in Darwinists to challenge Weismann for proof of his negative (i. e. that acquired characters are not transmitted), as it is in Weismann to challenge Darwinists for proof of the opposite negative (i.

e. that all seeming cases of such transmission are not due to natural selection). This dead-lock arises from the fact that in nature it is beyond the power of the followers of Darwin to exclude the abstract possibility of natural selection in any given case, while it is equally beyond the power of the followers of Weismann to exclude the abstract possibility of Lamarckian principles. Therefore at present the question must remain for the most part a matter of opinion, based upon general reasoning as distinguished from special facts or crucial experiments.

The evidence available on either side is presumptive, not demonstrative[32]. But it is to be hoped that in the future, when time shall have been allowed for the performance of definite experiments on a number of generations of domesticated plants or animals, intentionally s.h.i.+elded from the influences of natural selection while exposed to those of the Lamarckian principles, results will be gained which will finally settle the question one way or the other.

[32] Note A.

Meanwhile, however, we must be content with the evidence as it stands; and this will lead us to the second division of our subject. That is to say, having now dealt with the antecedent, or merely logical, state of the question, we have next to consider what actual, or biological, evidence there is at present available on either side of it. Thus far, neither side in the debate has any advantage over the other. On grounds of general reasoning alone they both have to rely on more or less dogmatic a.s.sumptions. For it is equally an unreasoned statement of opinion whether we allege that all the phenomena of organic evolution can be, or can not be, explained by the theory of natural selection alone. We are at present much too ignorant touching the causes of organic evolution to indulge in dogmatism of this kind; and if the question is to be referred for its answer to authority, it would appear that, both in respect of number and weight, opinions on the side of having provisionally to retain the Lamarckian factors are more authoritative than those _per contra_[33].