Volume Ii Part 17 (1/2)

[146] No one has supposed that the tendency need be ”strong”: it has only to be persistent.

[147] Of course it must be observed that degeneration of complexity involves also degeneration of size, so that a more correct statement of the case would be--Why, under the cessation of selection, does an organ of extreme complexity degenerate much more rapidly than one of much less complexity? For example, under domestication the brains of rabbits and ducks appear to have been reduced in some cases by as much as 50 per cent.

(Darwin, and Sir J. Crichton Browne.) But if it is possible to attribute this effect--or part of it--to an artificial selection of stupid animals, I give in the text an example occurring under nature. Many other cases, however, might be given to show the general rule, that under cessation of selection complexity of structure degenerates more rapidly--and also more thoroughly--than size of it. This, of course, is what Mr. Galton and I should expect, seeing that the more complex a structure the greater are the number of points for deterioration to invade when the structure is no longer ”protected by selection.” (On the other hand, of course, this fact is opposed to the view that degeneration of useless structures below the ”birth-mean” of the first generations, is exclusively due to the reversal of selection; for economy of growth, deleterious effect of weight, and so forth, ought to affect size of structure _much more_ than complexity of it.) But I choose the above case, partly because Professor Lloyd Morgan has himself alluded to ”the eyes of crustacea,” and partly because Professor Ray Lankester has maintained that the loss of these eyes in dark caves is due to the reversal of selection, as distinguished from the cessation of it. In view of the above parenthesis it will be seen that the point is not of much importance in the present connexion; but it appears to me that cessation of selection must here have had at least the larger share in the process of atrophy.

For while the economy of nutrition ought to have removed the relatively large _foot-stalks_ as rapidly as the _eyes_, I cannot see that there is any advantage, other than the economy of nutrition, to be gained by the rapid loss of hard-coated _eyes_, even though they have ceased to be of use.

To sum up. There is now no question in any quarter touching the fact that panmixia, or the cessation of selection, is a true cause of degeneration. The only question is as to the amount of degeneration which it is able to effect when not a.s.sisted by the reversal of selection, or any other cause of degeneration. Moreover, even with regard to this question of amount, there is no doubt on any side that panmixia alone causes degeneration _more rapidly_ where it has to do with complexity of organization, than it does where it is concerned with a mere reduction of ma.s.s.

The question as to the amount of degeneration that is caused by the cessation of selection alone is without any practical importance where species in a state of nature are concerned, because here the cessation of selection is probably always a.s.sociated more or less with the reversal of it; and it is as impossible as it is immaterial to determine the relative shares which these two co-operating principles take in bringing about the observed results. But where organisms in a state of domestication are concerned, the importance of the question before us is very great. For if the cessation of selection alone is capable of reducing an organ through 10 or 12 per cent. of its original size, nearly all the direct evidence on which Darwin relied in favour of use-inheritance is destroyed. On the other hand, if reduction through 5 per cent. be deemed a ”very liberal estimate” of what this principle can accomplish, the whole body of Darwin's direct evidence remains as he left it. I have now given my reasons for rejecting this lower estimate on the one band, and what seems to me the extravagant estimate of Weismann on the other. But my own intermediate estimate is enough to destroy the apparent proof of use-inheritance that was given by Darwin.

Therefore it remains for those who deny Lamarckian principles, either to accept some such estimate, or else to acknowledge the incompatibility of any lower one with the opinion that there is no evidence in favour of these principles.

APPENDIX II.

ON CHARACTERS AS ADAPTIVE AND SPECIFIC.

It is the object of this Appendix to state, more fully than in the text, the opinions with regard to this subject which have been published by the two highest authorities on the theory of natural selection--Darwin and Professor Huxley. I will take first the opinion of Professor Huxley, quoted _in extenso_, and then consider it somewhat more carefully than seemed necessary in the text.

As far as I am aware, the only occasion on which Professor Huxley has alluded to the subject in question, is in his obituary notice of Darwin in the _Proceedings of the Royal Society_, Vol. XLIV, No. 269, p. xviii.

The allusion is to my paper on _Physiological Selection_, in the _Journal of the Linnaean Society_, Zool. Vol. XIX, pp. 337-411. But it will be observed that the criticism has no reference to the theory which it is the object of that paper to set forth. It refers only to my definition of the theory of natural selection as primarily a theory of the origin, or c.u.mulative development, of adaptations. This criticism, together with my answer thereto at the time, is conveyed in the following words.

”Every variety which is selected into a species is favoured and preserved in consequence of being, in some one or more respects, better adapted to its surroundings than its rivals. In other words, every species which exists, exists in virtue of adaptation, and whatever accounts for that adaptation accounts for the existence of the species. To say that Darwin has put forward a theory of the adaptation of species, but not of their origin, is therefore to misunderstand the first principles of the theory. For, as has been pointed out, it is a necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities, in virtue of the advantage conferred by which it has fought through the crowd of its compet.i.tors, and achieved a certain duration. In this sense, it is true that every species has been 'originated' by selection.”

Now, in the first place, I have nowhere said that ”Darwin has put forward a theory of the adaptation of species, but not of their origin.” I said, and continue to say, that he has put forward a theory of _adaptations in general_, and that where such adaptations appertain to species only (i.e. are peculiar to particular species), the theory becomes ”_also_ a theory of the origin of the species which present them.” The only possible misunderstanding, therefore, which can here be alleged against me is, that I fail to perceive it as a ”necessary consequence of the theory of selection that _every_ species _must_ have some one or more structural or functional _peculiarities_” of an adaptive or utilitarian kind.

Now, if this is a misunderstanding, I must confess to not having had it removed by Mr. Huxley's exposition.

The whole criticism is tersely conveyed in the form of two sequent propositions--namely, ”Every species which exists, exists in virtue of adaptation; and whatever accounts for that adaptation accounts for the existence of the species.” My answer is likewise two-fold.

First, I do not accept the premiss; and next, even if I did, I can show that the resulting conclusion would not overturn my definition. Let us consider these two points separately, beginning with the latter, as the one which may be most briefly disposed of.

I. Provisionally conceding that ”every species which exists, exists in virtue of adaptation,” I maintain that my definition of the theory of natural selection still holds good. For even on the basis of this concession, or on the ground of this a.s.sumption, the theory of natural selection is not shown to be ”_primarily_” a theory of the origin of species. It follows, indeed, from the a.s.sumption--is, in fact, part and parcel of the a.s.sumption--that all species have been originated by natural selection; but why? _Only because natural selection has originated those particular adaptive features in virtue of which (by the hypothesis) species exist as species._ It is only in virtue of having created these features that natural selection has created the species presenting them--just as it has created genera, families, orders, &c., in virtue of _other_ adaptive features extending through progressively wider areas of taxonomic division. Everywhere and equally this principle has been ”primarily” engaged in the evolution of adaptations, and if one result of its work has been that of enabling the systematist to trace lines of genetic descent under his divisions of species, genera, and the rest, such a result is but ”secondary” or ”incidental.”

In short, it is ”_primarily_” a theory of adaptations _wherever these occur_, and only becomes ”_also_” or ”_incidentally_” a theory of species in cases where adaptations happen to be restricted in their occurrence to organic types of a certain order of taxonomic division.

II. Hitherto, for the sake of argument, I have conceded that, in the words of my critic, ”it is a necessary consequence of the theory of selection that every species must have some one or more structural or functional peculiarities” of an adaptive kind. But now I will endeavour to show that this statement does not ”follow as a necessary consequence” from ”the theory of selection.”

Most obviously ”it follows” from the theory of selection that ”every variety which is selected into a species is favoured and preserved in consequence of being, in some one or more respects, better adapted to its surroundings than its rivals.” This, in fact, is no more than a re-statement of the theory itself. But it does _not_ follow that ”every species which exists, exists in virtue of adaptation” _peculiar to that species_; i.e. that every species which exists, exists _in virtue of having been ”selected_.” This may or may not be true as a matter of fact: as a matter of logic, the inference is not deducible from the selection theory. Every variety which is ”_selected into_” a species must, indeed, present some such peculiar advantage; but this is by no means equivalent to saying, ”in other words,” that every variety which _becomes_ a species must do so. For the latter statement imports a completely new a.s.sumption--namely, that every variety which _becomes_ a species must do so because it has been ”_selected into_” a species.

In short, what we are here told is, that if we believe the selection principle to have given origin to some species, we must further believe, ”as a necessary consequence,” that it has given origin to all species.

The above reply, which is here quoted _verbatim_ from _Nature_, Vol. 38, p. 616-18, proceeded to show that it does not belong to ”the first principles of the theory of natural selection” to deny that no other cause than natural selection can possibly be concerned in the origin of species; and facts were given to prove that such unquestionably has been the case as regards the origin of ”local” or ”permanent” _varieties_.

Yet such varieties are what Darwin correctly terms ”incipient” species, or species in process of taking _origin_. Therefore, if Professor Huxley's criticism is to stand at all, we must accept it ”as a necessary consequence of the theory of selection,” that every such _variety_ ”which exists, exists in virtue of adaptation”--a statement which is _proved_ to be untrue by the particular cases forthwith cited. But as this point has been dealt with much more fully in the text of the present treatise, I shall sum up the main points in a few words.

The criticism is all embodied in two propositions--namely, (_a_) that the theory of natural selection carries with it, as a ”necessary consequence,” the doctrine that survival of the fittest has been the cause of the origin of _all_ species; and (_b_) that therefore it amounts to one and the same thing whether we define the theory as a theory of species or as a theory of adaptations. Now, as a mere matter of logical statement, it appears to me that both these propositions are unsound. As regards the first, if we hold with Darwin that other causes have co-operated with natural selection in the origination of some (i.

e. many) species, it is clearly no part of the theory of natural selection to a.s.sume that none of these causes can ever have acted independently. In point of fact, as we have seen in the foregoing chapters, such has probably and frequently been the case under the influences of isolation, climate, food, s.e.xual selection, and laws of growth; but I may here adduce some further remarks with regard to yet another possible cause. If the Lamarckian principles are valid at all, no reason can be shown why in some cases they may not have been competent _of themselves_ to induce morphological changes of type by successive increments, until a trans.m.u.tation of species is effected by their action alone--as, indeed, Weismann believes to have been the case with all the species of Protozoa[148]. That such actually has often been the case also with numberless species of Metozoa, is the belief of the neo-Lamarckians; and whether they are right or wrong in holding this belief, it is equally certain that, _as a matter of logical reasoning_, they are not compelled by it to profess any _disbelief_ in the agency of natural selection. They may be mistaken as to the facts, as Darwin in a lesser degree may have been similarly mistaken; but just as Darwin has nowhere committed himself to the statement that _all_ species must _necessarily_ have been originated by natural selection, so these neo-Lamarckians are perfectly logical in holding that _some_ species may have been wholly caused by the inheritance of acquired characters, as _other_ species may have been wholly caused by the natural selection of congenital characters. In short, unless we begin by a.s.suming (with Wallace and against Darwin) that there _can be no other cause_ of the origin of species than that which is furnished by natural selection, we have no basis for Professor Huxley's statement ”that every species has been originated by selection”; while, if we do set out with this a.s.sumption, we end in a mere tautology. What ought to be done is to prove the validity of this a.s.sumption; but, as Professor Huxley makes no attempt to do this, his criticism amounts to mere begging of the question.

[148] Since the above was written Professor Weismann has transferred this doctrine from the Protozoa to their ancestors.