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 Linnæan 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 cumulative 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 competitors, 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 assumption, the theory of natural selection is not shown to be "primarily" a theory of the origin of species. It follows, indeed, from the assumption—is, in fact, part and parcel of the assumption—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 assumption—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 assume 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, sexual 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 transmutation 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 assuming (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 assumption, we end in a mere tautology. What ought to be done is to prove the validity of this assumption; but, as Professor Huxley makes no attempt to do this, his criticism amounts to mere begging of the question.
And now, as regards the second point (b), even if we grant the assumption that natural selection is the only possible cause of the origin of species—or, which is the same thing, that every species has been originated by natural selection,—is it likewise the same thing whether we define the theory of natural selection as a theory of species or as a theory of adaptations? Professor Huxley's criticism endeavours to show that it is; but a little consideration is enough to show that it is not. What does follow from the assumption is, that, so far as specific characters are concerned, it is one and the same thing to say that the theory is a theory of species, and to say that it is a theory of adaptations. But specific characters are not conterminous with adaptive characters; for innumerable adaptive characters are not distinctive of species, but of genera, families, orders, classes, and sub-kingdoms. Therefore, if it is believed (as, of course, Professor Huxley believes) that the theory in question explains the evolution of all adaptive characters, obviously it is not one and the same thing to define it indifferently as a theory of species or as a theory of adaptations.
Now, all this is not merely a matter of logic chopping. On the contrary, the question whether we are to accept or to reject the deduction that all species must necessarily have owed their origin to natural selection, is a question of no small importance to the general theory of evolution. And our answer to this question must be determined by that which we give to the ulterior question—Is the theory of natural selection to be defined as a theory of species, or as a theory of adaptations?