6i6 



NATURE 



[Oct. 25, 1888 



Definition of the Theory of Natural Selection. 



In his Presidential address before Section D of the British 

 Association, Mr. Dyer is reported to have said, while alluding 

 to myself: — "He has startled us with the paradox that Mr. 

 Darwin did not, after all, put forth, as I conceive it was his own 

 impression that he did, a theory of the origin of species, but only 

 of adaptations. And inasmuch as Mr. Romanes is of opinion 

 that specific differences are not adaptive, while those of genera 

 are, it follows that Mr. Darwin only really accounted for the 

 origin of the latter, while for an explanation of the former we 

 must look to Mr. Romanes himself" (Nature, September 13, 

 p. 47.6). 



It is here stated : (1) that in my opinion specific differences are 

 not adaptive ; (2) that I regard Mr. Darwin's theory as explain- 

 ing the origin of genera, but not the origin of species ; and 

 (3) that, consequently, biologists are virtually invited by me 

 to accept the theory of physiological selection as a substitute for 

 Mr. Darwin's theory of natural selection, in so far, at all events, 

 as the origin of species is concerned. 



In direct contradiction to all these statements I will now quote 

 passages from the paper with reference to which they are made. 

 It would be easy for me to add further quotations to the same 

 effect under each of the three heads, but the following will be 

 sufficient to serve the double purpose which I have in view — 

 namely, first to correct misrepresentations, and next to furnish a 

 basis for further remarks upon the subject. The italics have 

 reference only to the former purpose. 



(1) and (2). — -"It [the theory of natural selection] is not, 

 strictly speaking, a theory of the origin of species : it is a theory 

 of the origin — or, rather, of the cumulative development — of 

 adaptations, whether these be morphological, physiological, or 

 psychological, and whether they occur in species only, or like- 

 wise in genera, families, orders, and classes. 



" These two things are very far from being the same ; for, on 

 the one hand, in an enormously preponderating number of 

 instances, adaptive structures are common to numerous species ; 

 while, on the other hand, the features which serve to distinguish 

 species from species are, as we have just seen, by no means 

 invariably — or even generally — of any adaptive character. Of 

 course, if this were not so, or if species ahvays and only differed 

 from one another in respect of features presenting some utility, 

 then any theory of the origin of such adaptive features would 

 also become a theory of the origin of the species which presented 

 them. As the case actually stands, however, not only are 

 specific distinctions very often of no utilitarian meaning ; but, as 

 already pointed out, the most constant of all such distinctions is 

 that of sterility, and this the theory of natural selection is con- 

 fessedly unable to explain. . . . In so far as natural selection 

 has had anything to do with the genesis of species, its operation 

 has been, so to speak, incidental : it has only helped in the work 

 of originating species in so far as some among the adaptive 

 variations which it has preserved happen to have constituted 

 differences of only specific value. But there is an innumerable 

 multitude of other such differences with which natural selection 

 can have had nothing to do — particularly the most general of 

 all such differences, or that of mutual sterility ; while, on the 

 other hand, by far the larger number of adaptations which it has 

 preserved are now the common property of numberless species. 

 But let me not be misunderstood. In saying that the theory of 

 natural selection is not, properly speaking, a theory of the origin 

 of species, I do not mean to say that the theory has no part at 

 all in explaining such origin. Any such statement would be in 

 the last degree absurd. What I mean to say is that the theory 

 is one which explains the origin or the conservation of adapta- 

 tions, whether structural or instinctive, and whether these occur 

 in species, genera, families, orders, or classes. In so far, there- 

 fore, as useful structure! are likewise species-distinguishing 

 structures, so far is the theory of their origin also a theory of the 

 origin of the species which present them. " 



(3) "Let it, therefore, be clearly understood that it is the 

 office of natural selection to evolve adaptations — not therefore 

 or necessarily to evolve species. Let it also be clearly under- 

 stood that in thus seeking to place the theory of natural selection 

 on its true I gical footing, I am in nowise detracting from the 

 importance of that theory. On the contrary, I am but seeking 

 to release it from the difficulties with which it has been hitherto 

 illegitimately surrounded. ... I cannot feel that I am turning 

 traitor to the cause of Darwinism. On the contrary, I hope thus 

 to remove certain difficulties in the way of Darwinian teaching ; 



and I well know that Mr. Darwin himself would have been the 

 first to welcome my attempt at suggesting another factor in the 

 formation of species, which, a'lhough quite independent of ' nattiral 

 selection, is in no zvay opposed to natural selection, and may there- 

 fore be regarded as a factor supplementary to natural selection. . . . 

 And here, as elsewhere, I believe that the co-operation enables 

 the two principles to effect very much more in the way of 

 species-making than either of them could effect if working 

 separately. On the one hand, without the assistance of physio- 

 logical selection, natural selection would, I believe, be all but 

 overcome by the adverse influences of free intercrossing — in- 

 fluences all the more potent under the very conditions which are 

 required for the multiplication of species by divergence of 

 character. On the other hand, vilhou? natural seLction, physio- 

 logical selection would be powerless to create any differences of 

 specific type, other than those of mutual sterility and trivia/ 

 details of structure, form, and colour — differences wholly without 

 meaning from a utilitarian point of view. But in their combina- 

 tion these two principles appear to me able to accomplish what 

 neither can accomplish alone — namely, a full and satisfactory 

 explanation of the origin of species." 



These quotations appear to me sufficient to prove the in- 

 accuracy of Mr. Dyer's remarks. But I should not have 

 taken the trouble to notice misinterpretations of so absurd a 

 kind, were it not that I have something more to say on the 

 subject of which they treat. For Mr. Dyer, in his address, 

 alludes to a recent criticism by Mr. Huxley, which also deals 

 with my "paradox," but does so in a very different manner. 

 That is to say, the passages which Mr. Huxley devotes to 

 this subject exhibit a much more careful consideration of 

 the points in it to which he alludes, as well as a manifest 

 desire to state the issue fairly. I will therefore pass on to 

 consider the criticism as it was originally presented by Mr. 

 Huxley, leaving behind the teralological reproduction of it by 

 Mr. Dyer as effectually disposed of by mere quotations from my 

 paper itself. 



The substance of Mr. Huxley's criticism, in so far as it 

 apparently applies to me, is conveyed in the following 

 words: — "'Favourable variations' are those which are better 

 adapted to surrounding conditions. It follows, therefore, that 

 every variety which is selected into a species is so 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 conse- 

 quence 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" (Proc. Roy. Soc, vol. xliv. No. 269, p. xviii.). 



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 par- 

 ticular species), the theory becomes " also a theory of the origin 

 of the species which present them." The only possible mis- 

 understanding, 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 over- 

 turn 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 





