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NA TURE 



\Pe£. 20, 1888 



question as " admissions wrung from a hostile witness," or as 

 due to Mr. Darwin "admitting the possibility of explanations to 

 which he really, however, did not attach much importance ;" 

 hut by thus endeavouring to belittle his judgment on these points, 

 tbe post-Darwinians are merely showing the weakness of their 

 own. These passages are due to Mr. Darwin having clearly 

 perceived that the doctrine of Mr. Wallace was neither sound in 

 logic nor true in fact. Not sound in logic, because it does not 

 follow as a " necessary deduction from the theory of natural 

 selection," that all specific characters must be adaptive— it being 

 sufficient for the theory if only some such characters are adaptive 

 in each case, as Mr. Huxley has recently shown ; not true in 

 fact, becaus;e any number of cases (such as those given by Mr. 

 Mivart) can be quoted to the contrary. Therefore, as I said 

 two years ago in the Nine'eenth Century, those who seek to 

 encumber the theory with this illogical deduction are merely 

 giving occasion to the enemy ; it is a gratuitous dogma, standing 

 like the feet of clay in a figure of iron.^ 



But to pass on to the second point. In my last letter I 

 challenged Mr. Dyer to justify his statements that I had roundly 

 denied the agency of natural selection as "the mechanical 

 means by which evolution has been brought about," and that 

 the theory of physiological selection " shrivels up the part 

 played by natural selection to very small dimensions." In 

 answer, he quotes a passage from my paper, and agrees with me 

 that what it says is substantially the same as what the Times 

 said. But he still fails to see that this is totally different from 

 what he himself said. In other words, unlike the 7'imes, he 

 does not perceive the validity of my distinction between natural 

 selection as a theory of species and as a theory of adaptations. 

 Physiological selection, he thinks, shrivels up natural selection, 

 because, if a true principle in Nature, it must play an important 

 part in the origination of species, and, in whatever measure it 

 does this, it must in a corresponding measure detract from the 

 importance of n.ntural selection. Having at last got him to 

 show that this is his way of regarding the matter, I must first 

 of all repeat that natural selection, besides being a theory of 

 the origin of species, is also something very much more : it 

 is a theory of the cumulative development of adaptations, 

 wherever these occur. In fact, it is, as I have before said, 

 primarily a theory of adaptations in all cases, and only becomes 

 incidentally a theory of species in those cases where the adapta- 

 tions happen to be of merely specific value. It is now perfectly 

 evident that Mr. Dyer fails to perceive this distinction ; hence 

 his misunderstanding of my views, and hence also the present 

 correspondence. He regards the "origin of species" as 

 synonymous with, and therefore as covering the whole field 

 of, "organic evolution": therefore he accuses me of roundly 

 denying natural selection as "the mechanical means by which 

 that evolution has been brought about," on the ground that 

 I have suggested a supplementary theory of the origin of 

 species. Such being manifestly the impression under which he 

 has read my paper, it is no wonder that in the process he has 

 been, as he says, "completely befogged." I will now en- 

 deavour to clarify the matter by explaining at length what I 

 had supposed the readers of my paper would have recognized 

 for themselves. 



It is quite true that the evolution of adaptations depends upon 

 the evolution of species, the serial succession of which, in any 

 given line of descent, is the necessary means (through the 

 struggle for existence) to the gradual development of adapta- 

 tions in that line. But it is of no consequence how many " in- 

 differfnl " characters these successive species display, provided 

 that they also display, in ever-improving degrees, the particular 

 adaptive charact<rs which are in cour.-e of evolution. A bird's 

 wing, for example, is an adaptive structure which cannot be 

 evolved as a merely specific character; it requires to be slowly 

 built up through the lives of an enormous number of successive 

 species, which ramify into genera, families:, &c., as the process 

 goes on. Now, throughout this process it is a matter of no 



'_ In connect'on with this po'.nt I have to express regret for a verba! error 

 which Mr. Dyer has already pointed out in my last letter. But, from what 

 he says of Pr, f. Nageli's essay, it appears to me that he cannot have read it. 

 At any mte, it is in no way confined to " the inutility of family characters," 

 and in considering it Mr. Darwin supplies more than two pages of instances 

 to ilhistrate his argument therewith, ending with the general conclusicn : — 

 ''We thus see that with plants many morphological changes may be attri- 

 buted to the laws oft grow h, and the interaction of parts, ird pendently of 

 natural selection." To ignore all such passages, or to regard them as 



admissions," is assuredly— once more 10 quote my critic's words against him- 

 self— " entirely to misapprehend their significance, or the spirit in whxh 

 they were made " 



consequence how many other features of a non-adaptive kind 

 arise among all these innumerable species : it is enough, as 

 regards the evolution of a wing, that at each stage of the pro- 

 cess some of the species should present slight improvements on 

 their predecessors in respect of this adaptive structure. Phy- 

 siological selection, sexual selection, geographical isolation, 

 "changed conditions" as to climate, &c., or any other "factor," 

 may all the while have been originating any number of species, 

 without reference to their wings, though, at the same time, 

 natural selection was continuously promoting the development 

 o[ ^'\ngs in genera, families, and orders. In short, species are 

 lil<e leave?, successive and transient crops of which are necessary 

 to the gradual building up of adaptations, while these, like the 

 woody and permanent branches, grow continuously in import- 

 ance and efficiency through all the tree of life. Now, it is the 

 office of natural selection to see to the growth of these perma- 

 nent branches : physiological selection has to do only with the 

 deciduous leaves. Hence, although natural selection has like- 

 wise an immensely la-ge share in the origination of species (i.e. 

 has to do with all species which are distinguished by adaptive 

 characters peculiar to themselves), this, in my view, is really 

 much the least important part of its work. Not as discovering 

 an agent in the differentiation of species, but as revealing the 

 agent in the genesis of adaptations, do T regad Mr. Darwin's 

 theory as the greatest generalization in the history of science. 

 If this view of the matter betrays on my part, as Mr. Dyer 

 says, a fundamental misunderstanding of that theory, I shall be 

 greatly obliged to him for showing me wherein the misunder- 

 standing consists. In the event of his doing so, I will cheerfully 

 renounce the inquiry on which I am engaged, for then, no doubt, 

 my theory would be found in opposition to — and not, as I sup- 

 pose, in co-operation with— the theory of natural selection. On 

 the other hand, should he fail to meet this request, I shall have 

 "reluctantly to arrive at the conclusion" that the "funda- 

 mental misunderstanding" in this matter, like the "strained 

 interpretation " previously considered, lies the other way. 



It will now, I trust, be sufficiently evident why I differ toto 

 coelo from Mr. Dyer where he concludes that the theory of 

 physiological selection shrivels up the theory of na'ural selection. 

 In point of fact, the former theory stands to the latter in precisely 

 the same relation as does the theory of sexual selection. In 

 both these supplementary theories, it is the origin of species that 

 is concerned, and so concerned with reference to characters that 

 are non-adaptive. The cases being thus precisely parallel, I 

 should like to know whether my present critic regards one of Mr. 

 Darwin's own theories as shrivelling up the other.^ Assuredly, 

 Mr. Darwin himself did not think so, because he clearly 

 perceived that the " origin of species " constituted but a small 

 part of the whole field of "organic evolution." It is true 

 that he entitled his work "The Origin of Species by means of 

 Natural Selection," and therefore in my paper I was careful to 

 p -int out that, " if it appeared somewhat presumptuous to have 

 insinuated that Mr. Darwin's great work on the ' origin of 

 species' has been mis-named," there were passage; in the work 

 itself which fully justified me in my definition of his theory. 

 My critic now quotes this explanation as justifying his 

 statement that I intended to deny the agency of natural 

 selection altogether ! I do not quite know how to meet an 

 opponent who resorts to such strange devices ; but I may 

 at any rate assure him that in my opinion no more appro- 

 priate title could have been chosen by Mr. Darwin for his 

 great work than the one which he did choose ; and if I spoke cf 

 that work as having been mis-named, I thought I had made it 

 clear enough that I was "strictly speaking," or speaking to a 

 point of logical definition. Moreover, at the time when the 

 work in question appeared, the problem as to the origin of 

 species was, as its author says, " the mystery of mysteries. " But 



' In this connection it is interesting to note that Mr. Wallace has always 

 been ihe principal opponent of the theory of sexual selection, as he now is of 

 the theory of physiological selection. Moreover, the reason of h s opposition 

 in b(th cases is that he thinks such subordinate the ries of the origin of 

 species must fail to find a locus stavdi in the presence of the greater theorv 

 of natural selection : the latter, in his view, must necessarily "shrivel up" 

 the former. Now, his arguments against sexual selectii n are incompar.nbly 

 stronger than those which he has advanced against physiological selec:imi 

 (compare "Tropical Nature," pp. 192-211, with the Fortnightly Re^iieiu for 

 ."September 1886, and the Nineteenth Century for January 1887); yet they 

 failed to influence the judgment of Mr. Darwin, whose very last words to 

 scie'ce — read a few hours before his death to a meeting of the Zoological 

 Society— were : " I may peihaps be here permitied to fay that, after hav ng 

 carefully weighed, to tjie best of my ability, the various arguments which 

 have been advanced against the principle of se-xual selection, I remain firmly 

 convinced of its truth." 



