Jan. 13, 1887] 



NA TURE 



247 



LETTERS TO THE EDITOR 



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Mr. Wallace on Physiological Selection 



In the September issue of the Fortnightly Rcriav, Mr. A. R. 

 Wallace published an article criticising the theory of physio- 

 logical selection, and subsequently published a letter in Nati're 

 conveying the substance of that criticism. Having now replied 

 to all my critics in the current issue of the Xiitelccnth Century, 

 I will here give the substance of my answer to Mr. Wallace. 



" (i) Mr. Romanes makes a great deal of the alleged ' inutility 

 of specific characters,' and founds upon it his e.xtraordinary 

 statement that, during his whole life, Darwin was mistaken in 

 supposing his theory to be ' a theory of the origin of species, ' 

 and that all Darwinians who have believed it to be so have 

 blindly fallen into the same error. I .allege, on the contrary, 

 that there is no proof worthy of the name that specific characters 

 are usually useless, and I adduce a considerable series of facts 

 tending to prove their general utility." 



Now, in this matter I not only " allege," but prove, that 

 I have upon my side Darwin himself (" Origin of Species," 

 pp. 171, 176, 421 ; "Descent of Man," p. 61) and more or less 

 "all Darwinians." Moreover, I have shown that the arguments 

 whereby Mr. Wallace seeks to justify his own individual views 

 are quite unworthy of their distinguished author. 



"{2) In support of his view as to the swamping effects of 

 intercrossing, Mr. Romanes objects to the assumption of Darwin, 

 ' that the same variation occuis simultaneously in a number of 

 individuals,' adding : ' Of course, if this assumption were 

 granted, there would be an end of the present difficulty ' ; and 

 his whole argument on this branch of the question rests on the 

 assumption being false. X adduce evidence — copious evidence — 

 that the supposed assumption represents a fact, which is now one 

 of the best -established facts of natural history." 



The "copious evidence" here alluded to consists merely in a 

 reference to the well-known observations of Mr. J. A. Allen 

 upon the kinds and degrees of individual variation exhibited by 

 certain species of American birds. I am able to show that 

 none of these observations have any bearing upon the "diffi- 

 culty" in question ; and that so far from the "assumption" in 

 question representing " a fact which is now one of the best- 

 established facts of natural history," even so accomplished an 

 ornithologist as Mr. .Seebohm displays so sublime an ignorance 

 of its establishment as to affirm, in his criticism of my paper, 

 that " it is seldom that the difficulties of natural selection from 

 fortuitous variations have been so clearly, so impartially, but so 

 candidly set forth." And he adds, speaking specially of birds, 

 " So far as is known, no species has ever been differentiated 

 without the aid of geographical i.solation," i.e. without some 

 check upon free intercrossing. 



" (3) Mr. Romanes states, as the special feature of his physio- 

 logical varieties, that ' they cannot escape the preserving agency 

 of physiological selection.' He gives no particle of proof of 

 this, while I show that, on the contrary, it is hardly possible for 

 them to survive to a second or third generation." 



The objection here is that the chances must be greatly against 

 the " physiological complements " (or the suitably varied indivi- 

 duals of opposite sexes) happening to mate, and, even if they 

 did, that their progeny should likewise do so often enough to 

 start a permanent variety. 



In answer to this objection I first of all adopt my critic's 

 assumption, namely, that in all cases physiological selection must 

 depend on the chance unions of physiological complements, 

 relatively very few in number, and scattered over areas occupied 

 by large species. Upon this assumption I agree that the sexual 

 variation, "wlienever it occurs, is almost certain to die out 

 immediately," after which the paper proceeds as follows : — 



"Granting it is shown that the union of these physiological 

 varieties of opposite se.xes is a matter of enormously rare occur- 

 rence, is it not also true that tie origin of a neio species is an 

 enormously rare event ? Not a few existing species have remained 

 unchanged from remote geological time ; the life of all species 



is incalculably long as compared with that of their constituent 

 individuals ; and in every generation of individuals there are, in 

 the case of most species, millions of fertile unions. Therefore, 

 so far as we can form any estimate on a subject where all pro- 

 portion seems to fail, we may safely conclude that the ratio 

 between the number of species which have appeared upon this 

 earth, and the number of fertile unions between their constituent 

 individuals, can only be represented by unity to billions. 



"In view of this fact I am not afraid of any calculation that 

 can be made, in order to ^how how many chances there are 

 against the confluence of those conditions on the occurrence of 

 which my theory supposes the origin of a species to depend. 

 According to Mr. Wallace's estimate, the chances against the 

 suitable mating of these physiological varieties ' may be any 

 number of thousands to one ' ; so that, in view of the considera- 

 tions above given, and the large number of species existing at 

 any one time, we might conclude that Mr. Wallace supposes the 

 birth of a new species to be an event of almost daily occurrence. 

 Therefore, looking to what we all know are the real facts of the 

 case, even if it were true that whenever one of these physiological 

 varieties occurs, 'it is almost certain to die out,' this almost may 

 be here quite sufficient for all that is required. Thus, upon the 

 whole, and under my temporary acceptance of Mr. Wallace's 

 assumptions, I confess it appears to me a somewhat feeble 

 criticism to represent that the conditions which my theory 

 requires for the origin of a new species are probably about as 

 rare in their occurrence as is the result which they are supposed 

 to produce. 



"So much, then, for my first answer. My lecond answer 

 simply is that from its beginning to its end this criticism is 

 wholly in the air. Hitherto I have been considering his assump- 

 tions merely for the sake of argument. But they are not jny 

 assumptions ; they form no part of my theory ; and, therefore, 

 I repudiate them in toto. The paper which Mr. Wallace is 

 criticising clearly and repeatedly sets forth that I do not suppose 

 the mating of physiological varieties to be wholly a matter of 

 chance. Whether or not it is a matter of chance will depend on 

 the cau.ses which determine the variation. When these causes 

 are of a kind which act simultaneously on many, on most, or 

 even on all individuals occupying the same area, the element o f 

 chance is proportionally excluded. One very obvious, and 

 prohably frequent, instance of what may be termed collective 

 variation in the reproductive system — or a variation due to a 

 common cause acting on many individuals simultaneously — is 

 actually quoted from my paper by Mr. Wallace himself, namely, 

 changes in the season of flowering or of pairing, which insure 

 that any section of a species so affected shall be fertile only 

 within itself. Collective variation of this kind may be directly 

 due to the incidence of some common cause, such as changed 

 conditions of life with respect to food, climate, station, Src. ; or, 

 as in the case of bud-variation, it may be due to a single ' sport ' 

 affecting all the blossoms growing upon the same branch. But 

 besides such direct action of a common cause, it is easy to see 

 that natural selection, use and disuse, &c., by operating in the 

 production of organic changes elsewhere, may not unfrequently 

 react on the sexual system indirectly, and so induce the sexual 

 change required in a number of individuals simultaneously. All 

 the parts of an organism are so intimately tied together, and the 

 reproductive system in particular is known to be so extraordinarily 

 sensitive to slight changes in tlie conditions of life, or to slight 

 disturbances of the organic system generally, that in their work 

 of adapting organisms to changes of their environment all 

 causes of an ' equilibrating ' kind must be calculated more or 

 less frequently to affect the reproductive system in the way 

 required. ... 



"If I have succeeded in making myself intelligible, it will 

 have been seen that Mr. Wallace's objection to my theory 

 admits of a twofold answer. In the first phace, it is impossible 

 for him to 'show' that the origin of a species is any more 

 frequent than it ought to be, even upon the assumption which he 

 has imputed to me — namely, that such origin is always due to 

 the chance mating of more or less extremely rare varieties. 

 And, in the next place, this a.ssumption on his part is wholly 

 gratuitous — or rather, I should say, directly opposed both to my 

 own statements and to all the probabilities of the case. 



" From which it is easy to perceive the inevitable inference, 

 or, if not, by stating it I will furnish a cue to future critics. The 

 real difficulty against my theory is precisely the opposite of that 

 which Mr. Wallace has advanced. This real difficulty is that the 

 differentiation of specific types has not been of nearly so frequent 



