246 



NA TURE 



[July 16, 1896 



•wrangle in regard lo the proper uses or improper uses of the 

 word "cause." But I did remember that Mill says that the 

 most vulgar form of "the fallacy of generalisation" is that 

 which is expressed by the phrase "post hoc or ciini lioc, ergo 

 propter hoc." I could not imagine how or why my friend Prof. 

 Woldon had been led to make himself the defiant, not to say 

 jubilant, champion of this fallacy. I have, on reading Prof. 

 Weldon's paper in the Proceedings of the Royal Society, vol. Ivii. 

 1894-95, found matter which throws light on the problem. It 

 would appear that Prof. Weldon, in discussing his measure- 

 ments of crabs, had already publicly adopted the logical position 

 which so much astonished those who heard him at the Linnean 

 Society. It appears that the fallacious process, which consists 

 in ignoring the possibility of two concomitant phenomena being 

 two independent consequences of one set of antecedents, gives 

 an apparent value to the laborious measurement of crabs which, it 

 ■seems, they would not possess if treated in a rational way. Prof. 

 Weldon says (loc. cit., p. 380) : " It is the object of the present 

 remarks to discuss the effect of small variations, as it may be 

 •deduced from the study of two organs in a single species. The 

 •case chosen is the variation, during growth and in adult life, of 

 the dimensions of female Carciiuis iiiienas." 



Further on he speaks of " the effect of small variations upon 

 "the chance of survival," and in close proximity occurs this 

 passage : " The law of growth having been ascertained, the rate 

 ■of destruction may be measured, and in this way an estiinate of 

 the advantage or iiisadvan/age of a variation may be obtained." 

 And again : " Knowing that a given deviation from the mean 

 character is associated with a greater or less percentage death- 

 rate in the animals possessing it, the importance of such a 

 ■deviation can be estimated without the necessity of inquiring 

 how that increase or decrease in the death-rate is brought about 

 so that all ideas of functional adaptation become unnecessary." 

 The title of the paper drawn up by a Committee, of which Prof. 

 Weldon is a member, and in reference to which his own paper 

 is written, stands : "An attempt to measure the death-rate due 

 to the selective destruction of Carcinus mcenas with respect to a 

 (particular dimension." 



(The italics in these citations are mine.) 



It appears to me that the language which I have italicised 

 indicates that Prof. Weldon — in his interpretation of the fact 

 ascertained by him, viz. that crabs with a particular proportion 

 of frontal breadth are commoner in the adult condition than in 

 younger stages — has deliberately departed from the simple 

 statement which his observations warranted, viz. that such- 

 and-such a proportion of frontal measurement accompanies 

 survival, and has unwarrantably (that is to say unre.isonably) 

 proceeded to speak of the " effect " of this frontal proportion, 

 to declare it to be a cause of survival, to estimate the "advan- 

 tage " and " disadvantage " of this .same proportion, and finally 

 to maintain that its " importance " may be estimated without 

 .troubling ourselves to inquire how it operates, or whether indeed 

 ■it is operative at all. 



Such methods of attempting to penetrate the obscurity which 

 veils the interactions of the immensely complex bundle of 

 .phenomena which we call a crab and its environment, appear to 

 me not merely inadequate, but in so far as they involve perver- 

 sion of the meaning of accepted terms and a deliberate rejection 

 ■of the method of inquiry by hypothesis and verification, injurious 

 ■to the progress of knowledge. E. Ray Lankf.stI';r. 



O.xford, June 30. 



Are Specific Characters the Result of " Natural 

 Selection" .' 



The last meeting— on June 18 — of the Linnean Society was 

 one of very exceptional interest, because the survivor of the two 

 illustrious naturalists who, on the same night —more than thirty- 

 seven years ago — first enunciated in that Society's rooms the 

 ■doctrine of the origin of species by " natural selection," read 

 a highly interesting paper on that very subject. 



The title of the paper, by Dr. Alfred R. Wallace, F.R.S., how- 

 ever, was " The Problem of Utility: Are specific characters always 

 or generally u.seful?" But the author, in treating the question, 

 expressly took for granted (as might surely have Ijeen expected 

 ■of him) the doctrine common to him and the late Mr. Darwin. 

 •So the question was implicitly answered at once ; for if species 

 arise by " natural selection," then those characters which con- 

 stitute them species must be due to the same cause, /.<•. to utility. 

 Thus the question really raised by Dr. Wallace was the old one, 

 " Do species arise through ' natural selection ' ? " 



NO. 1394, VOL. 54] 



This old question having been thus again started by its oldest 

 advocate, a few words in reply to it may be permitted to one of 

 itsoldest opponents. Not that I was always an opponent. The 

 doctrine of Messrs. Darwin and Wallace, as advocated by the 

 late Prof. Huxley, was held by me from i860 onwards for 

 several years. There was no antecedent reason why it should 

 be unwelcome to me, and, in fact, it was not at all so. It was 

 whilst working at Lemuroids that doubts first suggested them- 

 selves, which afterwards became, for me, certainties. 



It is one of those animals — the Potto — which has a specific 

 character, the least likely of any that I know of to have been 

 produced by "natural" or ".sexual" selection — one which I 

 cannot believe was ever occasioned by "utility," though it may 

 ha\e been so by another now suggested cause. It appears to 

 me to be an indisputable fact that in certain groups of animals 

 there are, .somehow, present, innate tendencies to development 

 along certain lines ; different degrees of the realisation of which 

 tendencies are characteristic of different species ; and this 

 without affecting the preservation of life. Thus amongst the 

 Lemuroids there appears to be a tendency to diminish the 

 size of the index finger, and this tendency culminates in the 

 Potto. 



In a section of the Marsupialia there seems to be a similar 

 tendency to diminish the size of two digits of the foot, though I 

 cannot believe that life has been saved at either the initial or 

 the extreme stages of this progressive degradation. 



Our own species supplies another example similar in character. 

 The penial bone of the lower apes is a considerable structure, 

 but in the Anthropoids it becomes so rudimentary, that the 

 chimpanzee was believed to have none till the late Mr. Crisp 

 exhibited the rudimentary representative of that structure at a 

 meeting of the Zoological Society, as I well remember. In man 

 it has, at least normally, entirely disappeared, and yet it is 

 impossible to suppose that its progressive disappearance has 

 been progressively useful as regards any form of "natural 

 selection." 



The existence of a latent tendency in a group of animals seems 

 to us peculiarly well marked in the Birds of Paradise. The 

 exceptional abnormalities of their plumage are so different in 

 different species, that these could never have sprang from a 

 common origin, but must have independently arisen in different 

 modes in different species.' 



Dr. Wallace said : "Accessory plumes and other ornaments 

 originate at points of great nervous and muscular excitation." 

 But the points of origin of abnormalities of plumage in these 

 birds are so numerous and diverse, that such local excitations 

 seem a very inadequate cause to account for them. Yet even if 

 they were adequate, what would account for such varied localities 

 of excitation in this particular group of birds alone ? 



But Dr. Wallace affirmed that such characters were utilised 

 "for purposes of recognition," . . . "each ornament being 

 really a ' recognition mark,' and therefore essential to both the 

 first production and .subsequent well-being of every species." 



Let us suppose that a certain group of birds (\) have begun 

 to vary in such a way that the males have acquired incipient 

 secondary sexual markings or growths in their plumage, and that 

 another group of birds (B) have begun to vary so that new tints, 

 or plumage growths, appear equally in both sexes. The change 

 must be small at first, and, indeed, Dr. Wallace said "the 

 transition" is an "almost imperceptible process." But what 

 influence can, at the same time, induce the males of the group 

 (A) to seek for females newly modified but difterent from them- 

 .selves, and the males of the group (B) to seek for females 

 newly modified but like themselves ? Why should the slightly 

 modified new varieties object to mate with members of the 

 hardly different parent stock ? Vet if they did not so object in a 

 majority of cases the new variety would soon disappear. Dr. 

 Wallace told us that such marks must have been specially needed 

 during the earlier stages of differentiation, yet at such early 

 stages the much-needed " recognition marks" must have been at 

 their minimum. This innate spontaneous impulse to breed 

 together, thus supposed to arise in members of every incipient 

 new variety whence every new species has arisen, is surely a very 

 mysterious impulse. No doubt Dr. Wallace has evidence that it 

 does in fact exist ; but if so, we must admit that somehow a 

 quasi voluntary process — a psychical character — has been pre- 

 caused (if we must not say pre-ordained), which is a sine qua 



1 I c-illed attention to this fact in my " Genesis of Species " in 1870. Since 

 then the discovery of new species with new abnormalities has intensified the 

 force of the aigunient. 



