534 



NATURE 



[October 6, 1892 



who accept the Darvvinian principle do not expect 

 heredity to achieve more than this — to offer the mate- 

 rials which can be accumulated by natural selection. 



Interesting as is the study of such "marked varia- 

 tions" and the statistics of their occurrence in nature, the 

 great principle of natural selection, whether applied to 

 the evolution of animal colouring or to any other 

 character, is not greatly affected thereby, but rather 

 demands such exact numerical investigations as those 

 published by Galton upon man, by Wallace upon various 

 animals, by Weldon upon Crangon, and by Lloyd Morgan 

 upon bats. 



Another objection to the natural selection argument 

 is given on p. 25, and it too turns on the author's concep- 

 tion of the mode in which this principle operates. Re- 

 capitulating Weismann's argument that longitudinal 

 stripes have been replaced by oblique ones in certain 

 larvas, on account of the more perfect concealment 

 afforded by the latter, he points out that some species 

 " have, on the contrary, remained at a stage of coloration 

 which is, ex hypothesi, disadvantageous." The longitu- 

 dinal striping was never disadvantageous, but only rela- 

 tively less advantageous, in certain species, and under 

 certain conditions. The failure of a species to take this 

 line of evolution may have been due to many causes, the 

 development of other modes of defence, the nature of 

 its peculiar environment, or may be solely due to the 

 kind of selection exercised by its foes. 



The author sees far-reaching conclusions against the 

 principle of natural selection in the admission that 

 pigment as a cause of colour was originally non-significant, 

 and is so still in many cases (colours of certain lower 

 forms, colours of blood, fat, &c.). He argues (pp. 68-70) 

 that as colour did not arise by natural selection, it must 

 be a normal product of the organization, and its disappear- 

 ance in cave-dwelling forms cannot be due to the cessation 

 of selection, but must follow as the direct effect of sur- 

 roundings, although he does not even hint at the mode in 

 which such effects are supposed to be wrought. But 

 these conclusions are by no means warranted by the 

 original admission. The first appearance of pigment in 

 the skin of the ancestor of a group of species which are 

 now coloured was certainly a normal product of the 

 organization ; but the fact that this variation subsequently 

 spread over all the individuals of the ancestral species, 

 and of those to which it gave rise, will be claimed by 

 Darwinians as the result of selection. And so strong are 

 the tendencies of variation in other directions that 

 partially or completely albino races can be produced by 

 man in a relatively short period of time, while such in- 

 dividuals are far from uncommon in nature in spite of 

 selection. The facts support the opposite conclusion 

 that the absence of colour from the skin would be the 

 normal result of organization for the average individual, 

 were it not for the strong and continuous action of selec- 

 tion. There are other instances of the disappearance of 

 colour in addition to that which has occurred in caves, 

 and in some of these the conclusion appears inevitable. 

 The whiteness of birds' eggs laid in dark holes certainly 

 cannot be traced to the direct action of surroundings, any 

 more than the colour of eggs laid in open nests ; and 

 natural selection being prevented by man, the colour is 

 NO. I 197, VOL. 46] 



disappearing from the eggs of the domestic fowl, just as- 

 it is lost in other species when prevented by darkness. 



It is certainly true that colour " must be there before it 

 can be acted upon, and modified in this or that direction 

 according to the needs of the animal." But this objection^ 

 which has been familiar since the earliest days of natural 

 selection, is less formidable than it appears to be. Colour 

 must have been present in the skin of some individual 

 ancestor certainly, but its existence, as well as its modifi- 

 cation, in the normal individual of the species is to be 

 explained by selection. 



It is hardly necessary to point out that this argument 

 does not apply to colours which still remain non-signifi- 

 cant and are excluded from selection ; but these are 

 precisely the colours which are unaffected by the changes 

 of environment alluded to above ; the blood of cave- 

 dwelling vertebrates remains red like that of others • 

 the yolk of eggs laid in holes does not differ in appear- 

 ance from that of those laid in open nests. 



A similar argument as to seasonal change of colour in 

 arctic animals may be answered in the same manner. 



The author's difficulties appear to arise in part from his 

 inadequate conceptions of the struggle for existence. 

 Speaking of certain night- feeding caterpillars, he says 

 (p 102) : " It may be suggested that they prefer to feed 

 early in the evening, when their colours, if conspicuous, 

 would be readily seen. If this is so, it does not much 

 matter, for the birds would— the bulk of them at any rate 

 — have gone to roost." Or speaking of Mimnonectes, an 

 Amphipod crustacean which bears a remarkable likeness 

 to a Medusa well defended by stings, he objects to at- 

 tribute any significance to so wonderful and detailed a 

 superficial resemblance, because " a school of whales or 

 a shoal of pelagic fish, rushing through the water and 

 devouring all before them, could hardly be supposed to 

 stop and analyze carefully the advantages or disadvant- 

 ages of selecting or rejecting a given animal as food." 

 On p. lis he remarks: "If Mr. Poulton is right in 

 assigning a protective value to the bright-coloured wings 

 of butterflies, ' as a conspicuous mark easily seized by an 

 enemy, and yet readily tearing without much injury to the 

 insect,' it seems unnecessary to pay much attention to the 

 supposed utility of protective colours, such as are shown 

 by the Kallima or the Green Hairstreak." 



The author scoffs at natural selection as an " easy " 

 road to an explanation, as "the very simple hypothesis 

 of a need for resemblance to the environment " : it may 

 at any rate be maintained that this method of meeting 

 it is very far from profound. 



It is only possible to give a very brief account of the 

 causes which the author would propose to substitute for 

 natural selection. The merits of each proposal lie in its 

 application, and the consideration of this means a dis- 

 cussion of each particular case. 



In support of the " effects of food upon colour," a 

 number of examples are quoted, many of which are so 

 inherently improbable and so imperfectly supported by 

 details, that it is impossible to accept them as evidence. 

 I am very far from disputing that changes of colour may 

 be directly produced by certain foods, although the signifi- 

 cance of such changes in the evolution of animal colouring 

 is a very different matter. When the author proceeds to 



