1873.] COLOURING IN INSECTS. 159 



first four classes, are in my belief well explained by natural selection. 

 In applying this principle to the cases of Classes III. and IV. the 

 law of " inheritance at corresponding periods of life," as developed 

 by Darwin*, should be borne in mind. Such cases as have been 

 included in Class V. have hitherto been regarded as due to the 

 direct action of external conditions ; but I am strongly of the opinion 

 that we see also in these cases a result attributable, in great part at 

 least, to the action of " the survival of the fittest." 



Let us consider, for instance, in what manner natural selection 

 acts in an ordinary case of protective resemblance, say in a larva 

 which simulates its food-plant in colour. Those who maintain the 

 descent theory believe that in such a case varieties which in any 

 way resembled their food-plant in colour more frequently escaped 

 detection, while their less fortunately coloured brethren were de- 

 stroyed, generation after generation, by the rigorous persecution of 

 their foes ; this selecting action, continued through many genera- 

 tions, results at length in the disguise we now behold. It is to be 

 observed that in this explanation no account is taken of the cause 

 of the original variations in the colour of the larva. The variations 

 may, and most probably do, arise in many such cases by the direct 

 action of the colouring-matter of the leaves on the tissues of the 

 larvaf. Thus it is well known that no internal feeding-larva is 

 green, while legions of arboreal feeders are so coloured. It might 

 be argued, therefore, that such colouring is due to the presence of 

 chlorophyl in the insect's food, and has, consequently, nothing to 

 do with natural selection. It can be shown, however, that the green 

 colouring is advantageous to the species that possess it, by rendering 

 them inconspicuous to their enemies ; whence it follows that any 

 variety departing from this mode of colouring (that is, any variety 

 in which the chlorophyl was not discernible through the skin) would 

 be weeded out by natural selection, whose function would therefore 

 be in this case to maintain the green colour of the insects, regardless 

 of the cause of such colour. 



An analogous example presents itself in the case of brightly 

 coloured larvae. The colours have been shown by Mr. Wallace to 

 be due in such cases to the survival of the more brightly coloured 

 individuals through many series of generations. Now, as the colours 

 in these larvae serve merely as a signal of distastefulness %, it is plain 

 that the elaborate and regular patterns we so often behold on these 

 creatures are quite superfluous for the purpose of warning, and are 

 therefore independent of natural selection. While attributing, then, 

 the production of the pattern to the unknown laws of growth, we 

 are justified in regarding the general production of colour as a pro- 



* ' Variation of Animals and Plants under Domestication,' vol. ii. chap. xiv. 

 pp. 75-80. 



f [In a recently published memoir on cholophyl, by M. J. Chantard (Comptes 

 Rendus hebdomadaires des seances de l'Academie des Sciences, Jan. 13tb, 1873), 

 the author announces the discovery of this substance in an unaltered state in the 



tissues of certain leaf-feeding insects. — March 6th, 1873.] 

 % See Wallace's ' Contributions to the theory of Natural 



Selection,' p. 117. 



