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Show 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 a m 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 destroyed, generation after generation, by the rigorous persecution of their foes; this selecting action, continued through many generations, 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. A n analogous example presents itself in the case of brightly coloured larvae. The colours have been shown by M r . 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. 13th, 1873), the author announces the discovery of this substance in an unaltered state in the tissues of certain leaf-feeding insects.-March 6tb, 1873.] % See Wallace's ' Contributions to the theory of Natural Selection,' p. 117. |