82 THE EVOLUTION THEORY 



further ! How should it be otherwise, when the colouring of the 

 parts just beside these is indifferent for the species, so that any 

 variations in these parts in the direction of protective colouring 

 never survive to be transmitted and accumulated ? 



It is precisely this restriction to what is absolutely necessary 

 that is the surest sign, here and elsewhere, that the character in 

 question has been brought about by natural selection. And if this is 

 the only possible, and at the same time quite sufficient explanation of 

 the remarkably well-defined colour deliminations in all Lepidoptera, 

 there can be no reason why we should try to drag in any other factor 

 to explain the case of Tropidoderus, the less so as here again selection 

 alone can account for the green of the exposed surfaces ; and further- 

 more, the modification, common in other Phasmidae, of the most anterior 

 green stripe of the posterior wing into a firm cover protecting the 

 soft abdomen, also points to natural selection ; the cover- wings 

 proper have here become too short, and so the edge of the posterior 

 wing has been modified into a hard rib, which protects the soft body 

 of the insect (Fig. n, H. horn). No differences in illumination, 

 and no direct effect of any external influence whatever could have 

 brought that about. 



How much more I might adduce in this connexion ! The 

 manifold diversity of colour and form adaptation is so great 

 among insects, to which protection from their enemies is so necessary, 

 and especially among butterflies, that I should never come to an 

 end if I were to try to give even an approximate idea of it. Let 

 us, therefore, turn now from such cases to a higher — the highest — 

 grade of adaptation, that in which there is not only a mimicry of 

 special and complex coloration, but in which the whole animal has 

 become like some external object, and is thereby secured from 

 discovery. 



We must first consider the case of our lappet moth (Gastrojxicha 

 quercifolia), which in its copper-red colour and in the remarkable 

 shape and dentated edges of its wings, and finally in the quite 

 extraordinary clucking-hen-like attitude of the wings when at rest, 

 greatly resembles some dry oak-leaves lying one above the other. 



Not unlike this is a ' shark ' moth found in this country, Xyllna 

 obsoleta, which, as the name indicates, looks when at rest like 

 a broken bit of half -rotten wood (Fig. 10, p. 77). It 'feigns death/ 

 as we commonly say, that is. it draws the legs and antennae close to the 

 body, and does not move ; indeed, one may lift it up and throw it 

 on the ground without its betraying by a single twitch that it lives. 

 Only after it has been left undisturbed for some time does it show 



