THE COLORATION OF ANIMALS 77 



it is always on the posterior wings, which are covered when at rest. 

 This can best be observed in the species of the genus Catocala. 



Let us now, however, interrupt our survey of the facts for 

 a moment, and let us inquire whether all the cases of protective 

 colouring in Lepidoptera we have considered can be referred to natural 

 selection, or whether it is not conceivable that other causes may have 

 evoked them. 



The first thing to be said is that the Lamarckian principle of 

 the inherited effects of use and disuse cannot here be taken into 

 account, as the colours of the surface of the body do not exercise any 

 active function at all; their effect is due simply to their presence, 

 and it is for them quite indifferent whether and how often they have 

 opportunity to protect their bearers from enemies, or whether no 

 enemies ever chance to appear. It has frequently been suggested, 

 too, that these colorations are associated with the differences in the 

 strength of the illumination to which the different parts and surfaces 

 are exposed. But this again is untenable, as is proved even by the 



Fig. 10. Xijlina vetusta, after Rosel. A, in flight. B, at rest. 



dimorphism frequently occurring in caterpillars, for the green and the 

 brown individuals are exposed to precisely the same light ; and still 

 more clearly by the sympathetic colouring, which is so exactly 

 defined and yet so different on the under surface of the diurnal 

 butterflies. Yet there are isolated cases in which it seems as if the 

 direct influence of the light had brought about certain striking 

 differences in the colouring of the parts of an insect, and I shall 

 describe perhaps the prettiest of these cases, to which Brunner von 

 Wattenwyl directed attention. It concerns one of the Orthoptera of 

 Australia, a Phasmid, Tropidoderus childrenl, Grey, which has 

 a general colouring of leaf -green, but with singular deviations from 

 it on certain areas of the body. In this insect the anterior wings 

 which form the wing covers or elytra (Fig. 1 1 , V) are so short that they 

 scarcely cover the half of the long abdomen. Their place is taken by 

 the anterior margin of the posterior wing (H. horn), which is hard and 

 horny like the elytra, and in the resting position protects the whole 

 abdomen. All these covering parts are grass-green, except at the 

 places where they overlap : on these areas they have a faded look, and 



