PEOTECTIVE RESEMBLANCES DIMOKPHISM, ETC. 45 



is mature, the hairs become black and the body of a 

 much darker tint, and the animal is then well pro- 

 tected by General Eesemblance to the dark surface 

 over which it moves (see fig. 13). 



Although the bark of large birch trees is chiefly 

 white, the caterpillar is, upon the whole, better con- 

 cealed by becoming dark-coloured. It lives on small 

 birches and alders with dark bark, as well as on 

 large birches, and in the latter case it probably 

 wanders among the wide dark chinks rather than 

 over the smooth wide expanses, for it would certainly 

 burrow in the former rather than the latter. 



Just before pupation the colours of caterpillars 

 nearly always become dull, and it is in every way 

 probable that such incidental changes have been 

 seized upon by natural selection, and have been ren- 

 dered advantageous to the species. Such alterations 

 of colour are entirely different from those which will 

 be described below, in which an animal can modify its 

 appearance into correspondence with its individual 

 surroundings. The larva of the Privet Hawk Moth 

 almost invariably wanders over the earth when it 

 has come down from its food-plant ; but if it were to 

 descend upon turf, the brown colour would still be 

 assumed, although green would conceal it more effec- 

 tually. The change to brown is, however, far safer 

 for the average caterpillar, and is beneficial to the 

 species on the whole, although it must lead to some in- 

 dividual failures. In the far higher form of Variable 



