iv SHIFTS FOR A LIVING 53 



leaves and herbage ; tawny animals harmonise with sandy 

 soil ; and the hare escapes detection among the clods. 

 So do spotted animals such as snakes and leopards live 

 unseen in the interrupted light of the forest, and the 

 striped tiger is lost in the jungle. Even the eggs of 

 birds are often well suited to the surroundings in which 

 they are laid. There can be no doubt that this resem- 

 blance between the colour of an animal and that of its 

 surroundings is sometimes of protective and also aggres- 

 sive value in the struggle for existence, and where this 

 is the case, natural selection w r ould foster it, favouring 

 with success those variations which were best adapted, 

 and eliminating those which were conspicuous. 



5. Variable Colouring. Some animals, such as the 

 ptarmigan and the mountain-hare, become white in 

 winter, and are thereby safer and warmer. In some 

 cases, as in the mountain-hare, the pigmented hairs seem 

 to become white ; in other cases, as in the ermine, the old 

 hairs drop off and are replaced by white ones ; some- 

 times the whiteness is the result of both these processes. 

 The whiteness is directly due to the formation of gas 

 bubbles inside the hairs or feathers. In some cases, e.g. 

 Ross's lemming and the American hare (Lepus americanus), 

 it has been shown experimentally that it is the cold that 

 pulls the trigger of the change to white, but the possi- 

 bility of the change, like that of other seasonal variations, 

 doubtless depends on a constitutional peculiarity. 



To several naturalists, but above all to Prof. Poulton, 

 we are indebted for much precise information in regard 

 to the variable colouring of many caterpillars and chrysa- 

 lids, which adjust their colours to those of the surround- 

 ings. Prof. Poulton experimented with the caterpillars 

 of the peacock butterfly (Vanessa io), small tortoise-shell 

 (Vanessa urticce], garden whites (Pieris brassicce and 

 Pieris rapce), and many others. Caterpillars of the small 

 tortoise - shell in black surroundings tend to become 

 darker as pupae ; in a w r hite environment the pupae are 

 lighter ; in gilded boxes they tend to become golden. 

 The surrounding colour seems to influence the caterpillar 

 " during the twenty hours immediately preceding the last 



