CHAP. IV Shifts for a Living 49 



visible in the sea ; white animals, such as the polar bear, 

 the arctic fox, and the ptarmigan in its winter plumage, 

 are inconspicuous upon the snow ; green animals, such 

 as insects, tree-frogs, lizards, and snakes, hide among the 

 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 resemblance between the colour 

 of an animal and that of its surroundings is sometimes of 

 protective and also aggressive value in the struggle for 

 existence, and where this is the case, natural selection 

 would foster it, favouring with success those variations 

 which were best adapted, and eliminating those which were 

 conspicuous. 



But there are many instances of resemblance to sur- 

 roundings which are hard to explain. Thus Dr. A. Seitz 

 describes a restricted area of woodland in South Brazil, where 

 the great majority of the insects were blue, although but 

 a few miles off a red colour was dominant. He maintains 

 that the facts cannot in this case be explained as due either 

 to general protective resemblance or to mimicry. 



I have reduced what I had written in illustration of 

 advantageous colouring of various kinds, because this 

 exceedingly interesting subject has been treated in a readily 

 available volume by one who has devoted much time and 

 skill to its elucidation. Mr. E. B. Poulton's Colours of 

 Animals (International Science Series, London, 1890) is a 

 fascinating volume, for which all interested in these aspects 

 of natural history must be grateful. With this a forth- 

 coming work {Anhnal Coloration, London, 1892) by Mr. 

 F. E. Beddard should be compared. 



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 it 

 is certain that the pigmented feathers and hairs become 

 white, in other cases the old feathers and hairs drop 



E 



