66 THE EVOLUTION THEOKY 



^\ animals which become white in winter is thus so organized that the 

 setting in of cold weather acts as a stimulus which incites the skin 

 tq the production of white hairs. This predisposition also we must 

 refer to the influence of natural selection, since it must have been 

 very useful to the species that the winter coat should grow just when 

 it was necessary as a protection against cold. This explains at the 

 same time why the predisposition to respond to the stimulus of cold 

 by a growth of winter fur finds expression earlier in those colonies of 

 Arctic animals, such as the hare, which live in Lapland, than in those 

 which live in the south of Norway. 



But that it is not the direct influence of cold which colours the 

 hair of a furred animal white we can see from our common hare 

 {Leiius timidus), which, in spite of the winter's cold, does not become 

 white, but retains its brown coat, and not less so from the mountain 

 hare (Lepus variabilis), which in the south of Sweden also remains 

 brown, although the winter there may be exceedingly cold. But as the 

 covering of the ground with snow is not so uninterrupted there as in 

 the higher North, a white coat would be not a better protection 

 than a brown one, but a worse. The white colouring of Arctic animals 

 is therefore not directly due to the influence of the climate, as has 

 often been maintained, but is due to it indirectly, that is, through the 

 operation of natural selection. I have tried to make this clear by 

 means of this example, so that we may not have to repeat it in 

 considering those which are to follow. 



But all attempts at any other explanation are even more decidedly 

 excluded when we turn our attention to more complicated cases of colour- 

 adaptation, which are not confined to the simple, general coloration, but 

 are helped by markings and colour-patterns, that is, by schemes of colour. 



Thus numerous caterpillars exhibit definite lines and spots on 

 their ground-colouring, which, in one way or another, aid in pro- 

 tecting them from their enemies. 



The green grass-eating caterpillar of many of our Satyridoe 

 has two or more darker or lighter lines running down the sides of its • 

 body, which make it much less conspicuous among the grasses on 

 which it feeds than if it were a uniform green mass (Fig. a). Not 

 infrequently the colour and form present a remarkably close resem- 

 blance to the inflorescences or fruit-ears of the grasses. Caterpillars 

 marked thus are never found on the leaves of trees, where they 

 would immediately catch the eye. It is true that longitudinal 

 striping often occurs on caterpillars which live on other plants 

 besides grass, but as these other plants grow among the grasses the 



