COLORATION AFFECTED BY THE ENVIRONMENT. 77 



the fur in an Arctic fox (one of several) in the Zoological 

 Gardens, seems rather to indicate the persistence of an 

 acquired habit than the direct effect of environment ; but 

 it may, perhaps, be explained on the assumption that the 

 stimulus (i.e. the cold) was not sufficiently great to affect one 

 individual, but was sufficient to affect another of (presumably) 

 a slightly different constitution. Moreover, the experiment of 

 Sir John Ross was so far inconclusive that the cold was 

 administered in a sudden dose, which may have produced an 

 effect analogous to a nervous shock : we know that in human 

 beings the hair may turn white from the effects of a sudden 

 shock . 



The fact that the change to white at the approach of winter 

 is not seen in all the inhabitants of the polar regions, appears 

 to be an objection to the view that the change is a direct effect 

 of the environment. It is not, however, a fatal objection, 

 for it is a reasonable assumption that different animals are 

 differently affected ; if individuals differ in their susceptibility, 

 we need not be surprised to find that different species vary 

 in the same way. Another view of the matter is that the 

 change is one which has been brought about by natural 

 selection ; the white coat being useful for concealment in 

 the winter, and useless or rather harmful when the snow has 

 largely melted, we get the need for such a change. Here the 

 exceptions seem to prove the rule. The sable, the musk-ox, 

 and the raven retain their summer colouring throughout the 

 winter ; but the sable and the raven are strong and active 

 animals, which have no enemies to fear ; the agility of the 

 sable in pursuing birds among the boughs of trees, and 

 perhaps also the dark colour of the boughs, renders un- 

 necessary the adoption of any concealment ; while, as regards 

 the raven, it is mainly a carrion feeder. 



