VIII ORIGIN AKD USES OF COLOUR IN AXIMALS 191 



ever, been suggested. One is, that the prevalent white of the 

 arctic regions has a direct effect in producing the white colour 

 in animals, either by some photographic or chemical action on 

 the skin or by a reflex action through vision. The other is, 

 that the white colour is chiefly beneficial as a means of checking 

 radiation and so j^reserving animal heat during the severity of 

 an arctic A^nter. The first is part of the general theory that 

 colour is the eflect of coloured light on the objects — a pure 

 hypothesis which has, I believe, no facts whatever to support 

 it. The second suggestion is also an hypothesis merely, 

 since it has not been proved by experiment that a white 

 colour, per se, independently of the fur or feathers which is so 

 coloured, has any effect whatever in checking the radiation of 

 low-grade heat like that of the animal body. But both alike 

 are sufficiently disproved by the interesting exceptions to the 

 rule of white coloration in the arctic regions, which exceptions 

 are, nevertheless, quite in harmony with the theory of pro- 

 tection. 



Whenever we find arctic animals which, from whatever 

 cause, do not require protection by the white colour, then 

 neither the cold nor the snow-glare has any effect upon their 

 coloration. The sable retains its rich bro^vn fur throughout 

 the Siberian winter ; but it frequents trees at that season and 

 not only feeds partially on fruits or seeds, but is able to 

 catch birds among the branches of the fir-trees, Avith the bark 

 of which its colour assimilates. Then we have that thoroughly 

 arctic animal, the musk-sheep, which is brown and conspicuous ; 

 but this animal is gi-egarious, and its safety depends on its 

 association in small herds. It is, therefore, of more im- 

 portance for it to be able to recognise its kind at a distance 

 than to be concealed from its enemies, against which it can 

 well protect itself so long as it keeps together in a compact 

 body. But the most striking example is that of the common 

 raven, which is a true arctic bird, and is found even in 

 mid-^^dnter as far north as any known bird or mammal. 

 Yet it always retains its black coat, and the reason, from our 

 point of view, is obvious. The raven is a powerful bird 

 and fears no enemy, while, being a carrion-feeder, it has no 

 need for concealment in order to approach its prey. The 

 colour of the raven and of the musk -sheep are, therefore. 



