VIII 
ORIGIN AND USES OF COLOUR IN ANIMALS 
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 preserving animal heat during the severity of 
an arctic winter. The first is part of the general theory that 
colour is the effect 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 brown 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, with 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 gregarious, 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 
Avell 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-winter 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, 
