Those which are permanently white remain
among the snow nearly all the year round, while those which change their
colour inhabit regions which are free from snow in summer. The obvious
explanation of this style of coloration is, that it is protective,
serving to conceal the herbivorous species from their enemies, and
enabling carnivorous animals to approach their prey unperceived. Two
other explanations have, however, 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.
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