nstitutional rhythm which leads to white hair in winter has been
fostered and fixed for a reason quite apart from protection. The fact is
that for a warm-blooded creature, whether bird or mammal, the
physiologically best dress is a white one, for there is less radiation
of the precious animal heat from white plumage or white pelage than from
any other colour. The quality of warm-bloodedness is a prerogative of
birds and mammals, and it means that the body keeps an almost constant
temperature, day and night, year in and year out. This is effected by
automatic internal adjustments which regulate the supply of heat,
chiefly from the muscles, to the loss of heat, chiefly through the skin
and from the lungs. The chief importance of this internal heat is that
it facilitates the smooth continuance of the chemical processes on which
life depends. If the temperature falls, as in hibernating mammals (whose
warm-bloodedness is imperfect), the rate of the vital process is slowed
down--sometimes dangerously. Thus we see how the white coat helps the
life of the creature.

Sec. 3

Rapid Colour-change

Bony flat-fishes, like plaice and sole, have a remarkable power of
adjusting their hue and pattern to the surrounding gravel and sand, so
that it is difficult to find them even when we know that they are there.
It must be admitted that they are also very quick to get a sprinkling
of sand over their upturned side, so that only the eyes are left
showing. But there is no doubt as to the exactness with which they often
adjust themselves to be like a little piece of the substratum on which
they lie; they will do this within limits in experimental conditions
when they are placed on a quite artificial floor. As these fishes are
very palatable and are much sought after by such enemies as cormorants
and otters, it is highly probably that their power of self-effacement
often saves their life. And it may be effected within a few minutes, in
some cases within a minute.

In these self-effacing flat-fishes we know with some precision what
happens. The adjustment of colour and pattern is due to changes in the
size, shape, and position of mobile pigment-cells (chromatophores) and
the skin. But what makes the pigment-cells change? The fact that a blind
flat-fish does not change its colour gives us the first part of the
answer. The colour and the pattern of the surroundings must affect the
eye. The message travels by the optic nerve to the brain; fr