his eye, and thus dazzling his sight, both in air and beneath the
water; which must have happened, if that surface had been white like the
rest of his feathers.

There is a still more wonderful thing concerning these colours adapted to
the purpose of concealment; which is, that the eggs of birds are so
coloured as to resemble the colour of the adjacent objects and their
interfaces. The eggs of hedge-birds are greenish with dark spots; those of
crows and magpies, which are seen from beneath through wicker nests, are
white with dark spots; and those of larks and partridges are russet or
brown, like their nests or situations.

A thing still more astonishing is, that many animals in countries covered
with snow become white in winter, and are said to change their colour again
in the warmer months, as bears, hares, and partridges. Our domesticated
animals lose their natural colours, and break into great variety, as
horses, dogs, pigeons. The final cause of these colours is easily
understood, as they serve some purposes of the animal, but the efficient
cause would seem almost beyond conjecture.

First, the choroid coat of the eye, on which the semitransparent retina is
expanded, is of different colour in different animals; in those which feed
on grass it is green; from hence there would appear some connexion between
the colour of the choroid coat and of that constantly painted on the retina
by the green grass. Now, when the ground becomes covered with snow, it
would seem, that that action of the retina, which is called whiteness,
being constantly excited in the eye, may be gradually imitated by the
extremities of the nerves of touch, or rete mucosum of the skin. And if it
be supposed, that the action of the retina in producing the perception of
any colour consists in so disposing its own fibres or surface, as to
reflect those coloured rays only, and transmit the others like
soap-bubbles; then that part of the retina, which gives us the perception
of snow, must at that time be white; and that which gives us the perception
of grass, must be green.

Then if by the laws of imitation, as explained in Section XII. 3. 3. and
XXXIX. 6. the extremities of the nerves of touch in the rete mucosum be
induced into similar action, the skin or feathers, or hair, may in like
manner so dispose their extreme fibres, as to reflect white; for it is
evident, that all these parts were originally obedient to irritative
motions during their growth