There can be no doubt that
these colours have been acquired as a protection, when we see that in
all the temperate regions, where the leaves are deciduous, the ground
colour of the great majority of birds, especially on the upper surface,
is a rusty brown of various shades, well corresponding with the bark,
withered leaves, ferns, and bare thickets among which they live in
autumn and winter, and especially in early spring when so many of them
build their nests.
Nocturnal animals supply another illustration of the same rule, in the
dusky colours of mice, rats, bats, and moles, and in the soft mottled
plumage of owls and goatsuckers which, while almost equally
inconspicuous in the twilight, are such as to favour their concealment
in the daytime.
An additional illustration of general assimilation of colour to the
surroundings of animals, is furnished by the inhabitants of the deep
oceans. Professor Moseley of the Challenger Expedition, in his British
Association lecture on this subject, says: "Most characteristic of
pelagic animals is the almost crystalline transparency of their bodies.
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