VIII 
ORIGIN AND USES OF COLOUR IN ANIMALS 
193 
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. So perfect is this trans¬ 
parency that very many of them are rendered almost entirely 
invisible when floating in the water, while some, even when 
caught and held up in a glass globe, are hardly to be seen. 
The skin, nerves, muscles, and other organs are absolutely 
hyaline and transparent, but the liver and digestive tract 
often remain opaque and of a yellow or brown colour, and 
exactly resemble when seen in the water small pieces of 
floating seaweed.” Such marine organisms, however, as 
are of larger size, and either occasionally or habitually float 
on the surface, are beautifully tinged with blue above, thus 
harmonising with the colour of the sea as seen by hovering 
birds; while they are white below, and are thus invisible 
against the wave-foam and clouds as seen by enemies beneath 
the surface. Such are the tints of the beautiful nudibranchiate 
mollusc, Glaucus atlanticus, and many others. 
General Theories of Animal Colour. 
We are now in a position to test the general theories, or, 
to speak more correctly, the popular notions, as to the origin 
of animal coloration, before proceeding to apply the principle 
of utility to the explanation of some among the many 
extraordinary manifestations of colour in the animal world. 
The most generally received theory undoubtedly is, that 
brilliancy and variety of colour are due to the direct action 
of light and heat; a theory no doubt derived from the 
abundance of bright - coloured birds, insects, and flowers 
which are brought from tropical regions. There are, however, 
O 
