VIII ORIGIN AND USES OF COLOUR IN ANIMALS 193 



they live in autumn and "winter, and especially in early spring 

 Avhen 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 Avater, Avhile 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 opacj^ue and of a yellow or bro^wai colour, and 

 exactly resemble when seen in the water small pieces of 

 floating seaAveed." Such marine organisms, however, as 

 are of larger size, and either occasionally or habitually float 

 on the surface, are beautifully tinged Avith blue above, thus 

 harmonising Avitli the colour of the sea as seen by hovering 

 birds ; Avhile they are Avhite beloAv, and are thus invisible 

 against the Avave-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 noAv 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 Avorld. 

 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 floAvers 

 Avhich are brought from tropical regions. There are, hoAvever, 







