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, two strong arguments against this
theory. We have already seen how generally bright coloration is wanting
in desert animals, yet here heat and light are both at a maximum, and if
these alone were the agents in the production of colour, desert animals
should be the most brilliant. Again, all naturalists who have lived in
tropical regions know that the proportion of bright to dull coloured
species is little if any greater there than in the temperate zone, while
there are many tropical groups in which bright colours are almost
entirely unknown. No part of the world presents so many brilliant birds
as South America, yet there are extensive families, containing many
hundreds of species, which are as plainly coloured as our average
temperate birds. Such are the families of the bush-shrikes and
ant-thrushes (Formicariidae), the tyrant-shrikes (Tyrannidae), the
American creepers (Dendrocolaptidae), together with a large proportion
of the wood-warblers (Mniotiltidae), the finches, the wrens, and some
other groups.
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