Vv COLOURS OF ANIMALS 341 
Theory of Heat and Light as producing Colour 
In commencing our study of the great mass of facts 
relating to the colours of the organic world, it will be neces- 
sary to consider, first, how far the chief theories already 
proposed will account for them. One of the most obvious and 
most popular of these theories, and one which is still held, in 
part at least, by many eminent naturalists, is, that colour is 
due to some direct action of the heat and light of the sun— 
thus at once accounting for the great number of brilliant birds, 
insects, and flowers which are found between the tropics. 
But before proceeding to discuss this supposed explanation 
of the colours of living things, we must ask the preliminary 
question,—whether it is really the fact that colour is more 
developed in tropical than in temperate climates in propor- 
tion to the whole number of species ; and even if we find this 
to be so, we have to inquire whether there are not so many 
and such striking exceptions to the rule as to indicate some 
other causes at work than the direct influence of solar light 
and heat. As this is a most important branch of the inquiry, 
we must go into it somewhat fully. 
It is undoubtedly the case that there are an immensely 
greater number of richly-coloured birds and insects in tropical 
than in temperate and cold countries, but it is by no means 
so certain that the proportion of coloured to obscure species is 
much or any greater. Naturalists and collectors well know 
that the majority of tropical birds are dull-coloured; and 
there are whole families, comprising hundreds of species, not 
one of which exhibits a particle of bright colour. Such are, 
for example, the Timaliide or babbling thrushes of the eastern, 
and the Dendrocolaptide or tree-creepers of the western 
hemispheres. Again, many groups of birds which are uni- 
versally distributed are no more adorned with colour in the 
tropical than in the temperate zones; such are the thrushes, 
wrens, goat-suckers, hawks, grouse, plovers, and snipe ; and if 
tropical light and heat have any direct colouring effect, it is 
certainly most extraordinary that in groups so varied in form, 
structure, and habits as those just mentioned, the tropical 
should be in no wise distinguished in this respect from the 
temperate species. 
