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 Timaliidse or babbling thrushes of the eastern, 

 and the Dendrocolaptidae 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. 



