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COLOURS OF ANIMALS 341Theory of Heat and Light as producing ColourIn commencing our study of the great mass of factsrelating to the colours of the organic world, it will be necessaryto consider, first, how far the chief theories alreadyproposed will account for them. One of the most obvious andmost popular of these theories, and one which is still held, inpart at least, by many eminent naturalists, is, that colour isdue to some direct action of the heat and light of the sunthus 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 explanationof the colours of living things, we must ask the preliminaryquestion, whether it is really the fact that colour is moredeveloped in tropical than in temperate climates in proportionto the whole number of species ;and even if we find thisto be so, we have to inquire whether there are not so manyand such striking exceptions to the rule as to indicate someother causes at work than the direct influence of solar lightand heat. As this is a most important branch of the inquiry,we must go into it somewhat fully.It isundoubtedly the case that there are an immenselygreater number of richly-coloured birds and insects in tropicalthan in temperate and cold countries, but it isby no meansso certain that the proportion of coloured to obscure species ismuch or any greater. Naturalists and collectors well knowthat the majority of tropical birds are dull- coloured ;andthere are whole families, comprising hundreds of species, notone 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 westernhemispheres. Again, many groups of birds which are universallydistributed are no more adorned with colour in thetropical 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 iscertainly most extraordinary that in groups so varied in form,structure, and habits as those just mentioned, the tropicalshould be in no wise distinguished in this respect from thetemperate species.