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Show 194 DARWINISM CI!Al'. two strong arguments against this theory. vV c have already seen how generally bright coloration is wanting in desert animals, yet here heat and light arc both at a maximnm, and if these aloue were the agents in the production of colour, desert animals shonld 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 arc many tropical groups in which bright colours arc almost en tirely unknown. No part of the world presents so many brilliant birds as South America, yet there arc extc11siv f<tmilies, containing many hundreds of species, which are a~ plainly coloured as our average temperate birds. Such arc the families of the bush-shrikes and ant-thrushes (Formicariidro), the tyrant-shrikes (Tyrannidro), the American creepers (Dcndrocolaptidro), together with a large proportion of the woodwarblers (Mniotiltidro), the finches, the wrens, and some oth01· groups. In the eastern hemisphere, also, we have the babblingthrushes (Timaliidro), the cuckoo-shrikes (Campcphagiclro), tho honey-suckers (Mcliphagidre), and several other smaller gronps which are certainly not coloured above the avemgc stambrd of temperate birds. Again, there are mn.ny families of birds which spread over the whole world, tempera.tc and tropical, and among these the tropical species rarely present any exceptional brilliancy of colour. Such arc the thrushes, goatsuckcrs, hawks, plovers, and ducks; and in the bst-named group it is the temperate and arctic zones that afford the most brilliant coloration. The same general facts are found to prevail among insects. Although tropical insects present some of the mo t gorgcom; coloration in the whole realm of nature, yet there arc thousands and tens of thousands of species which arc as dull coloured as any in our cloudy land. The extensive family of the carnivorous ground-beetles (Carabidre) attains its greatest brilliancy in the temperate zone; while by far the larger proportion of the great families of the longicorns and tho weevils, are of obscure colours even in the tropics. In butterflies, there is undoubtedly a larger proportion of brilliant colour in the tropics; but if we compare families which arc almost equally developed over the globe-as the Pieridro or VIII ORIGIN AND USES OF COLOUR IN ANIMALS 195 whites and yellows, <tnd the Satyridro or ringlets- we sktll fin<l no great di:proportion in colour between those of temperate and tropical regions. The variou: facts which have now briefly been noticed are sufficient to indicate that the light and heat of the sun arc not the di rcct causes of the colour. of animals, although they m:1y favour the production of colour when, n.s in tropical regions, the persistent high tcmpemturc favours the development of the maximum of life. \Vc will now consider the llCxt suggestion, that light reflected from surrounding coloured objects tends to produce corresponding colour. in the animal world. This theory is founded on a nnmhcr of very curious facts which prove, that such a change docs sometimes occur and is directly dependent on the colours of surronnding objects ; but these facts arc comparatively rare and exceptional in their nature, and the same theory will certainly not apply to the infinitely varied colours of the higher animals, many of which are exposed to a constantly varying amount of light and colour during their active existence. A brief sketch of these dependent changes of colour may, however, he advantageously given here. Vm·iable Protective Colou1·in,q. There arc two distinct kinds of change of colour in animals duo to the colouring of the environment. In one case the change is caused by reflex action set up by the animal seeing the colour to be imitated, and tho' change prodnccd can he n.ltcrcd or repeated as the animal changes its position. In the other case the change occurs but once, and is probably not dnc to any conscious or sense action, but to some direct inflncncc on the surface tissues while the creature is undergoing a moult or change to the pupa form. The most striking example of the first class is that of the chameleon, which change: to white, brown, yellowish, or green, according to the colour of the object on which it rests. This change is brought about by means of two layers of pigment cells, deeply seated in the skin, and of bluish and yellowish colours. By snitahlc muscles these cell. can be forced upwards so as to modify the colour of the skin, which, |
