| OCR Text |
Show 190 DARWINISM ORAl', great variability. We see this in our horses and cattle, our dogs and cats, our pigeons and poultry. Now, the essential difference between the conditions of life of domesticated and wild animals is, that the former are protected by man, while the latter have to protect themselves. Thc •cxtremc variations in colour that immediately arise under domestication indicate a tendency to vary in this way, and the occasional occurrence of white or piebald or other exceptionally coloured individuals of many species in a state of nature, shows that this tendency exists there also ; and, as these exceptionally coloured individuals rarely or never increase, there must be some constant power at work to keep it in check This power car1 only be natural selection or the survival of the fittest, which again implies that some colours are useful, some injurious, in each particular case. With this principle as our gnide, let us see how far we can account both for the general and special colours of the animal world. Colo~ur ancl Environment. The fact that first strikes us in our examination of the colours of animals as a whole, is the close relation that exi ts between these colours and the general environment. Thus, white prevails among arctic animals; yellow or brown in des 'rt species; while green is only a common colour in tropical evergreen forests. If we consider these cases somewhat carefully we shall find, that they afford us excellent materials for forming a judgment on the various theories that have been suggested to account for the colours of the animal world. In the arctic regions there arc a number of animals which nrr wholly white all the year round, or which only turn white in winter. Among the former arc the polar hear and the Amcric:tn polar hare, the snowy owl and the GrcenhnCl falcon j among the latter the arctic fox, the arctic ha,rc, the ermine, and tlw ptarmigan. Those which are permanently white remain among the snow nearly all tho year round, while those which chan ~e their colour inhabit regions which arc free from snow iu summer. The obvious explanation of this style of coloration is, that it is protective, serving to conceal the hcrbivoron species from their enemies, and enabling carnivorous a,nimals to approach their prey unperceived. Two other explana,tions have, how- VIII ORIGIN AND USES OF COLOUR IN ANIMALS 191 ever, been suggested. One is, that the prevalent white of the arctic regions has a direct cffc<.:t in producing the white colour in animals, either by some photographic or chemical action on the kin or by a reflex action through vision. The other i:, that the white colour is chiefly beneficial as a means of checking radiation and so preserving animal heat during the severity of an arctic winter. The first is part of the general theory that colour is the effect of coloured light on the objects- a pure hypothesis which has, I believe, no facts whatever to support it. The second suggestion is also an hypothesis merely, since it has not been proved by experiment that a white colour, per e, independently of the fur or feathers which is so coloured, has any effect whatever in checking the radiation of low-grade heat like that of the animal body. But both alike are sufficiently disproved by the interesting exceptions to the rule of white coloration in the arctic regions, which exceptions are, nevertheless, quite in harmony with the theory of protection. Whenever we find arctic animals which, from whatever cause, do not require protection by the white colour, then neither the cold nor the snow-glare bas any effect upon their coloration. The sable retains its rich brown fur throughout the Siberjan winter; but it frequents trees at that sea ·on and not o11ly feeds partially on fmits or seeds, bnt i: able to catch birds among the branches of the fir-trees, with the bark of which its colour assimilates. Then we have that thoroughly arctic animal, the musk-sheep, which is brown and conspicuous; but this animal is gregarious, and its safety depends on its association in small herds. It is, therefore, of more importance for it to be able to recognise its kind at a distance than to be concealed from its enemies, against which it can well protect itself so long as it keeps together in a compact body. But the most striking example is that of the common raven, which i · a true arctic bird, and is fonnd even in mid-winter as far north as any known bird or mammal. Yet it always retains its black coat, and the reason, from our point of view, is obvious. The raven is a powerful bird and fears no enemy, while, being a carrion-feeder, it has no need for concealment in order to approach its prey. The colour of the raven and of the musk-sheep are, therefore, |
