Sympathetic Coloration 51 



It is part of the essence of selection, that it not only causes a part to 

 vary till it has reached its highest pitch of adaptation, but that it 

 maintains it at this pitch. This conserving influence of natural 

 selection is of gi*eat importance, and was early recognised by Darwin ; 

 it follows naturally from the principle of the sui'vival of the fittest. 



We understtmd from this how it is that a species which has 

 become fully adapted to certain conditions of life ceases to vary, 

 but remains " constant," as long as the conditions of life for it remain 

 unchanged, whether this be for thousands of years, or for whole 

 geological epochs. But the most convincing proof of the poAver 

 of the principle of selection lies in the innumerable multitude of 

 phenomena which cannot be explained in any other way. To this 

 category belong all structures which are only passively of advantage 

 to the organism, because none of these can have arisen by the alleged 

 LamarcJcian princij)le. These have been so often discussed that 

 we need do no more than indicate them here. Until quite recently 

 the sympathetic coloration of animals — for instance, the whiteness 

 of Arctic animals — was referred, at least in part, to the direct 

 influence of external factors, but the facts can best be explained 

 by referring them to the processes of selection, for then it is un- 

 necessary to make the gratuitous assumption that many species are 

 sensitive to the stimulus of cold and that others are not. The great 

 majority of Arctic land-animals, mammals and birds, are white, and 

 this proves that they were all able to present the variation which 

 was most useful for them. The sable is brown, but it lives in trees, 

 where the brown colouring protects and conceals it more effectively. 

 The musk-sheep (Ovlbos moschatus) is also brown, and contrasts sharply 

 with the ice and snow, but it is protected from beasts of prey by its 

 gregarious habit, and therefore it is of advantage to be visible fi-om 

 as gi'eat a distance as possible. That so many species have been 

 able to give rise to white varieties does not depend on a special 

 sensitiveness of the skin to the influence of cold, but to the fact that 

 Mammals and Birds have a general tendency to vary towards white. 

 Even with us, many birds — starlings, blackbirds, swallows, etc. — 

 occasionally produce white individuals, but the white variety does 

 not persist, because it readily falls a victim to the carnivores. This 

 is true of white fauTis, foxes, deer, etc. The whiteness, therefore, 

 arises from internal causes, and only persists when it is useful. 

 A gi'cat many animals living in a green environment have become 

 clothed in gieen, especially insects, caterpillars, and Mantidae, both 

 persecuted and persecutors. 



Tliat it is not the direct effect of the environment which calls 

 forth the green colour is shown by the many kinds of caterpillar 

 which rest on leaves and feed on them, but are nevertheless brown. 



4—2 



