190 DARWINISM chap. 



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. The extreme 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 in- 

 dividuals rarely or never increase, there must be some con- 

 stant power at work to keep it in check. This power can 

 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 guide, let 

 us see how far we can account both for the general and 

 special colours of the animal world. 



Colour and Environment. 



The fact that first strikes us in our examination of the 

 colours of animals as a whole, is the close relation that exists 

 between these colours and the general environment. Thus, 

 white prevails among arctic animals ; yellow or brown in desert 

 species ; while green is only a common colour in tropical ever- 

 green 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 are a number of animals which are 

 wholly white all the year round, or which only turn white in 

 winter. Among the former are the polar bear and the American 

 polar hare, the snowy owl and the Greenland falcon ; among 

 the latter the arctic fox, the arctic hare, the ermine, and the 

 ptarmigan. Those which are permanently white remain among 

 the snow nearly all the year round, while those which change 

 their colour inhabit regions which are free from snow in 

 summer. The obvious explanation of this style of coloration 

 is, that it is protective, serving to conceal the herbivorous species 

 from their enemies, and enabling carnivorous animals to approach 

 their prey unperceived. Two other explanations have, how- 



