342 THE BIOLOGY OF THE SEASONS 



that a very slight change of alkalinity or acidity will change 

 the colour of a flower, and that boiling water makes the 

 blue lobster red. But when a hair or feather becomes 

 white slowly, the change may be of a quite different kind. 

 For Metchnikoff has shown that the wandering amoeboid 

 cells the phagocytes of the body, which discharge so 

 many useful offices, pass into the hair and down again, 

 carrying a microscopic burden of pigment material. 

 Whether the change be quick or slow, there is an associated 

 physical change in the accumulation of gas vacuoles. 

 For the whiteness of grey hair is like the whiteness of foam. 



When we remember that albinos often crop up as the 

 result of inborn variations in all sorts of animals, even 

 the blackest of them ; that albinism occurs in all possible 

 degrees of thoroughness ; that mammals periodically 

 moult their hair and birds their feathers there seems no 

 particular difficulty in understanding how species may 

 have arisen with the constitutional idiosyncrasy of assuming 

 a white dress in winter, provided always that the change 

 can be shown to be distinctly advantageous. 



There appear to be various advantages in a white dress 

 in very cold snowy regions. For a hot-blooded animal, 

 with a temperature high above that of the surrounding world, 

 the loss of heat is less with white hairs or white feathers 

 than with any other colour of dress. It is physiologically 

 the most comfortable dress, putting least strain on the 

 complex mechanism that regulates the body-temperature 

 in warm-blooded animals. This is probably the chief 

 advantage of turning white in winter, but it must also be 

 admitted that a white dress is the least conspicuous in 

 snowy regions as well as the most comfortable. 



Where the struggle for existence is keen, it may be but 

 a little thing that turns the balance towards survival 

 or elimination. Professor Davenport had 300 chickens in 



