HOW ARCTIC ANIMALS TURN WHITE 67 



viduals. The species in question is the Arctic fox, of 

 which the beautiful fur, in both the white and the blue 

 phase, is, as mentioned in a later article, now much 

 affected by ladies. That both the white and the blue 

 individuals of this species are in the winter dress will 

 be evident to every one who examines such furs carefully, 

 the length and thickness of the hair being quite decisive 

 on this point. 



With the single exception of Iceland, where they are 

 always blue, it appears that the white and the blue phase 

 are met with throughout the habitat of the species. In 

 other words, the animal is " dimorphic," if it be permis- 

 sible to apply this term to a case where the difference 

 between the two phases of a species is restricted to 

 coloration. 



What makes the matter so puzzling is this : if blue foxes 

 are able to thrive during winter in a snow-clad country, 

 what necessity is there for their fellows — and, indeed, for 

 any species — to turn white at that season of the year ? 

 An explanation of the case of the blue foxes has been 

 attempted in the article already referred to. 



Since the present article was written important additional 

 information with regard to the manner in which hair 

 bleaches has been afforded by a communication from 

 Mr. E. Metchnikoff, published in the Proceedings of the 

 Royal Society for 1902. It is there stated that the all- 

 devouring cells known as phagocytes are the cause of 

 the mischief. These cells, which frequently have amoeba- 

 like processes, are developed in the central or medullary 

 part of the hair, whence they make their way into the 

 outer or cortical layer, where they absorb, and thus destroy, 

 the pigment-granules. Numbers of these phagocytes may 

 be seen in hair which is commencing to turn white. 



