288 COLOUR IN NATURE chap. 



silk." This shows that these markings are not 

 wholly determined by the amount of pigment pre- 

 sent in the hairs, there must be also some additional 

 cause. 



Albino varieties occur occasionally as a sport, 

 especially under domestication, but many mammals 

 are naturally white. As is well known, certain 

 Arctic animals, e.g. the polar bear, are always white, 

 others only turn white in winter, e.g. the Arctic fox. 

 The change of colour in these cases is associated 

 with the development of numerous air-bubbles in the 

 hair. It would seem that in some cases this is not 

 accompanied by a destruction of the pigment, which 

 is merely concealed by the air-bubbles. 



For further particulars as to the characters of the 

 colours in mammals, reference should be made to 

 the text-books, and for the markings to Elmer's 

 papers. 



The pigments of mammals have been relatively 

 little investigated, but there is probably great uni- 

 formity throughout the group. Leydig describes 

 uric acid compounds as occurring in the skin of 

 Chrysochloris, and regards them as factors in the 

 coloration, but in general the colours are apparently 

 due only to the melanins. 



Colour of the Hair and Skin in Man 



In connection with the pigments, a few remarks 

 upon the colour of the skin and hair in our own 

 species may not be out of place, especially as the 

 questions connected with it have considerable bear- 

 ing upon general problems. As is well known, the 



