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 Eimer’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 
