VAKIABLE EESEMBLANCE IN VERTEBRATA, ETC. 103 



nearly all the year retain the white coat permanently, 

 and there is no need for the mechanism by which the 

 change is produced. And yet in certain species we 

 may feel sure that such a mechanism existed under 

 former conditions. Thus the Arctic Hare {Lepus 

 glacialis) usually remains white all the summer ; oc- 

 casionally, however, it becomes greyish, the change 

 of colour being limited to the points of the hair : 

 the young are born grey, but change to white at 

 their first winter (Welch). The latter change 

 appears to be independent of cold, for Sir J. Boss 

 speaks of a young hare turning white as early as 

 those running wild, although in a temperature not 

 much below freezing. This observation forms an in- 

 teresting contrast with the behaviour of species pos- 

 sessing an efficient nervous mechanism (the Hudson's 

 Bay Lemming and the American Hare) when shielded 

 from a low temperature. 



The white winter coat chiefly for concealment, but 

 may also help to retain heat 



Certain northern animals, especially those fre- 

 quenting trees, do not become white in winter : this 

 is true of the Glutton {Gulo luscus). Occasionally 

 dark winter individuals occur in species which as a 

 rule change their colour regularly : thus, a black 

 Arctic Fox is well known, but its rarity (Sir J. Eoss 

 found three individuals out of fifty white ones) prob- 



