VAEIABLE KESEMBLANCE IN VERTEBEATA, ETC. 101 



of the nervous system which presides over the nu- 

 tritive and chemical changes involved in the growth 

 of hair and the appearance of the bubbles. 



Probable variation in susceptibility to stimulus of 

 cold in different districts 



In the northern part of an animal's range, natural 

 selection would favour great delicacy in the adjustment 

 of the mechanism by which such changes are produced, 

 so that the winter coat would be ready in time to 

 harmonise with the mantle of snow. Conversely, 

 extreme delicacy would be a disadvantage in the 

 southern part of the range, if the climate were such 

 that the snow did not lie on the ground for any great 

 part of the winter. There is abundant evidence of 

 variations in the delicacy of adjustment, upon which 

 natural selection could operate. 



Mr. F. E. Beddard has directed my attention to 

 three Arctic Foxes (Cam's lagopus) from Iceland, which 

 have been in the Zoological Gardens since 1887. 

 One l of these turns perfectly white every winter, while 

 the other two remain dark. 



The stoat always becomes white in the alpine 

 districts of Scotland,' frequently in the north of Eng- 



1 When I examined this fox on October 14, 1889, the change in 

 colour was nearly complete : there was, however, a grey patch of hair 

 on the back which was certainly moulting. It is possible, therefore, 

 that the change is effected in an entirely different manner in this 

 species. 



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