VARIABLE RESEMBLANCE IN VERTEBKATA, ETC. 97 



colouring process and in the thickening of the fur. 

 There is no indication of shedding : an increase in 

 length ensues over the whole body.' 



There is considerable individual difference in the 

 time of change : it sometimes commences before the 

 first fall of snow, indicating that the stimulus is the fall 

 of temperature affecting the skin rather than the 

 colour affecting the eyes. Great differences are seen 

 when the same species is followed into other localities. 

 ' On the seaboard it (the winter change) is postponed 

 in comparison with inland districts in the same lati- 

 tudes.' In Hudson's Bay Territory it changes early 

 and carries the winter coat till June, while no change 

 of colour takes place in the winter in the southern 

 parts of the United States. An individual kept in a 

 warm barn at St. John's, New Brunswick, retained the 

 summer colours. 



The consideration of the Hudson's Bay Lemming 

 and the American Hare lead to the conclusion that all 

 species in which the northern change does not occur 

 in the southern individuals, possess the power of Vari- 

 able Eesemblance. It is possible that the change is 

 merely contemporaneous when it occurs uniformly in 

 all individuals of the species, and it is at any rate 

 probable that it would soon become so, because the 

 extreme complexity of the mechanism by which Vari- 

 able Resemblance is brought about would need the 

 constant operation of natural selection to keep it in 



