328 



THE EFFECTS OF CONDITIONS 



first three are the widest ranging species. Allen 

 found that, in respect of skull measurement, the com- 

 mon wolf is fully a fifth larger in the northern parts of 

 British America and Alaska than it is in northern 

 Mexico, the southern limit of its habitat, whilst, as we 

 see in the accompanying table, specimens from inter- 

 mediate regions show a gradual intergradation between 

 these extremes.* The common fox from Alaska is 

 about 10 per cent, larger than that in 'New England, 



whilst the gray fox probably varies considerably more 

 in size with locality, but the number of skulls obtained 

 for measurement (15 in all) is insufficient to warrant 

 any generalisation in its case. 



This increase in size on passing from South to North 

 is not universal, however. Thus lynxes and wild cats, 

 though series of skulls were obtained from such widely 

 separated localities as Alaska, California, and Northern 

 Mexico, revealed no appreciable variation of size with 



* U. S. Geol. and Geographical Survey, vol. ii. p. 309, 1876. 



