308 Evolution and Adaptation 



others to Old World species ; and in these the largest indi- 

 viduals of the species of a genus are northern. Conversely, 

 in the exceptional cases of increase in size toward the south, 

 it can be shown that the forms have probably had a southern 

 origin. 



The Canidse (wolves and foxes) have their largest repre- 

 sentatives, the world over, in the north. " In North America 

 the family is represented by six species, the smallest of which 

 (speaking generally) are southern and the largest northern." 

 The three species that have the widest ranges (the gray 

 wolf, the common fox, and the gray fox) show the most 

 marked differences in size. The skull, for instance, of " the 

 common wolf is fully one-fifth larger in the northern parts 

 of British America and Alaska than it is in northern 

 Mexico, where it finds the southern limit of its habitat. 

 Between the largest northern skull and the largest southern 

 skull there is a difference of about thirty-five per cent of 

 the mean size. Specimens from the intermediate region 

 show a gradual intergradation between the extremes, although 

 many of the examples from the upper Missouri country are 

 nearly as large as those from the extreme north." The 

 common fox is about one-tenth larger, on the average, in 

 Alaska than it is in New England. The gray fox, whose 

 habitat extends from Pennsylvania southward to Yucatan, 

 has an average length of skull of about five inches in the 

 north, and less than four in Central America— about ten 

 per cent difference. 



The Felidae, or cats, " reach their greatest development as 

 respects both the number and the size of the species in the 

 intertropical regions. This family has sent a single typical 

 representative, the panther (Felis concotor), north of Mexico, 

 and this ranges only to about the northern boundary of the 

 United States. The other North American representatives 

 of the family are the lynxes, which in some of their varieties 

 range from Alaska to Mexico." Although they vary greatly 



