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 Canidz (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 Felidz, 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 (Fe/is concolor), 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 
