ON VARIATION. 49 



is confined to high latitudes. The three widest-rang- 

 ing species (the gray wolf, the common fox, and the 

 gray fox) are those which present the most marked 

 variation in size. Taking the skull as the basis of 

 comparison, it is found that the common wolf is fully 

 one-fifth larger in the northern parts of British Amer- 

 ica 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 these extremes, 

 although many of the examples from the upper Mis- 

 souri country are nearly as large as those from the ex- 

 treme North. 



" The common fox, though occurring as far north 

 as the wolf, is much more restricted in its southward 

 range, especially along the Atlantic coast, and presents 

 a correspondingly smaller amount of variation in size. 

 The Alaskan animal, however, averages about one- 

 tenth larger than the average size of specimens from 

 New England. In the gray fox, whose habitat ex- 

 tends from Pennsylvania southward to Yucatan, the 

 average length of the skull decreases from about five 

 inches in Pennsylvania to considerably less than four 

 in Central America — a difference equal to about thirty 

 per cent, of the mean size for the species. 



"The Felidae, unlike the Canidae, reach their great- 

 est development, as respects both the number and 

 the size of the species, in the intertropical regions. 

 This family has but a single typical representative — 

 the panther (^Felis concolor)- — north of Mexico, and this 

 ranges only to about the northern boundary of the 

 United States. The other North American represen- 



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