ALLEN ON GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 319 



Summary. 



Tctal number of genera 54 



Subcosmopolitan 5 



Circumpolar 27 



Shores of North America and Asia 5 



Shores of North America and Europe 5 



Exclusively either American or Europa3o- Asiatic 12 



Peculiar to America 8 



-Peculiar to the Europaeo- Asiatic continent 4 



The above-given statistics show most clearly that the mammals of 

 the northern third of the northern hemisphere present few generic or 

 subgeneric forms that are peculiar to either North America or to the 

 Europa30-Asiatic continent. In many cases, these are closely representa- 

 tive forms 5 in other cases, the peculiar genera extend but a short dis- 

 tance into the region, being temperate forms rather than hyperboreal. 



The close relationship of the mammalian life of the northern lands, as 

 compared with the diversity met with between that of the northern and 

 southern portions of the two northern continents, is further shown by a 

 tabulation of the genera met with in the region intervening between the 

 cold-temperate and sub-tropical zones of life, the northern and southern 

 boundaries of which may be considered respectively as the isotherms of 

 36 and 08 to 70 F. Rather more than one-half of the above-enu- 

 merated genera extend also over a large portion of this more southern 

 belt, and impart thereby a general similarity to the fades of the mam- 

 malian faunas of the two regions. In addition to these, however, we find 

 in North America thirty -one genera and seven subgenera that are not 

 found much, if any, to the northward of the isotherm of 36 F., and 

 about the same proportion of new generic and subgeneric types make 

 their appearance in the corresponding region of the Old World. Turn- 

 ing first to North America, we find that of these added forms one has 

 so wide a distribution that it may be properly considered as subcos- 

 mopolitan, being found in the corresponding region of the Europseo- 

 Asiatic continent as well as far to the southward of the region under 

 notice. One other occurs also in Eastern Asia and six more belong 

 rather to Tropical America than to Temperate North America. Exclud- 

 ing these, leaves about thirty as strictly American and twenty- two that 

 are almost wholly restricted to Temperate North America ; there is, hence, 

 twice as great a difference between the mammalian fauna? of the middle 

 temperate region of North America and the colder portion of the same 

 continent as there is between those of the colder parts of the two north- 

 era continents, or the northern portions of the so-called " Nearctic " 

 and "Palaaarctic Regions". But we get in Temperate North America 

 not only twenty-two generic and subgeneric forms peculiar to this 

 region, but a differentiation of this region into three well-marked faunal 

 areas, differing more from each other than do the boreal parts of the 

 New World (''Nearctic Region") from the boreal parts of the Old World 

 (" Palsearctic Region"). While thirteen of the genera, or about one- 



