ALLEN ON GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 321 



Between the warm-temperate belt we have been considering and the 

 zone next to the southward the subtropical the faunal differences are 

 far greater than between the warm-temperate and colder zones. Aside 

 from the few subcosmopolitan genera still present, and the few essen- 

 tially tropical genera that range northward into the warmer temperate 

 zone, there is little in common to the mammalian faunae of these two 

 regions. At or near this boundary (the isotherm of about 68 F. say 

 68 to 70 F.) several strictly tropical families first make their appear- 

 ance, and tropical genera begin largely to replace those of the colder 

 region to the northward. 



In respect to the Europaeo- Asiatic continent, we have already seen 

 how small a proportion of the genera of mammals met with north of 

 the thirty-sixth isotherm are really peculiar to this region, the number 

 being less than twelve per cent., the remainder being circumpolar. Pass- 

 ing, however, to the warm-temperate division of this Europa30- Asiatic 

 continent, or that portion between the isotherms of 36 and 68 to 70 

 F., and we meet with many genera not found to the northward. While 

 many circumpolar genera still prevail, at least three-fourths of the 

 whole number are here first met with. A considerable proportion (about 

 one-fifth) are properly southern or subtropical, and extend far to the 

 southward of the warm-temperate zone. About one-half, however, are 

 peculiar to this zone, and belong to groups (families of subfamilies) espe- 

 cially characteristic of the North Temperate Kealm. In adopting the 

 isotherm of 70 F. as its southern boundary, we include not only the Medi- 

 terranean Province (and hence Northern Africa), but all of Asia north 

 of the great Himalayan cfaain, together with Northern China and the* 

 Persian -Peninsula. Hence quite a number of such southern forms occur 

 as Macacus, Herpestes, Genetta, Hycena, Hystrix, etc., that are more prop- 

 erly members of the intertropical fauna. Owing to the great extent 

 of this region, we meet with many genera peculiar to special districts, 

 giving a higher proportion of peculiar forms than is met with in the 

 corresponding portion (but far more limited in area) of North America. 

 Of about fifty genera met with here that do not occur to the northward, 

 about one-fourth may be thrown out as more properly tropical, since 

 they in most cases barely enter the southern border. 



Of the remainder, fully one-half are restricted in their range wholly 

 or almost wholly to this region, the rest extending far into or through- 

 out the Old World tropics. There is thus more than thrice as great a 

 difference between the mammalian fauna of the boreal parts of the 

 Europseo- Asiatic continent and that of the warmer parts of the same con- 

 tinent as between the fauna of the boreal parts of the Europseo- Asiatic 

 continent and the corresponding region of North America. The differ- 

 entiation is here again, as in North America, from the north southward, 

 not through the rapid increase of land-area and diversity of physical 

 structure, but purely from climatic conditions, through the multipli- 

 cation of life in consequence of increase of temperature and means of 



