ALLEN OX GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 325 



Summary. 



Total number 50 



Peculiar (or almost wholly restricted) to the region 22 



Subcosmopolitan 13 



Represented in the American tropics (only) 4 



Occurring in the Old World north of the tropics 23 



Tropical 29 



It thus appears that only about three-fifths as many families of mam- 

 mals occur in the intertropical parts of the New World as in the cor- 

 responding parts of the Old World. The disproportion in the same 

 direction in respect to genera and species is still greater. This is 

 obviously due to the difference in size and configuration of the two 

 areas. The Old World intertropical land- surface is not only several 

 times greater than the American (embracing thrice as great a breadth 

 longitudinally), but is differentiated into one continental (Africa), two 

 large peninsular (India and China) areas, and a group of large, highly 

 differentiated islands (Malay Archipelago), while the intertropical re- 

 gion of America forms a single unindented region, with a single narrow 

 isthmic prolongation. In the one case (America) we have a striking 

 uniformity of mammalian life throughout, corresponding with the gen- 

 eral uniformity of the climatic conditions characteristic of this area, 

 contrasting with well-marked subdivisions in the other, and a much 

 greater diversity of environing circumstances, originating geologically 

 far back in the history of these several land-masses. As Mr. W r allace 

 has remarked, " To those who accept the theory .of development as 

 worked out by Mr. Darwin, and the views as to the general permanence 

 and immense antiquity of the great continents and oceans so ably de- 

 veloped by Sir Charles Lyell, it ceases to be a matter of surprise that 

 the tropics of Africa, Asia, and America should differ in their produc- 

 tions, but rather that they should have anything in common. Their 

 similarity, not their diversity, is the fact that most frequently puzzles 

 us."* 



In the foregoing remarks, no reference has been made to Madagascar 

 or to Australia, for the reason that they belong to distinct primary life- 

 regions having little in common with the great Europaeo- Asiatic land- 

 area (of which Africa, on the other hand, is an inseparable appendage), 

 which, with America, form the regions to which the discussion has thus 

 far been intentionally limited. As will be more fully considered later, 

 the intertropical Old World area is divisible into secondary regions, 

 which for the present need not enter into the questions immediately at 

 issue. These are, first, Does that portion of the northern hemisphere 

 north of the northern subtropical zone admit of division into two pri- 

 mary life-regions, conforming in their boundaries to the configuration of 

 the two great northern land-areas ? And, secondly, lu accordance with 

 what principle does the life of the northern hemisphere become differ- 

 entiated from the homogeneity characteristic of the northern regions 



* Geogr. Dist. Anirn., vol. i, p. 51. 



