ALLEN ON GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 375 



period, of forms adapted to a high temperature, and in part to the high 

 rate of differentiation favored by tropical conditions of climate. Hence, 

 given : 1. Arctic and cold-temperate conditions of climate, and we have 

 a fauna only slightly or moderately diversified ; 2. A moderate increase 

 of temperature, giving warm -temperate conditions of climate, and we 

 have the addition of many new types of life; 3. A high increase of 

 temperature, giving tropical conditions of climate, and we have a rapid 

 multiplication of new forms and a maximum of differentiation. Again, 

 given : 1. A long-continued continuity of laud surface, and we have 

 an essential identity of fauna ; 2. A divergence and partial isolation of 

 land-areas, and we find a moderate but decided differentiation of faunaB; 

 3. A total isolation of land-areas, and we have a thorough and radical 

 differentiation of faunae, proportioned to the length of time the isola- 

 tion has continued. Hence, the present diversity of life is correlated 

 with two fundamental conditions : 1. Continuity or isolation, past as 

 well as present,* of land surface; and, 2. Climatic conditions, as deter- 

 mined mainly by temperature.* 



In accordance with these principles, which rest on incontrovertible 

 facts of distribution, it follows that the nearly united lands of the North 

 present a continuous, almost homogeneous, arctopolitan fauna ; that 

 farther southward, in the warmer temperate latitudes, we begin to find 

 a marked differentiation on the two continents ; that this differentiation 

 is still further developed in the tropical continuations of these same 

 land-areas, till an almost total want of resemblance is reached, except 

 that there is what may be termed, in contrast with the more northern 

 regions, a " tropical fades 77 common to the two. The small amount of 

 land surface belonging to these primary land regions south of the trop- 

 ics have no more in common (a few marine species excepted) than have 

 these two tropical areas, but it is hardly possible for them to have much 

 less. The Antarctic (mainly oceanic) region has a fauna strongly recall- 

 ing the marine fauna of the Arctic, but has no resemblance to that of 

 the intervening area. 



The northern circumpolar lands may be looked upon as the base or 

 centre from which have spread all the more recently developed forms of 

 mammalian life, as it is still the bond that unites the whole. Of the 

 few cosmopolitan types that in a manner bind together and connect the 

 whole mammalian fauna of the globe (the Lemurian and Australian 

 Bealms in part excepted), nearly all have either their true home or be- 

 long to groups that are mainly developed in the northern lands. A few 



* In illustration of the above, it may be added that the circumpolar lands north of 

 the mean annual of 36 F., or, in general terms, north of the fiftieth parallel, with ap- 

 proximately an area of about 12,500,000 square miles, have representatives of about 

 fifty-four genera of mammals ; Tropical America, with an approximate area of about 

 5,000,000 square miles, has about ninety genera ; the Indo- African Realm, with an 

 approximate area of about 15,000,000 square miles, has about two hundred and fifty 

 genera. Hence the tropical lands are four to five times richer in genera, iu proportion 

 to area, than those of the Cold-temperate and Arctic regions. 



