8 BIOLOGICAL SOCIETY OF WASHINGTON. 



temperate oceanic regions are recognizable as a primary region, 

 characterized by a peculiar general fades of life, that more strongly 

 recalls that of the corresponding portions of the northern hemis- 

 phere than of any other portion of the earth. It has been further 

 shown that the Australian Realm is divisible into temperate and 

 tropical portions, and also that the land surface is separable into 

 zones of even still narrower limits, corresponding in a general way 

 with those recognized by Dana for marine life. 



"The almost total absence of identical genera, or even of families, 

 excepting such as are essentially cosmopolitan, in the American and 

 Old World tropics, as well as the distinctness of the Lemurian 

 Realm, and the almost total isolation of the Australian Realm, evi- 

 dently require for their explanation other causes than merely the 

 existing climates. The geological history of these land-areas and 

 their faunae must be of course considered in order to understand 

 their present relationships. As the northern hemisphere at present 

 most clearly shows,- nearly continuous land surface and similarity of 

 climatic conditions implies identity of fauna, while isolation, especi- 

 ally when joined with diverse climatic conditions, implies diversity 

 of life, and a differentiation proportionate to the degree of isolation, 

 and the length of time such isolation has existed ; in other words, 

 that the present want of affinity between the life of the Lemurian 

 and Australian Realms and that of the rest of the world is due 

 rather to their long geographical isolation than to present climatic 

 conditions, and that we here find, for reasons perhaps not wholly 

 apparent, the remnants of a somewhat primitive or early fauna that 

 was formerly shared more largely by other areas than at present — 

 that these regions became isolated before the development of many 

 of the higher and now prevalent types of the larger and more diversi- 

 fied land-areas, and that here differentiation has proceeded less 

 rapidly and along fewer and narrower lines than elsewhere ; further- 

 more, that the present highly diversified fauna of the chief tropical 

 areas, in comparison with the fauna of the north-circumpolar lands, 

 is due in part to the southward migration, near the close of the 

 Tertiary period, of forms adapted to a high temperature, and in 

 part to the high rate of differentiation favored by tropical condi- 

 tions of climate. Hence, given: i. Arctic and cold-temperate 

 conditions of climate, and we have a fauna only slightly or moder- 

 ately diversified ; 2. A moderate increase of temperature, giving 

 warm-temperate conditions of climate, and we have the addition of 



