374 BULLETIN UNITED STATES GEOLOGICAL SURVEY.. 



thirdly, it has been shown, so far as the northern hemisphere is con- 

 cerned, that the life of the tropical and temperate regions of the same 

 continent is more widely different than is the life of corresponding por- 

 tions of the temperate and colder parts of the (so-called) Old World and 

 the New ; fourthly, that the life of Tropical America has very little in 

 common with that of the tropical portions of Asia and Africa ; fifthly, 

 that the life of the South Temperate Zone presents a, fades distinct from 

 that of the tropics, and has still less in common with that of the North 

 Temperate Zone ; sixthly, that Australasia is so highly differentiated 

 as to form a distinct primary region, having little in common with other 

 lauds, even with those of contiguous regions, or those having a similar 

 geographical position ; seventhly, that Madagascar and its contiguous 

 islands, while to some extent African in affinity, form also a highly 

 specialized region ; lastly, that the antarctic and cold south-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 hemisphere 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, corre- 

 sponding in a general way with those recognized by Dana for marine life. 

 The almost total absence of identical genera, or even of families, ex- 

 cepting 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 Eealm, evidently 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, especially when joined with diverse 

 climatic conditions, implies diversity of life, and a differentiation propor- 

 tionate 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 

 diversified 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 jto the southward migration, near the close of the Tertiary 



