142 



GEOGRAPHICAL DISTRIBUTION 



of the sea which now separates island and main- 

 land, shallow seas dividing portions of land that 

 have only recently been disconnected, and deeper 

 seas separating those which have been longer 

 apart. 



The most remarkable case of isolation is pre- 

 sented by the Australian region, the fauna and 

 flora of which are the most peculiar in the world. 

 In the widest sense, this region includes not only 

 the vast island of Australia itself, but also New 

 Guinea and all the Malayan and Pacific islands 

 to the east of a deep channel between the islands 

 of Bali and Lombok a channel the significance 

 of which, as a boundary line for plants and 

 animals, was first pointed out by Wallace, the 

 great authority on animal distribution, and hence 

 known as Wallace's Line. The great feature of 

 this region (so far as animal distribution is con- 

 cerned ) is ' the almost total absence of all the 

 forms of mammalia which abound in the rest of 



the world, their place being taken by a great 

 variety of marsupials.' The family just mentioned, 

 though now restricted in the manner stated at the 

 beginning of this article, was at one time spread 

 over the whole world, but has in most parts 

 become extinguished by the competition of later 

 types ; thus presenting one of the best examples 

 of what are known as discontinuous areas of 

 distribution, and offering an illustration of the 

 mode in which such discontinuity is usually 

 brought about. The early severance of the 

 Australian region from the Asiatic continent (a 

 severance which must be referred to some period 

 in the Secondary Age of geologists) saved the 

 Australian marsupials from the competition which 

 almost extinguished the group elsewhere. 



Turning now to marine distribution, we find 

 evidence of the former absence of a land-barrier at 

 the Isthmus of Panama in the identity of many 

 species of fish on both sides of the isthmus. 



Sub-regions of Palsearctic Region 



1. European. 



2. Mediterranean. 



3. Siberian. 



4. Manehurian. 

 Sub-regions of Ethiopian Region 



1. East African. 



2. West ii 



3. South ii 



4. Malagasy. 



The Zoogeographies 1 Regions according to A. R. "Wallace : 



Sub-regions of Oriental Region 



1. Indian. 



2. Ceylonese. 



3. Indo-Chinese. 



4. Indo-Malayan. 

 Sub-regions of Australian Region 



1. Austro-Malayan. 



2. Australian. 



3. Polynesian. 



4. New Zealand. 



Sub- regions of Neotropical Region- 



1. Chilian. 



2. Brazilian. 



3. Mexican. 



4. Antillean. 

 Sub-regions of Nearctic Region 



1. Californian. 



2. Rocky Mountain. 



3. Alleghanian. 



4. Canadian. 



Changes in the climatic barrier have also had an 

 important influence on geographical distribution ; 

 and it is by such changes, combined with changes 

 in the continuity of land in the north polar regions, 

 that the affinities between the floras of Japan and 

 eastern North America must be explained. When 

 these affinities were first pointed out by Asa Gray, 

 that distinguished botanist divined the true ex- 

 planation viz. that in former geological epochs a 

 genial climate must have prevailed even within the 

 polar circle, so as to allow of the existence of a 

 remarkably uniform flora, suitable to such a 

 climate, all round the pole in very high latitudes ; 

 and that as the climate became colder in the north 

 this flora was driven southwards, and became 

 differentiated according to the differences of climate 

 in the more southerly latitudes to which it 

 advanced. Hence the eastern parts of America 

 and Asia, as they correspond pretty much in 



climate, came to correspond also more closely than 

 other tracts in the same latitude in the character 

 of their floras. The soundness of this surmise was 

 afterwards confirmed by the discovery of abundant 

 plant remains of the Miocene age, indicating a 

 warm climate in Greenland, Spitzbergen, and else- 

 where. The effects on distribution of the changes 

 of climate belonging to the period known as the 

 Glacial Period (q.v.) or Ice Age must be alluded 

 to here, but there is no space to do more. 



As the result of all the processes of dispersal 

 across the various barriers to migration, and of the 

 changes in these barriers, we have the present dis- 

 tribution of plants and animals, which is such as 

 to enable us to divide the terrestrial surface of the 

 globe into more or less well-marked regions. For 

 animals the regions adopted by Wallace are 

 nearly the same as those first suggested by 

 Sclater as applicable to the distribution of birds ; 



