CHAP. XIII.] THE AUSTRALIAN EEGION. 403 



that undoubtedly occur, are probably due to three distinct 

 causes. 



First, we have the American Didelphyidte, among Mammals, 

 and the Cracida?, among birds, allied respectively to the Marsu- 

 pials and the Megapodiida; of Australia. This is probably more 

 a coincidence than an affinity, due to the preservation of ancient 

 wide-spread types in two remote areas, each cut off from the 

 great northern continental masses, in which higher forms were 

 evolved leading to the extinction of the lower types. In each 

 of these southern isolated lands the original type would undergo 

 a special development ; in the one case suited to an arboreal 

 existence, in the other to a life among arid plains. 



The second case is that of the tree-frogs, and the genus Ostco- 

 (jlossum among fishes; and is most likely due to the extension 

 and approximation of the two southern continents, and the exist- 

 ence of some intermediate lands, during a warm period when 

 facilities would be afforded for the transmission of a few or- 

 ganisms by the causes which have led to the exceptional diffusion 

 of fresh- water productions in all parts of the world. As however 

 Osteoglossum occurs also in the Sunda Islands, this may be a case 

 of survival of a once wide-spread group. 



The third case is that of the same genera and even species of 

 fish, and perhaps of frogs, in the two countries ; which may 

 be due to transmission from island to island by the aid of float- 

 ing ice, with or without the assistance of more intervening 

 lands than now exist. 



Having arrived at these conclusions from a consideration of 

 the vertebrata, we shall be in a position to examine how far 

 the same causes will explain, or agree with, the distribution of the 

 invertebrate groups, or elucidate any special difficulties we may 

 meet with in the relations of the sub-regions. 



Insects. 



The insects of the Australian region are as varied, and in 

 some respects as peculiar as its higher forms of life. As we 

 have already indicated in our sketch of the Oriental region, a 

 vast nimiber of forms inhabit the Austro-Malay sub-region 



D D 2 



