CHAP. XIII. ] THE AUSTRALIAN REGION. 403 
that undoubtedly occur, are probably due to three distinct 
causes. 
First, we have the American Didelphyide, among Mammals, 
and the Cracid, among birds, allied respectively to the Marsu- 
pials and the Megapodiid# 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- 
glossum 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 
Osteoglosswm 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 number of forms inhabit the Austro-Malay sub-region 
