Distribution of Primitive Types 63 



and the shy selvas (Coenolestes) of South America, 

 the marsupials are now confined to Australasia. It is 

 a reasonable hypothesis that they had established 

 themselves in part of Australia while that region was 

 connected with the continent of Asia by a great land- 

 bridge across the Java Sea, and that insulation oc- 

 curred before any of the placental mammals ap- 

 peared on the scene. Thus, through no virtue of their 

 own, but by a geological subsidence, many of the 

 marsupials were isolated in Australia, and thus saved 

 from a competition for which they were not equal. 

 They have evolved in Australia in many different 

 directions. Geography and evolutionism join hands. 



As everyone knows, there are two kinds of islands 

 in the sea. First, there are oceanic islands, which are 

 usually of volcanic origin. They have a peculiar 

 fauna and flora which can be accounted for, as Alfred 

 Russel Wallace showed, by fortuitous contributions 

 borne by water-currents, wind, and birds. Thus there 

 are no amphibians on oceanic islands, for it is very 

 unusual for an amphibian to be able to endure any 

 trace of salt. Secondly, there are continental islands, 

 which are isolated pieces of an adjacent continent, 

 and they always have a fauna and flora with a pre- 

 dominant resemblance to those of the mainland. The 

 f aunistic and floristic differences between oceanic and 

 continental islands are readily intelligible to the 

 evolutionist. 



Another interesting fact in regard to geographical 

 distribution is the widespread representation of some 

 of the primitive types. The lung-breathing fishes or 



