26 Darwinism and Other Essays. 



elongation of its fingers and the growth of a web 

 like skin between them. 



If we consider the most general features of the 

 geographical distribution and geological succes 

 sion of organisms, we find the evidence hardly 

 less complete and convincing. Generally speak 

 ing, the contemporary species found in any geo 

 graphical area most closely resemble the species 

 that inhabited the same area in former ages. 

 Thus in the Miocene age Australia abounded in 

 marsupials, and marsupials specifically different, 

 though nearly allied to these, make up to-day the 

 greater part of the mammalian fauna of Austra 

 lia. There is no imaginable reason why this 

 should be so, unless the contemporary marsupials 

 are descended from the earlier forms. It cannot 

 be urged that marsupials are better adapted to 

 the conditions of life in Australia than placental 

 mammals ; for the placental mammals lately in 

 troduced there are already beginning to supplant 

 and exterminate the marsupials. The only possi 

 ble explanation is that, whereas marsupials once 

 covered the terrestrial globe, and have been sup 

 planted by better adapted forms in the Old 

 World and (with the exception of the opossum) 

 in America, on the other hand the isolation of 

 Australia has allowed them there to go on repro- 



