iv THE AUSTRALIAN FAUNA 107 



tioned, that the American fauna, especially in regard to 

 markings, has remained at an earlier stage of evolution ; 

 thus the differentiation has been partly accomplished by 

 genepistasis. 



Species representing one another in this way (vicarious 

 species) exist also, as is well known, in abundance in the 

 faunas of various parts of the world ; only both the separation 

 and the relationship are not as a rule so clearly expressed as 

 they are between the European and American forms. 



The most striking example, however, of the importance of 

 climate, character of the soil, and isolation in the moulding 

 of the organic world, is supplied by the very peculiar fauna 

 and flora of Australia. Australia affords us an enclosed area 

 of evolution in nature on a large scale than which no better 

 could be wished. Marsupial animals represent there the 

 most diverse groups of our mammals in a series of parallel 

 forms. There are marsupials which are proxies for our 

 Rodents, others for our Euminants, and so forth, and these 

 have a dentition similar to that of the Rodents and Ruminants 

 respectively ; nay more, the representatives of the Ruminants 

 have a multiple stomach. 



Darwinism of course says this similarity has arisen 

 through selection based on the same requirements : since in 

 Australia no other animals except marsupials have developed 

 out of the lower forms, therefore the marsupials have adapted 

 themselves to the diverse possibilities of the country. In 

 other countries, however, the evolution of mammals proceeded 

 in various directions from the original marsupials, and so 

 arose the diversity of the mammalian kingdom. But, without 

 doubt, the direct influence of the uniformity of the Australian 

 area ought to be held responsible for the uniformity of the 

 animals and plants belonging to it, and the fact that the mar- 

 supials have assumed in their organisation similar relationships 

 to those of the other mammalian groups outside Australia is 



