HISTORY OF ZOOLOGY. 51 



instead, the remarkable bircllike animals (having a beak 

 and a cloaca), and the Marsupials, which in the Old World, 

 as also in America (with the exception of the Opossums], 

 have become quite extinct. The phenomenon is explained 

 by the geological fact that in the earth's history Australia, 

 with its surrounding islands, was certainly the earliest to 

 lose all its connection with the other continents. While 

 in the other four parts of the earth the higher vertebrates, 

 which were developed from the marsupials and their lower 

 contemporaries, came, through the lands connecting the 

 various continents, to have a wide or even a cosmopolitan 

 distribution, in isolated Australia this process of evolution 

 did not go on, and its ancient faunal character was preserved. 

 (2) As Wallace has shown, the Malay Archipelago is divided 

 faunally into an eastern and a w r estern half ; within each group 

 there are islands which, in spite of a different sort of cli- 

 mate, have a very similar fauna. On the other hand, the 

 faunal boundary passes between the two islands Bali and 

 Lombok, which have the same climate and geographically 

 are very close together. But the depth of the strait in this 

 region shows that here runs a boundary of extraordinarily 

 long geological duration, and that in the earth's history 

 Bali has developed in connection with the western, Lom- 

 bok with the eastern chain of islands. (3) Even a long 

 time before Darwin, the renowned geologist Leopold von 

 Buch, from studying the distribution of plants on the 

 Canary Islands, had come to a conclusion in regard to the 

 change of species into new species; viz., on islands peculiar 

 species develop in secluded valleys, because high mountain- 

 chains isolate plants more effectually than do wide areas of 

 water. M. Wagner has collected many instances which 

 prove that localities inhabited by certain species of beetles 

 and snails have been sharply divided by wide rivers or 

 by mountain chains, while in neighboring regions so-called 

 " vicarious species " are found. 



Causal Foundation of the Theory of Descent. The 

 Darwinian theory, so far as the above exposition shows, is 

 fundamentally like the theories of descent advocated at the 



