42 GENERAL PRINCIPLES OF ZOOLOGY. 



these two changes the changes in the earth's surface and in the 

 animal world established upon it have gone on hand in hand 

 -there follows necessarily the consequence that greater differences 

 in the faunal character of two lands must result the longer they 

 have developed independently of one another, without interchange 

 of their animal populations, and the longer the inhabitants have 

 been separated by impassable barriers. For the various groups 

 the character of the barriers is different ; terrestrial animals, which 

 cannot fly, are hindered in their distribution by arms of the sea; 

 marine forms by land barriers; for terrestrial molluscs high moun- 

 tain ranges, which are dry and barren, or constantly snow-capped, 

 are effectual. 



Instances of Proofs. Since attention has been called to these 

 conditions, many geographical facts favorable to the theory of 

 evolution have been ascertained: (1) Of the various continents 

 Australia has faunally an independent character; when discovered 

 it contained almost none of the higher (placental) mammals., 

 except such as can fly (Chiroptera), or marine forms (Cetacea), or 

 .such as are easily transported by floating wood (small rodents), or 

 .such as could be introduced by man (dingo, the Australian dog); 

 instead, it had remarkable birdlike animals (with beak and cloaca), 

 .and the marsupials, which have become extinct in the Old World 

 and the opossums excepted, in America as well. 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 its connexion 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, by way of 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 

 i'aunal character was preserved. (2) As Wallace has shown, the 

 Malay Archipelago is divided faunally into an eastern and a western 

 half; within each group there are islands which, in spite of a 

 different climate, have a very similar fauna. On the other hand, 

 the faunal boundary (' Wallace's line') 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 connexion with the western, Lombok with the eastern 

 chain of islands. More recent studies make it probable that there 



