THE DISTRIBUTION OF SPECIES. 



I 99 



mon with that of Africa, and it is, moreover, surrounded 

 by transition districts which lead on the north to the 



holarctic and on the west to the Ethiop- 

 Indian realm. . . , T ,. . . 



ian. On the east the Indian realm is 



lost in the islands of Polynesia, which represents each 

 one its own degree of transition and isolation. 



The Australian realm of Australia and its islands is 

 more isolated than any of the others. It shows a sin- 

 gular development of low or primitive 

 types of vertebrate life, as though in the 

 progress of evolution this continent had 

 been left a whole geological age behind the others. It 

 is certain that, could the closely competing fauna of the 

 holarctic or Indian realms have been able to invade 

 Australia, the dominant mammals and birds of that re- 

 gion would not have been marsupials and parrots. Un- 

 specialized types abound only where barriers have pre- 

 vented competition. The larger the land area the greater 

 the competition and the more specialized its character- 

 istic forms. As part of this specialization is in the di- 

 rection of hardiness, the species of the large experience 

 are the more persistent and less easy of extermination. 

 The rapid multiplication which certain holarctic animals 

 and plants have shown when transported to the Aus- 

 tralian realm, demonstrates what might have taken place 

 if impassable barriers had not previously shut them out. 



Each of these great realms may be indefinitely sub- 

 divided into provinces and sections, for there is no end 

 to the possibility of analysis. No township or school 

 district has exactly the same animals or plants as any 

 other ; and, finally, in ultimate analysis, no two animals 

 or plants are alike. Modification comes with the growth 

 of each new individual, and steadily increases with the 

 individual's separation in time or space from the parent 

 stock. Moreover, we observe apparent anomalies of 



