160 MEANING OF THE SYSTEM. 



order to find an historical explanation of the known facts of distribu- 

 tion. Although in this sense the science of animal geography is still 

 in its infancy, yet numerous and important phenomena of geographical 

 distribution receive a satisfactory explanation according to the theory 

 of transmutation of species on the supposition of migrations and 

 gradual changes brought about by natural selection. 



It is a most important fact that neither the resemblance nor the 

 want of resemblance of the animals inhabiting different localities 

 can be completely explained as the result of climatic and physical 

 conditions. Closely allied species of plants and animals often appear 

 under very different natural conditions, while a completely different 

 fauna and flora can exist in a similar climate and on a similar soil. 

 On the other hand, the extent of the difference between two fauna 

 is closely connected with the limitations of space and the barriers 

 and hindrances to free migration. The Old and New Worlds, which, 

 leaving out of consideration the polar connection, are completely 

 separated, have in part a very different fauna and flora, although 

 with regard to the climatic and physical conditions of existence there 

 are innumerable parallels which would equally favour the prosperity 

 of the same species. 



In particular if we compare the districts of South America with 

 regions situated in the same latitude and possessing the same climate 

 in South Africa and Australia, we find three fauna and flora which 

 differ considerably, while the natural productions from different 

 latitudes of South America with entirely different climates are 

 closely allied. Here the northern animals are indeed specifically 

 different from the southern, but belong to similar or nearly allied 

 genera with the peculiar stamp characteristic of South America. 



Zoological Provinces. The surface of the earth can be divided 

 into from six to eight regions according to the general features of 

 the terrestrial and fresh-water fauna. These regions can indeed only 

 be considered as a relative expression for large natural districts of 

 distribution, since they cannot be applied to all groups of animals 

 in the same manner, and it is impossible that they should differ in 

 like degree and in the same direction. There must also be inter- 

 mediate regions combining the characteristics of the neighbouring 

 regions with peculiarities of their own ; and the question must arise 

 whether these should not be taken as independent regions. 



The merit of having first established a natural division of the 

 earth into zoological regions and sub-regions belongs to Sclater. This 

 naturalist founded his system on the distribution of birds, and dis- 



