306 Mr. A. R. Wallace on 



to account for the phenomena of distribution as he finds them 

 to exist. 



Zoological divisions of the earth's surface can only be true 

 and useful ones, in so far as they agree with the most ancient 

 and permanent natural barriers to the diffusion of species. 

 Those divisions marked out by diversity of physical character or 

 of climate are less important, because they are necessarily less 

 defined, less general in their effects, and have most likely fluc- 

 tuated much during short geological epochs. From some points 

 of view the productions of the pampas of Buenos Ayres may 

 differ more from those of the Brazilian forests than do these latter 

 from those of the forests of tropical Africa, yet all will admit that 

 the latter is a primary, while the former is a secondary zoological 

 division. However large the number of groups which can be 

 adduced as common to the equatoreal regions of Africa and 

 America, but which do not extend to the open plains of tempe- 

 rate South America, we should none of us admit the propriety of 

 forming for those groups a new division of the earth, in which 

 the tropics of America and Africa should form a single region. 

 All we can do is to endeavour to account for the anomaly, and try 

 to discover what special causes have affected the distribution of 

 these particular groups in a manner very different from that of 

 almost all the other productions of nature. The case we are con- 

 sidering is one almost exactly analogous to this. The distribu- 

 tion of mammals, of birds, of reptiles, and many facts in the dis- 

 tribution of insects, clearly indicate that the mass of land forming 

 Australia is but a portion of one of the primary zoological divi- 

 sions of the globe, although in it some of the peculiarities of that 

 division reach their maximum, while others are better exhibited 

 in New Guinea and the Moluccas. The peculiar climate. and 

 physical condition of Australia, however, have led to the develop- 

 ment of many groups peculiar to it, while the same causes have 

 prevented others from becoming established in it, wliicli have 

 spread rapidly in the damp wooded tropical islands to the north. 

 This localization of forms is carried to a greater extent in insects 

 than in any of the higher animals ; and so many extensive and 

 remarkable groups abound in Australia, and are to be seen in 

 every collection from that country and from no other, that the 

 Entomologist may easily be led to ignore the equally striking facts 

 that unite it to the surrounding islands by a strong band of 

 zoological affinity. 



The numerical proportion of species in islands and groups of 



