372 



be so from the other also, for however mucli they 

 might be adapted to the country, the law of close 

 affinity would not allow of their appearance, except 

 by a long succession of steps occupying an immense 

 geological interval. The species which at the time of 

 separation were found only in one country, would, by 

 the gradual introduction of species allied to them, give 

 rise to groups peculiar to that country. This separation of 

 New Guinea from Australia no doubt took place while Aru 

 yet formed part of the former island. lts separation must 

 have occurred at a very recent period , the number of spe- 

 cies common to the two showing that scarcely any extinc- 

 tions have since taken place, and probably as few 

 introductions of new species. 



If we now suppose the Aru Islands to remain un- 

 disturbed during a period equal to about one divisi- 

 on of the Tertiary epoch of geologists, we have reason 

 to believe that the change of species of Vertebrata will 

 beconie complete, an entirely new race having gradually 

 been introduced , but all more or less closely allied to 

 those now existing. During the same period a new 

 fauna will also have arisen in New Guinea, and then 

 the two will present the same comparative features that 

 North Australia and New Guinea do now. Let the 

 process of gradual change still go on for another period 

 regulated by the the same laws. Sorae species will 

 then have become exinct in the one country, and unre- 

 placed, while in the other a numerous series of mo~ 

 dified species may have been introduced. Then the 

 faunas will come to difFer not in species only, but in 

 generic groups. Then would be then the resemblance 

 between them that there is between the West India 



