of the Aru Islands. 483 



India Islands and Mexico. During another geological period, 

 let us suppose Aru to be elevated, and become a mountainous 

 country, and extended by alluvial plains, while New Guinea was 

 depressed, reduced in area, and thus many of its species perha])s 

 extinguished. New species might then be more rapidly intro- 

 duced into the modified and enlarged country; some groups, 

 which had been early extinct in the other, might thus become 

 very rich in species, and then we should have an exact counter- 

 part of what we see now in Madagascar, where the families and 

 some of the genera are African, but where there are many ex- 

 tensive groups of species forming peculiar genera, or even fami- 

 lies, but still with a general resemblance to African forms. In 

 this manner, it is believed, we may account for the facts of the 

 present distribution of animals, without supposing any changes 

 but what we know have been constantly going on. It is quite 

 unnecessary to suppose that new species have ever been created 

 " perfectly dissimilar in forms, habits, and organization^^ from 

 those which have preceded them ; neither do " centres of crea- 

 tion," which have been advocated by some, appear either neces- 

 sary or accordant with facts, unless we suppose a " centre" in 

 every island and in every district which possesses a peculiar 

 species. 



It is evident that, for the complete elucidation of the present 

 state of the fauna of each island and each country, we require a 

 knowledge of its geological history, its elevations and subsi- 

 dences, and all the changes it has undergone since it last rose 

 ^bove the ocean. This can very seldom be obtained; but a 

 knowledge of the fauna and its relation to that of the neighbour- 

 ing countries will often throw great light upon the geology, and 

 enable us to trace out with tolerable certainty its past history. 

 A consideration of the birds of Aru has led us at some length 

 into this subject, both on account of the interest attached to it, 

 and because we are not aware of any attempt to explain in detail 

 how the existing distribution of species has arisen, or strictly to 

 connect it with those changes of surface which all countries have 

 undergone. The Birds and Mammalia only have been used for 

 illustration, because they are much better known than any other 

 groups. The Insects, however, of which I have made a very ex- 

 tensive collection, furnish exactly similar results, and were these, 

 particularly the Coleoptera, well known, they would perhaps be 

 preferable to any group for such an inquiry, from the great 

 number of their genera and species, and the very limited range 

 which many of them attain. In imperfectly explored countries, 

 however, Birds are almost always better known than any other 

 group, as a larger proportion of the whole number of species may 

 be obtained in a limited time. I think it probable that 1 have 



