373 



Islands and Mexico. .During another geological period , 

 let us suppose Aru to be elevated plains, and become 

 a mountainous country, and extended byalkivial plains, 

 while New Guinea was depressed, reduced in area, 

 and tlius many of its species perhaps extinguished. New 

 species might then be more rapidly introduced into the 

 modified and enlarged country; some groups, which 

 had been early extinct in the other, might thus be- 

 come very rich in species , and then we should have 

 an exact counterpart of what we see now in Mada- 

 gascar, where the families and some of the genera are 

 African, but where there are many extensive groups 

 of species forming peculiar genera, or even families, 

 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 sup- 

 pose that new species have ever been created // per- 

 fectlv dissimilar in forms, habits, and oro^anization" 

 from those which have preceded them ; neither do // een- 

 tres of creation," which have been advocated bv some, 

 appear either necessary 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 his- 

 tory, its elevations and subsidences, and all the chan- 

 ges it has undergone since it last rosé above the ocean. 

 This can very seldom be obtained ; but a knowledge 

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



