Chap. XI. SINGLE CENTRES OF CREATION. 353 



nearly the same, so that a multitude of European animals 

 and plants have become naturalised in America and 

 Australia ; and some of the aboriginal plants are identi- 

 cally the same at these distant points of the northern 

 and southern hemispheres ? The answer, as I believe, 

 is, that mammals have not been able to migrate, whereas 

 some plants, from their varied means of dispersal, have 

 migrated across the vast and broken interspace. The 

 great and striking influence which barriers of every kind 

 have had on distribution, is intelligible only on the view 

 that the great majority of species have been produced 

 on one side alone, and have not been able to migrate to 

 the other side. Some few families, many sub-families, 

 very many genera, and a still greater number of sec- 

 tions of genera are confined to a single region ; and it 

 has been observed by several naturalists, that the most 

 natural genera, or those genera in which the species are 

 most closely related to each other, are generally local, 

 or confined to one area. What a strange anomaly it 

 would be, if, when coming one step lower in the series, 

 to the individuals of the same species, a directly oppo- 

 site rule prevailed ; and species were not local, but had 

 been produced in two or more distinct areas ! 



Hence it seems to me, as it has to many other natu- 

 ralists, that the view of each species having been pro- 

 duced in one area alone, and having subsequently mi- 

 grated from that area as far as its powers of migration 

 and subsistence under past and present conditions per- 

 mitted, is the most probable. Undoubtedly many cases 

 occur, in which we cannot explain how the same species 

 could have passed from one point to the other. But 

 the geographical and climatal changes, which have cer- 

 tainly occurred within recent geological times, must 

 have interrupted or rendered discontinuous the for- 

 merly continuous range of many species. So that 

 we are reduced to consider whether the exceptions to 



