GEOGRAPHICAL DISTRIBUTION 317 



space could not be easily passed over by migration, the 

 fact is given as something remarkable and exceptional. 

 The capacity of migrating across the sea is more dis- 

 tinctly limited in terrestrial mammals, than perhaps in 

 any other organic beings ; and, accordingly, we find no 

 inexplicable cases of the same mammal inhabiting dis- 

 tant points of the world. No geologist will feel any 

 difficulty in such cases as Great Britain having been 

 formerly united to Europe, and consequently possessing 

 the same quadrupeds. But if the same species can 

 be produced at two separate points, why do we not 

 find a single mammal common to Europe and Aus- 

 tralia or South America ? The conditions of life are 

 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 sections 

 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 

 naturalists, that the view of each species having been 

 produced in one area alone, and having subsequently 

 migrated from that area as far as its powers of migration 



