RECAPITULATION AND CONCLUSION 429 



descent with modification, most of the great leading 

 facts in Distribution. We can see why there should be 

 so striking a parallelism in the distribution of organic 

 beings throughout space, and in their geological suc- 

 cession throughout time ; for in both cases the beings 

 have been connected by the bond of ordinary genera- 

 tion, and the means of modification have been the 

 same. We see the full meaning of the wonderful 

 fact, which must have struck every traveller, namely, 

 that on the same continent, under the most diverse 

 conditions, under heat and cold, on mountain and 

 lowland, on deserts and marshes, most of the inhabit- 

 ants within each great class are plainly related ; for 

 they will generally be descendants of the same pro- 

 genitors and early colonists. On this same principle 

 of former migration, combined in most cases with 

 modification, we can understand, by the aid of the 

 Glacial period, the identity of some few plants, and 

 the close alliance of many others, on the most distant 

 mountains, under the most different climates ; and 

 likewise the close alliance of some of the inhabitants 

 of the sea in the northern and southern temperate 

 zones, though separated by the whole intertropical 

 ocean. Although two areas may present the same 

 physical conditions of life, we need feel no surprise at 

 their inhabitants being widely different, if they have 

 been for a long period completely separated from each 

 other ; for as the relation of organism to organism is 

 the most important of all relations, and as the two 

 areas will have received colonists from some third 

 source or from each other, at various periods and in 

 different proportions, the course of modification in the 

 two areas will inevitably be different. 



On this view of migration, with subsequent modifica- 

 tion, we can see why oceanic islands should be inhabited 

 by few species, but of these, that many should be 

 peculiar. We can clearly see why those animals 

 which cannot cross wide spaces of ocean, as frogs 

 and terrestrial mammals, should not inhabit oceanic 

 islands ; and why, on the other hand, new and peculiar 



