420 BECAPITULATION. • Chap. XIV. 



of life. Lastly, the wonderful law of the long endurance of 

 allied forms on the same continent — of marsupials in Australia, 

 of edenlatii in America, anct other such cases — is intelligible, 

 for generally within the same country the existing and the ex- 

 tinct will be; closely allied l)y descent. 



Looking to geographical distribution, if wc admit that 

 there has been during the long course of ages much migration 

 from one part of the world to another, owing to former clima- 

 tal and geographical changes and to the many occasional and 

 unknown means of dispersal, then wc can understand, on the 

 theory of 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 succession through- 

 out time ; for in both cases the beings have been connected by 

 the bond of ordinary generation, and the means of modifica- 

 tion have been the same. We see the full meaning of the 

 wonderful fact, which has struck every traveller, namely, that 

 on the same continent, under the most diverse conditions, 

 luider heat and cold, on mountain and lowland, on deserts and 

 marshes, most of the inhabitants within each great class are 

 plainly related ; for they are the descendants of the same pro- 

 genitors and early colonists. On this same principle of former 

 migration, combined in most cases with modification, we can 

 unclerstand, 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, and in the northern and southern tem- 

 perate zones ; and likewise the close alliance of some of the 

 inhabitants of the sea in the northern and southern temperate 

 latitudes, though separated by the whole intertropical ocean. 

 Although two countries may present physical conditions as 

 closely similar as the same species ever reqviire, we need feel 

 no surprise at their inhabitants being widely different, if they 

 have been for a long period completely sundered from each 

 other ; for, as the relation of organisn\ to organism is the most 

 important of all relations, and as the two countries will have 

 received colonists at various periods and in difi'erent propor- 

 tions, from some other country or from each other, the course 

 of modification in the two areas will ineWtably have been dif- 

 ferent. 



On this view of migration, with subsequent modification, 

 we see why oceanic islands arc inhabited by only few species, 

 but of these, why many arc peculiar or endemic forms. We 



