418 RECAPITULATION. [C"ap. XV. 



forms having retrograded in organisation, by having become at 

 each stage of descent better fitted for new and degraded habits 

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

 forms on the same continent, — of marsupials in Australia, of eden- 

 tata in America, and other such cases, — is intelligible, for within 

 the same country the existing and the extinct will be closely 

 allied by descent. 



Looking to geographical distribution, if we admit that there has 

 been during the long course of ages much migration from one part 

 of the world to another, owing to former climatal and geographical 

 changes and to the many occasional and unknown means of dis- 

 persal, then we can understand, on the theory of descent with 

 modification, most of the great leading facts in Distribution. "VYc» 

 can see why there should be so striking a parallelism in the dis- 

 tribution of organic beings throughout space, and in their geological 

 succession throughout time ; for in both cases the beings have been 

 connected by the bond of ordinary generation, and the means of 

 modification 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, under 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 progenitors 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, and in the northern 

 and southern temperate 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 require, 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 organism to organism is the most important of all 

 relations, and as the two countries will have received colonists 

 at various periods and in different proportions, from some other 

 country or from each other, the course of modification in the two 

 areas will inevitably have been different. 



On this view of migration, with subsequent modification, we 

 see why oceanic islands are inhabited by only few species, but of 

 these, why many are peculiar or endemic forms. "We clearly see 

 why species belonging to those groups of animals which cannot 



