RECAPITULATION AND CONCLUSION 427 



simple conditions of life; it is likewise compatible with some forms 

 having retrograded in organization, 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 edentata 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 climatical and 

 geographical changes and to the many occasional and unknown 

 means of dispersal, then we can understand, on the theory of 

 descent with modification, most cf the great leading facts in dis- 

 tribution. We can see why there should be so striking a parallel- 

 ism in the distribution 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 travel- 

 ler, 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 dis- 

 tant 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 in- 

 evitably have been different. 



On this view of migration, with subsequent modification, we see 

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



