.462 RECAPITULATION. 



functions. This fact is perfectly compatible with numerous 

 beings still retaining simple and but little improved struc- 

 tures, fitted for simple conditions of life ; it is likewise com- 

 patible 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 con- 

 tinent — 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 migra- 

 tion from one part of the world to another, owing to former 

 climatical and geographical changes and to the many occa- 

 sional and unknown means of dispersal, then we can under- 

 stand, 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 suc- 

 cession 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 trav- 

 eller, 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 like- 

 wise 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 impor- 

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

 received colonists at various periods and in different propor* 



