493 RECAPITULATION. 



more improved forms have conquered the older and less 

 improved forms in the struggle for life; they have also 

 generally had their organs more specialized for different 

 functions. This fact is perfectly compatible with numer- 

 ous beings still retaining simple and but little improved 

 structures, fitted for simple conditions of life; it is likewise 

 compatible with some forms having retrograded in organi- 

 zation, 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 migra- 

 tion 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 modi- 

 fication, 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 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 traveler, namely, 

 that on the same continent, under the most diverse con- 

 ditions, 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 descend- 

 ants 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 mount- 

 ains, 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 lati- 

 tudes, though separated by the whole intertropical ocean. 

 Although two countries may present physical conditions as 



