298 THE ORIGIN OF SPECIES 



and but little improved structures, fitted for simple con- 

 ditions of life; it is likewise compatible with some forms 

 having retrograded in organization, bj having become at 

 each stage of descent better fitted for new and degraded 

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

 endurance Qf allied forms on the same continent — of mar- 

 supials 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 mi- 

 gration from one part of the world to another, owing 

 to former climatal 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 ia 

 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 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 

 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, 



