BE8ULT8 OF NATURAL SELECTION. 99 



nental area, which has undergone many oscillations of 

 level, will have been the most favorable for the production 

 of many new forms of life, fitted to endure for a long time 

 and to spread widely. While the area existed as a'conti- 

 nent the inhabitants will have been numerous in individu- 

 als and kinds, and will have been subjected to severe com- 

 petition. When converted by subsistance into large 

 separate islands there will still have existed many indi- 

 viduals of the same species on each island: intercrossing 

 on the confines of the range of each new species will have 

 been checked: after physical changes of any kind immigra- 

 tion will have been prevented^ so that new places in the 

 polity of each island will have had to be filled up by the 

 modification of the old inhabitants; and time will have 

 been allowed for the varieties in each to become well modi- 

 fied and perfected. When, by renewed elevation, the 

 islands were reconverted into a continental area, there will 

 again have been very severe competition; the most favored 

 or improved varieties will have been enabled to spread; 

 there will have been much extinction of the less improved 

 forms, and the relative proportional numbers of the various 

 inhabitants of the reunited continent will again have been 

 changed; and again there will have been a fair field for 

 natural selection to improve still further the inhabitants, 

 and thus to produce new species. 



That natural selection generally acts with extreme slow- 

 ness I fully admit. It can act only when there are places 

 in the natural polity of a district which can be better occu- 

 pied by the modification of some of its existing inhabitants. 

 The occurrence of such places will often depend on physi- 

 cal changes, which generally take place very slowly, and 

 on the immigration of better adapted forms being pre- 

 vented. As some few of the old inhabitants become modi- 

 fied the mutual relations of others will often be disturbed; 

 and this will create new places, ready to be filled up by 

 better adapted forms; but all this will take place very 

 slowly. Although all the individuals of the same species 

 differ in some slight degree from each other, it would often 

 be long before differences of the right nature in various 

 parts of the organization might occur. The result would 

 often be greatly retarded by free intercrossing. Many will 

 exclaim that these several causes are amply suflicieut to 



