148 THE ORIGIN OF SPECIES 



whole amount of modification at each stage will not be 

 due to descent from a single parent. To illustrate wliat 

 I mean: our English racehorses differ from the horses of 

 every other breed; but they do not owe their difference 

 and superiority to descent from any single pair, but to 

 continued care in the selecting and training of many 

 individuals during each generation. 



Before discussing the three classes of facts, which 

 I have selected as presenting the greatest amount of 

 difficulty on the theory of "single centres of creation," 

 I must say a few words on the means of dispersal. 



Means of Dispersal 



Sir C. Lyell and other authors have ably treated this 

 subject. I can give here only the briefest abstract of the 

 more important facts. Change of climate must have had 

 a powerful influence on migration. A region now impas- 

 sable to certain organisms from the nature of its climate 

 might have been a highroad for migration, when the 

 climate was different. I shall, however, presently have 

 to discuss this branch of the subject in some detail. 

 Changes of level in the land must also have been highly 

 influential: a narrow isthmus now separates two marine 

 faunas; submerge it, or let it formerly have been sub- 

 merged, and the two faunas will now blend together, 

 or may formerly have blended. Where the sea now 

 extends, land may at a former period have connected 

 islands or possibly even continents together, and thus 

 have allowed terrestrial productions to pass from one to 

 the other. No geologist disputes that great mutations of 

 level have occurred within the period of existing organ- 

 isms. Edward Forbes insisted that all the islands in the 



