140 MBANS OP DISPBBSAL. [Chap. XII. 



amount of modification at each stage will not be due to 

 descent from a single parent. To illustrate what I 

 mean: our English race-horses 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 in- 

 dividuals during each generation. 



Before discussing the three classes of facts, which I 

 have selected as presenting the greatest amount of diffi- 

 culty 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 impassable to certain organisms from the nature of 

 its climate, might have been a high road for migration, 

 when the climate was different. I shall, however, pres- 

 ently 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 sepa- 

 rates two marine faunas; submerge it, or let it formerly 

 have been submerged, 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 organisms. Edward Forbes insisted that all 

 the islands in the Atlantic must have been recently 



