140 MEANS OF DISPERSAL. [Chap. XIL 



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 

 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 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, 

 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 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 to- 

 gether, and thus have allowed terrestrial productions to 

 pass from one to the other. No geologist disputes that 

 o-reat 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 



