Chap. XII.] MEANS OF DISPERSAL. 323 



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 tho 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 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 connected with Europe or Africa, 

 and Europe likewise with America. Other authors have thus 

 hypothetically bridged over every ocean, and united almost every 

 island to some mainland. If indeed the arguments used by Forbes 

 are to be trusted, it must be admitted that scarcely a single island 

 exists which has not recently been united to some continent. This 

 view cuts the Gordian knot of the dispersal of the same species to 

 the most distant points, and removes many a difficulty ; but to the 

 best of my judgment we are not authorized in admitting such 

 enormous geographical changes within the period of existing species. 

 It seems to me that we have abundant evidence of great oscillations 

 in the level of the land or sea ; but not of such vast changes in the 

 position and extension of our continents, as to have united them 

 within the recent period to each other and to the several intervening 

 oceanic islands. I freely admit the former existence of many islands, 

 now buried beneath the sea 3 which me y have served as halting- 



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