CHAI-. XII.] MEANS OF DISPERSAL. 303 



viduals or varieties of the same species ; so that, at each successive 

 stage of modification, all the individuals of the same form will be 

 descended from a single parent. But in the great majority of 

 cases, namely, with all organisms which habitually unite for each 

 birth, or which occasionally intercross, the individuals of the 

 same species inhabiting the same area will be kept nearly uniform 

 by intercrossing; so that many individuals will go on simul- 

 taneously changing, and the whole 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 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 migra- 

 tion, 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 con- 

 nected with Europe or Africa, and Europe likewise with America. 

 Other authors have thus hypothetically bridged over every ocean, 

 and united almost every island with some mainland. If indeed 

 the arguments used by Forbes are to be trusted, it must be ad- 

 mitted 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 authorised in admitting such enormous geographical 



