330 THE ORIGIN OF SPECIES 



planted each other, but have never blended with other individuals 

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

 of modification, all the individuals of the same form will be de- 

 scended 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 simultaneously 

 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 select- 

 ing and training of many individuals during each generation. 



Before discussing the three classes of facts, which I have se- 

 lected 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 nov/ impassible 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 nar- 

 row 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 ter- 

 restrial 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 au- 

 thors 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 admitted 

 that scarcely a single island exists which has not recently been 

 united to some continent. This view cuts the Gordian knot of the 



