MEANS OF DISPERSAL 403 



ated. With organic beings which never intercross, if such 

 exist, each species must be descended from a succession of 

 modified varieties, that have supplanted 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 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 dififer 

 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 sub- 

 ject. I can give here only the briefest abstract of the more 

 important facts. Change of climate must have had a power- 

 ful influence on migration. A region now impassable to cer- 

 tain organisms from the nature of its climate, might have 

 been a high road for migration, when the climate was dif- 

 ferent. 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 



