MEANS OF iJliSFERtiAL. 381 



ously created. Witli organic beings which never intercross*, 

 if such exist, each species must be descended from u suc- 

 cession of modified varieties, that liave supplanted eacii 

 other, but have never blended with other individuals or 

 varieties of the same species; so that, at each snocessivo 

 stage of modification, all the individuals of the same form 

 will be descended from a single parent. But in the groat 

 majority of cases, namely, with all organisms which habit- 

 ually 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 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 centers of creation/' I must say a 

 few words on the means of dispersal. 



MEAN'S 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 impass- 

 able 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, j)resently 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; sub- 

 merge 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 ^a^- 

 sibly even continents together, and thus have allowed terres- 

 trial productions to pass fi'om one to the other. No geolo-isi 



