356 GEOGEAPHICAL DISTRIBUTION. Chap. XI. 



a single parent. But in the majority of cases, namely, 

 with all organisms which habitually unite for each birth, 

 or which often intercross, I believe that during the slow 

 process of modification the individuals of the species will 

 have been kept nearly uniform by intercrossing ; so that 

 many individuals will have gone on simultaneously 

 changing, and the whole amount of modification will not 

 have been due, at each stage, to descent from a single 

 parent. To illustrate what I mean : our English race- 

 horses differ slightly from the horses of every other 

 breed ; but they do not owe their difference and supe- 

 riority to descent from any single pair, but to continued 

 care in selecting and training many individuals during 

 many generations. 



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 when its climate was different 

 may have been a high road for migration, but now be 

 impassable ; 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 influ- 

 ential: a narrow isthmus now separates two marine 

 faunas ; submerge it, or let it formerly have been sub- 

 merged, and the two faunas will now blend 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. 



