320 ON THE ORIGIN OF SPECIES 



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 inter- 

 crossing ; 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 superiority 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 diffi- 

 culty on the theory of i single centres of creation,' I 

 must say a few words on the means of dispersal. 



Meam 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 

 submerged, 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. No geologist will dispute that great mutations 

 of level have occurred within the period of existing 

 organisms. Edward Forbes insisted that all the islands 

 in the Atlantic must recently have been connected with 

 Europe or Africa, and Europe likewise with America. 

 Other authors have thus hypothetically bridged over 



