356 
GEOGEAPHIOAL DISTRIBUTION^. 
Chap. XL 
successive stage of modification and improvement^ all 
the individuals of each variety will have descended from 
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 diffi¬ 
culty on the theory of single centres of creation,” I 
must say a few words on the means of dispersal. 
Means of Dispersal ,—Sir 0. 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 
