Chap. XI. MEANS OF DISPERSAL. 329 



cics has come into existence coincident both in sjKice and time 

 with a preexist ino- closely-allied species." And I now know that 

 he attributes this coincidence to descent with modification. 



The discussion on " single and multiple centres of crea- 

 tion " does not directly l)ear on another allied question — 

 namely, whether all the individuals of the same species are 

 descended from a single pair, or single hermaphrodite, or 

 whether, as some authors suppose, from many individuals si- 

 uuiltan<'()usly created. With organic beings which never inter- 

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

 cession of modified varieties, which have sujiplanted each 

 other, but which have never blended with other individuals 

 or varieties of the same species ; so that, at each successive 

 stage of modification and improvement, all the individuals 

 of the same variety will be descended from a single parent. 

 IJut in the great majority of cases, namely, with all organisms 

 which habitually unite for each birth, or which occasionally inter- 

 cross, 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 chfler 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 difliculty on the 

 theory of " single centres of creation," I must say a few words 

 on the means of dispersal. 



Means of Dlq:)crsaL 



Sir C Lyell and other authors have ably treated this sub- 

 ject. I can give here only the l^riefest abstract of the more 

 important facts. Change of climate must have had a p(jwcr- 

 ful influence on migration ; an impassable region when its 

 climate was different from what it now is, may have been a 

 high-road for migration; 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 isthnuis now separates two marine faunas ; submerge 

 it, or let it fonncrly have been submerged, and the two faunas 



