GEOGRAPHICAL DISTRIBUTION 365 



discussed by Alph. de Candolle in regard to plants, 

 namely, that the lower any group of organisms is, the 

 more widely it is apt to range. 



The relations just discussed, — namely, low and 

 slowly -changing organisms ranging more widely than 

 the high, — some of the species of widely-ranging genera 

 themselves ranging widely, — such facts, as alpine, 

 lacustrine, and marsh productions being related (with 

 the exceptions before specified) to those on the sur- 

 rounding low lands and dry lands, though these stations 

 are so different, — the very close relation of the distinct 

 species which inhabit the islets of the same archipelago, 

 — and especially the striking relation of the inhabitants 

 of each whole archipelago or island to those of the 

 nearest mainland, — are, I think, utterly inexplicable 

 on the ordinary view of the independent creation of 

 each species, but are explicable on the view of colon- 

 isation from the nearest or readiest source, together 

 with the subsequent modification and better adaptation 

 of the colonists to their new homes. 



Summary of last and present Chapters. — In these 

 chapters I have endeavoured to show, that if we make 

 due allowance for. our ignorance of the full effects of 

 all the changes of climate and of the level of the land, 

 which have certainly occurred within the recent period, 

 and of other similar changes which may have occurred 

 within the same period ; if we remember how pro- 

 foundly ignorant we are with respect to the many 

 and curious means of occasional transport, — a subject 

 which has hardly ever been properly experimentised on ; 

 if we bear in mind how often a species may have ranged 

 continuously over a wide area, and then have become 

 extinct in the intermediate tracts, I think the diffi- 

 culties in believing that all the individuals of the same 

 species, wherever located, have descended from the 

 same parents, are not insuperable. And we are led to 

 this conclusion, which has been arrived at by many 

 naturalists under the designation of single centres of 

 creation, by some general considerations, more especially 



