406 GEOGRAPHICAL DISTRIBUTION. Chap. XII. 



quarters of the world, where they may have become 

 slightly modified in relation to their new conditions. 

 There is, also, some reason to believe from geological 

 evidence that organisms low in the scale within each 

 great class, generally change at a slower rate than the 

 higher forms ; and consequently the lower forms will have 

 had a better chance of ranging widely and of still re- 

 taining the same specific character. This fact, together 

 with the seeds and eggs of many low forms being Very 

 minute and better fitted for distant transportation, pro- 

 bably accounts for a law which has long been observed, 

 and which has lately been admirably 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, lacus- 

 trine, and marsh productions being related (with the 

 exceptions before specified) to those on the surrounding 

 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 spe- 

 cies, but are explicable on the view of colonisation 

 from the nearest and 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 



