Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 359 



class the lower organisms change at a slower rate than the higher ; 

 consequently they will have had a better chance of ranging widelj 

 and of still retaining the same specific character. This fact, 

 together with that of the seeds and eggs of most lowly organised 

 forms being very minute and better fitted for distant transportal, 

 probably accounts for a law which has long been observed, and 

 which has lately been discussed by Alph. de Candollc in regard tc 

 plants, namely, that the lower any group of organisms stands, the 

 more widely it ranges. 



The relations just discussed, — namely, lower organisms ranging 

 more widely than the higher, — some of the species of widely- 

 ranging genera themselves ranging widely, — such facts, as alpine, 

 lacustrine, and marsh productions being generally related to those 

 which live on the surrounding low lands and dry lands, — the 

 striking relationship between the inhabitants of islands and those 

 of the nearest mainland — the still closer relationship of the distinct 

 inhabitants of the islands in the same archipelago — are inexplicable 

 on the ordinary view of the independent creation of each species, 

 but are explicable if we admit colonisation from the nearest cr 

 readiest source, together with the subsequent adaptation of the 

 colonists to their new homes. 



Summary of the 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 changes of 

 climate and of the level of the land, which have certainly occurred 

 within the recent period, and of other changes which have probably 

 occurred, — if we remember how ignorant we are with respect to the 

 many curious means of occasional transport, — if we bear in mind, 

 and this is a very important consideration, how often a species 

 may have ranged continuously over a wide area, and then have 

 become extinct in the intermediate tracts, — the difficulty is not 

 insuperable in believing that all the individuals of the same 

 species, wherever found, are descended from common parents. 

 And we are led to this conclusion, which has been arrived at by 

 many naturalists under the designation of single centres of creation, 

 by various general considerations, more especially from the import- 

 ance of barriers of all kinds, and from the analogical distribution cf 

 sub-genera, genera, and families. 



With respect to distinct species belonging to the same genus, 

 which on our theory have spread from one parent-source ; if we 

 make the same allowances as befcre for our ignorance, and re- 

 member that some forms of life have changed very slowly, 



