424 SUMMART. 



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 surround- 

 ing 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 in- 

 habitants of the islands in the same archipelago — are inex- 

 plicable on the ordinary view of the independent creation 

 of each species, but are explicable if we admit colonization 

 from the nearest or readiest source, together with the sub- 

 sequent adaptation of the colonists to their new homes. 



SUMMARY OF THE LAST AXD PRESEXT CHAPTERS. 



In these chapters I have endeavored 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 occun-ed — if we re- 

 member 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 centers of creation, by 

 various general considerations, more especially from the 

 importance of barriers of all kinds, and from the ana- 

 logical distribution of subgenera, 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 before for our 

 ignorance, and remember that some forms of life have 

 changed very slowly, enormous periods of time having been 

 thus granted for their migration, the difficulties are far 

 from insuperable; though in this case, as in that of the 

 individuals of the same species, they are often great. 



