368 SUM.MAliY OF THE LAST Chap. XII. 



The relations just discussed — namely, lower oriranisms ran- 

 ging' more "widely than the higher — some of the species of widely- 

 ranging genera themselves ranging widely — such facts, as al- 

 l)ine, lacustrine, and marsh productions being generally related 

 to those which live on the surrounding low-lands and drylands 

 — the striking relationship between the inhabitants of islands 

 and tliose of the nearest main-land — 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, l)ut arc explicable if we admit coloni- 

 zation from the nearest or readiest source, together with the 

 subsequent adaptation of the colonists to their new homes. 



Summary of the last and 2)^'c^C}it Chapter. 



In these chapters I have endeavored to show that, if we 

 make due allowance for our ignorance of the full effects of the 

 changes of climate and of the level of the land, which have cer- 

 tainly occurred within the recent period, and of other changes 

 which have probably occurred — if Ave remember how ignorant 

 M'e are with respect to the many curious means of occasional 

 transport — 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 — the difficulty is not insuper- 

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

 logical distribution of sub-genera, genera, and families. 



With respect to distinct species belonging to the same 

 genus, Avhich on our theory must have spread from one parent- 

 source ; if Ave make the same allowance as before for our igno- 

 rance, and remember that some forms of life have changed A'cry 

 slowly, enormous periods of time having been thus granted for 

 tlieir migration, the difficulties are far from insuperable; though 

 in this case, and in that of the individuals of the same species, 

 they are often great. 



As exemplifying the effects of climatal changes on distribu- 

 tion, I liaA'c attempted to shoAV how important a part the Gla- 

 cial period has played, Avhich affected even the equatorial 

 regions, and Avhich, during the alternations of the cold in the 



