206 THE ORIGIN OF SPECIES 



Siimmary of the last and present Chapters 



In these chuptera I have endeavored to show that v 

 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 des'gnation of single centres of 

 creation, by various general considerations, more espe- 

 cially from the importance of barriers of all kinds, and 

 from the analogical distribution of 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 before our 

 ignorance, and remember that some forms of life have 

 changed very slowly, enormous periods of time haviug 

 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, tney are often great. 



As exemplifying the effects of climatal changes on 

 distribution, I have attempted to show how important a 

 part the last Glacial period has played, which affected 



