Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 197 



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 trans- 

 port, — 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 im- 

 portance 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 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. 



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 



