Chap. XII. SUMMARY. 407 



the changes of climate and of the level of the land, 

 which have certainly occurred within the recent period, 

 and of other similar changes which may have occurred 

 within the same period ; if we remember how pro- 

 foundly ignorant we are with respect to the many 

 and curious means of occasional transport, — a subject 

 which has hardly ever been properly experimentised 

 on ; 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, I think the 

 difficulties in believing that all the individuals of the 

 same species, wherever located, have descended from the 

 same parents, are not insuperable. And we are led to 

 tins conclusion, which has been arrived at by many 

 naturalists under the designation of single centres of 

 creation, by some general considerations, more especially 

 from the importance of barriers and from the analogical 

 distribution of sub-genera, genera, and families. 



With respect to the distinct species of the same genus, 

 which on my theory must have spread from one parent- 

 source ; if we make the same allowances as before for 

 our ignorance, and remember that some forms of life 

 change most slowly, enormous periods of time being 

 thus granted for their migration, I do not think that the 

 difficulties are insuperable ; though they often are in 

 this case, and in that of the individuals of the same spe- 

 cies, extremely grave. 



As exemplifying the effects of clirnatal changes on 

 distribution, I have attempted to show how important 

 has been the influence of the modern Glacial period, 

 winch I am fully convinced simultaneously affected the 

 whole world, or at least great meridional belts. As 

 showing how diversified are the means of occasional 

 transport, I have discussed at some little length the 

 means of dispersal of fresh- water productions. 



