Chap. XII. 
SUMMAEY. 
407 
the changes of climate and of the level of the land, 
Avhich 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 
this 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 
species, extremely great. 
As exemplifying the effects of climatal changes on 
distribution, I have attempted to show how important 
has been the influence of the modern Glacial period, 
which 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. 
