462 
KECAPITULATION. 
Chap. XIV. 
even to conjecture how this could have been effected. 
Yet, as we have reason to believe that some species have 
retained the same specific form for very long periods, 
enormously long as measured by years, too much stress 
ought not to be laid on the occasional wide diffusion of 
the same species; for during very long periods of time 
there will always have been a good chance for wide 
migration by many means. A broken or interrupted 
range may often be accounted for by the extinction of 
the species in the intermediate regions. It cannot be 
denied that we are as yet very ignorant of the full 
extent of the various climatal and geographical changes 
which have affected the earth during modern periods; 
and such changes will obviously have greatly facilitated 
migration. As an example, I have attempted to show 
how potent has been the influence of the Glacial period 
on the distribution both of the same and of representa¬ 
tive species throughout the world. We are as yet 
profoundly ignorant of the many occasional means of 
transport. With respect to distinct species of the same 
genus inhabiting very distant and isolated regions, as 
the process of modification has necessarily been slow, all 
the means of migration will have been possible during a 
very long period; and consequently the difficulty of the 
wide diffusion of species of the same genus is in some 
degree lessened. 
As on the theory of natural selection an interminable 
number of intermediate forms must have existed, linking 
together all the species in each group by gradations as 
fine as our present varieties, it may be asked. Why do 
we not see these linking forms all around us? Why 
are not all organic beings blended together in an inex¬ 
tricable chaos? With respect to existing forms, we 
should remember that we have no right to expect (ex¬ 
cepting in rare cases) to discover directly connecting 
