| → enormously 1859 1860 1861 |
| of time, enormously 1866 |
| of time, immensely 1869 1872 |
|
| → our present 1859 1860 1861 |
| are our present 1866 1869 |
| are our existing 1872 |
|
|
course of successive generations have
from some one
to
others. We are often wholly unable even to conjecture how this could have been effected. Yet, as we have reason to believe that some species have retained the same specific
for very long
→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
there will always
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
the full extent of the various climatal and geographical changes which have affected the earth during modern periods; and such changes
have
facilitated migration. As an example, I have attempted to show how potent has been the influence of the Glacial period on the distribution
of the same and of
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
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
of the same genus is in some degree lessened. |
|
| As
the theory of natural selection an interminable number of intermediate forms must have existed, linking together all the species in each group by
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 inextricable chaos? With respect to existing forms, we
|
