very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds
are rendered in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life, we need
not feel surprise at hybrids being in some degree sterile, for their
constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is supported
by another parallel, but directly opposite, class of facts; namely, that
the vigour and fertility of all organic beings are increased by slight
changes in their conditions of life, and that the offspring of slightly
modified forms or varieties acquire from being crossed increased vigour
and fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of
life and crosses between less modified forms, increase fertility.

Turning to geographical distribution, the difficulties encountered
on the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, must have descended from common parents; and
therefore, in however distant and isolated parts of the world they are
now found, they must in the course of successive generations have passed
from some one part to the 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 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 be 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 in