f the intercrossed species. We see the truth of this conclusion in
the vast difference in the result, when the same two species are crossed
reciprocally; that is, when one species is first used as the father and
then as the mother.

The fertility of varieties when intercrossed and of their mongrel offspring
cannot be considered as universal; nor is their very general fertility
surprising when we remember that it is not likely that either their
constitutions or their reproductive systems should have been profoundly
modified. Moreover, most of the varieties which have been experimentised on
have been {461} produced under domestication; and as domestication (I do
not mean mere confinement) apparently tends to eliminate sterility, we
ought not to expect it also to produce sterility.

The sterility of hybrids is a 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 {462}