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 produced under
domestication; and as domestication 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
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
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