pecies, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the most careful experimentalists have arrived at diametrically opposite
conclusions in ranking forms by this test. The sterility is innately
variable in individuals of the same species, and is eminently
susceptible to action of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed by several curious and complex laws. It is generally different,
and sometimes widely different in reciprocal crosses between the same
two species. It is not always equal in degree in a first cross and in
the hybrids produced from this cross.

In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally
of an unknown nature, in their vegetative systems, so in crossing,
the greater or less facility of one species to unite with another is
incidental on unknown differences in their reproductive systems. There
is no more reason to think that species have been specially endowed with
various degrees of sterility to prevent their crossing and blending
in nature, than to think that trees have been specially endowed with
various and somewhat analogous degrees of difficulty in being grafted
together in order to prevent their inarching in our forests.

The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it
seems to depend on several circumstances; in some instances in chief
part on the early death of the embryo. In the case of hybrids, it
apparently depends on their whole organisation having been disturbed by
being compounded from two distinct forms; the sterility being closely
allied to that which so frequently affects pure species, when exposed to
new and unnatural conditions of life. He who will explain these latter
cases will be able to explain the sterility of hybrids. This view is
strongly supported by a parallelism of another kind: namely, that,
firstly, slight changes in the conditions of life add to the vigour
and fertility of all organic beings; and secondly, that the crossing of
forms, which have been exposed to slightly different conditions of life,
or which have varied, favours the size, vigour and fertility of their
offspring. The facts given on the sterility of the illegit