degree of sterility of first crosses and of hybrids runs, to
a certain extent, parallel with the general or systematic affinity of the
forms which are united. For species belonging to distinct genera can
rarely, and those belonging to distinct families can never, be crossed. The
parallelism, however, is far from complete; for a multitude of closely
allied species will not unite, or unite with extreme difficulty, whilst
other species, widely different from each other, can be crossed with
perfect facility. Nor does the difficulty depend on ordinary {180}
constitutional differences, for annual and perennial plants, deciduous and
evergreen trees, plants flowering at different seasons, inhabiting
different stations, and naturally living under the most opposite climates,
can often be crossed with ease. The difficulty or facility apparently
depends exclusively on the sexual constitution of the species which are
crossed; or on their sexual elective affinity, _i. e._ _Wahlverwandtschaft_
of Gaertner. As species rarely or never become modified in one character,
without being at the same time modified in many, and as systematic affinity
includes all visible resemblances and dissimilarities, any difference in
sexual constitution between two species would naturally stand in more or
less close relation with their systematic position.

_Sixthly_, the sterility of species when first crossed, and that of
hybrids, may possibly depend to a certain extent on distinct causes. With
pure species the reproductive organs are in a perfect condition, whilst
with hybrids they are often plainly deteriorated. A hybrid embryo which
partakes of the constitution of its father and mother is exposed to
unnatural conditions, as long as it is nourished within the womb, or egg,
or seed of the mother-form; and as we know that unnatural conditions often
induce sterility, the reproductive organs of the hybrid might at this early
age be permanently affected. But this cause has no bearing on the
infertility of first unions. The diminished number of the offspring from
first unions may often result, as is certainly sometimes the case, from the
premature death of most of the hybrid embryos. But we shall immediately see
that a law of an unknown nature apparently exists, which causes the
offspring from unions, which are infertile, to be themselves more or less
infertile; and this at present is all that can be said.

_Seventhly_, hybrids and mongrels present, with the