t at first appears.

_On the Difference in Fertility between Varieties and Species when
crossed._

This work is not the proper place for fully treating the subject of
hybridism, and I have already given in my 'Origin of Species' a moderately
full abstract. I will here merely enumerate the general conclusions which
may be relied on, and which bear on our present point.

_Firstly_, the laws governing the production of hybrids are identical, or
nearly identical, in the animal and vegetable kingdoms.

_Secondly_, the sterility of distinct species when first united, {179} and
that of their hybrid offspring, graduates, by an almost infinite number of
steps, from zero, when the ovule is never impregnated and a seed-capsule is
never formed, up to complete fertility. We can only escape the conclusion
that some species are fully fertile when crossed, by determining to
designate as varieties all the forms which are quite fertile. This high
degree of fertility is, however, rare. Nevertheless plants, which have been
exposed to unnatural conditions, sometimes become modified in so peculiar a
manner, that they are much more fertile when crossed by a distinct species
than when fertilised by their own pollen. Success in effecting a first
union between two species, and the fertility of their hybrids, depends in
an eminent degree on the conditions of life being favourable. The innate
sterility of hybrids of the same parentage and raised from the same
seed-capsule often differs much in degree.

_Thirdly_, the degree of sterility of a first cross between two species
does not always run strictly parallel with that of their hybrid offspring.
Many cases are known of species which can be crossed with ease, but yield
hybrids excessively sterile; and conversely some which can be crossed with
great difficulty, but produce fairly fertile hybrids. This is an
inexplicable fact, on the view that species have been specially endowed
with mutual sterility in order to keep them distinct.

_Fourthly_, the degree of sterility often differs greatly in two species
when reciprocally crossed; for the first will readily fertilise the second;
but the latter is incapable, after hundreds of trials, of fertilising the
former. Hybrids produced from reciprocal crosses between the same two
species, likewise sometimes differ in their degree of sterility. These
cases also are utterly inexplicable on the view of sterility being a
special endowment.

_Fifthly_, the