s with respect to animals, comes to the
conclusion, that the laws of resemblance of the child to its parents are
the same, whether the two parents differ much or little from each other,
namely in the union of individuals of the same variety, or of different
varieties, or of distinct species.

Laying aside the question of fertility and sterility, {276} in all other
respects there seems to be a general and close similarity in the offspring
of crossed species, and of crossed varieties. If we look at species as
having been specially created, and at varieties as having been produced by
secondary laws, this similarity would be an astonishing fact. But it
harmonises perfectly with the view that there is no essential distinction
between species and varieties.

_Summary of Chapter._--First crosses between forms sufficiently distinct to
be ranked as species, and their hybrids, are very generally, but not
universally, sterile. The sterility is of all degrees, and is often so
slight that the two most careful experimentalists who have ever lived, have
come to diametrically opposite conclusions in ranking forms by this test.
The sterility is innately variable in individuals of the same species, and
is eminently susceptible 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
hybrid produced from this cross.

In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown differences
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 them crossing and blending in nature, than to think that trees have
been specially endowed with various and {277} somewhat analogous degrees of
difficulty in being grafted together in order to prevent them becoming
inarched in our forests.

The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances; in
some cases largely on t