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 breeding 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 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 the early death of the embryo. The sterility
of hybrids which have their reproductive systems imperfect, and which
have had this system and their whole organization disturbed by being
compounded of two distinct species, seems closely allied to that
sterility which so frequently affects pure species when their natural
conditions of life have been disturbed. This view is supported by a
parallelism of another kind: namely, that the crossing of forms, only