members of any two groups.

Mr. Darwin has discussed this question with singular ability and
circumspection, and his conclusions are summed up as follows, at page 276
of his work:--

"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 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.