ssed; while both animals and plants, which have
always been regarded by naturalists as of distinct species, turn out,
when the test is applied, to be perfectly fertile. Again, the sterility
or fertility of crosses seems to bear no relation to the structural
resemblances or differences of the members of any two groups.

Mr. Darwin has discussed this question with singular ability and
circumspection, and his conclusions are summed up as follow, 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.

"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 dist