ned from crosses are of no value; and the antipathy of wild
animals of the same species for one another, or even of wild and tame
members of the same species, is ordinarily so great, that it is hopeless
to look for such unions in Nature. The hermaphrodism of most plants,
the difficulty in the way of insuring the absence of their own, or the
proper working of other pollen, are obstacles of no less magnitude
in applying the test to them. And, in both animals and plants, is
superadded the further difficulty, that experiments must be continued
over a long time for the purpose of ascertaining the fertility of the
mongrel or hybrid progeny, as well as of the first crosses from which
they spring.

Not only do these great practical difficulties lie in the way of
applying the hybridization test, but even when this oracle can be
questioned, its replies are sometimes as doubtful as those of Delphi.
For example, cases are cited by Mr. Darwin, of plants which are more
fertile with the pollen of another species than with their own; and
there are others, such as certain 'fuci', whose male element will
fertilize the ovule of a plant of distinct species, while the males of
the latter species are ineffective with the females of the first. So
that, in the last-named instance, a physiologist, who should cross the
two species in one way, would decide that they were true species; while
another, who should cross them in the reverse way, would, with equal
justice, according to the rule, pronounce them to be mere races. Several
plants, which there is great reason to believe are mere varieties, are
almost sterile when crossed; 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 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 i