The constitution of many wild animals is so altered by confinement that
they will not breed even with their own females, so that the negative
results obtained from crosses are of no value; and the antipathy of wild
animals of different 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 super-added 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 hybridisation 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,_ the male element of which will fertilise 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