eans of distinguishing any two true species
from any two varieties. If a male and a female, selected from each
group, produce offspring, and that offspring is fertile with others
produced in the same way, the groups are races and not species. If, on
the other hand, no result ensues, or if the offspring are infertile with
others produced in the same way, they are true physiological species.
The test would be an admirable one, if, in the first place, it were
always practicable to apply it, and if, in the second, it always
yielded results susceptible of a definite interpretation. Unfortunately,
in the great majority of cases, this touchstone for species is wholly
inapplicable.

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 ensuring 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 cro