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