pose 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 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 gene