ny, a fact strongly suggestive of segregation. Now if the sterility of
the cross-bred be really the consequence of the meeting of two
complementary factors, we see that the phenomenon could only be produced
among the divergent offspring of one species by the acquisition of at least
_two_ new factors; for if the acquisition of a single factor caused
sterility the line would then end. Moreover each factor must be separately
acquired by distinct individuals, for if both were present together, the
possessors would by hypothesis be sterile. And in order to imitate the case
of species each of these factors must be acquired by distinct breeds. The
factors need not, and probably would not, produce any other perceptible
effects; they might, like the colour-factors present in white flowers, make
no difference in the form or other characters. Not till the cross was
actually made between the two complementary individuals would either factor
come into play, and the effects even then might be unobserved until an
attempt was made to breed from the cross-bred.

Next, if the factors responsible for sterility were acquired, they
would in all probability be peculiar to certain individuals and would
not readily be distributed to the whole breed. Any member of the breed
also into which _both_ the factors were introduced would drop out of
the pedigree by virtue of its sterility. Hence the evidence that the
various domesticated breeds say of dogs or fowls can when mated
together produce fertile offspring, is beside the mark. The real
question is, Do they ever produce sterile offspring? I think the
evidence is clearly that sometimes they do, oftener perhaps than is
commonly supposed. These suggestions are quite amenable to
experimental tests. The most obvious way to begin is to get a pair of
parents which are known to have had any sterile offspring, and to find
the proportions in which these steriles were produced. If, as I
anticipate, these proportions are found to be definite, the rest is
simple.

In passing, certain other considerations may be referred to. First,
that there are observations favouring the view that the production of
totally sterile cross-breds is seldom a universal property of two
species, and that it may be a matter of individuals, which is just
what on the view here proposed would be expected. Moreover, as we all
know now, though incompatibility may be dependent to some extent on
the degree to which the species are dissim