erly
relation to the pattern of the segmentation. Even if a model which would
do this could be constructed it might prove to be a useful beginning.

This may be looking too far ahead. If we had to choose some one piece of
more proximate knowledge which we would more especially like to acquire,
I suppose we should ask for the secret of interracial sterility. Nothing
has yet been discovered to remove the grave difficulty, by which Huxley
in particular was so much oppressed, that among the many varieties
produced under domestication--which we all regard as analogous to the
species seen in nature--no clear case of interracial sterility has
been demonstrated. The phenomenon is probably the only one to which the
domesticated products seem to afford no parallel. No solution of the
difficulty can be offered which has positive value, but it is perhaps
worth considering the facts in the light of modern ideas. It should be
observed that we are not discussing incompatibility of two species to
produce offspring (a totally distinct phenomenon), but the sterility of
the offspring which many of them do produce.

When two species, both perfectly fertile severally, produce on crossing
a sterile progeny, there is a presumption that the sterility is due to
the development in the hybrid of some substance which can only be formed
by the meeting of two complementary factors. That some such account is
correct in essence may be inferred from the well-known observation that
if the hybrid is not totally sterile but only partially so, and thus is
able to form some good germ-cells which develop into new individuals,
the sterility of these daughter-individuals is sensibly reduced or may
be entirely absent. The fertility once re-established, the sterility
does not return in the later progeny, 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 prob