ably 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 dissimilar, no such principle can be demonstrated to
determine sterility or fertility in general. For example, though all our
Finches can breed together, the hybrids are all sterile. Of Ducks some
species can breed together without producing the slightest sterility;
others have totally sterile offspring, and so on. The hybrids between
several genera of Orchids are perfectly fertile on the female side, and
some on the male side also, but the hybrids produced between the Turnip
(Brassica napus) and the Swede (Brassica campestris), which, according
to our estimates of affinity should be nearly allied forms, are totally
sterile. (See Sutton, A.W., "Journ. Linn. Soc." XXXVIII. pag