98 Heredity and Variation in Modern Lights 
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 acquisi- 
tion 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 ex- 
perimental 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 pro- 
ducing the slightest sterility ; others have totally sterile offspring, and 
soon. The hybrids between several genera of Orchids are perfectly 
