of doubt occur. Whenever the least trace of
sterility between two plants, however closely allied, was observed by
Koelreuter, and more especially by Gaertner, who counted the exact number
of seed in each capsule, the two forms were at once ranked as distinct
species; and if this rule be followed, assuredly it will never be proved
that varieties when crossed are in any degree sterile. We have formerly
seen that certain breeds of dogs do not readily pair together; but no
observations have been made whether, when paired, they produce the full
number of young, and whether the latter are perfectly fertile _inter se_;
but, supposing that some degree of sterility were found to exist,
naturalists would simply infer that these breeds were descended from
aboriginally distinct species; and it would be scarcely possible to
ascertain whether or not this explanation was the true one.

The Sebright Bantam is much less prolific than any other breed of fowls,
and is descended from a cross between two very distinct breeds, recrossed
by a third sub-variety. But it would be extremely rash to infer that the
loss of fertility was in any manner connected with its crossed origin, for
it may with more probability be attributed either to long-continued close
interbreeding, or to an innate tendency to sterility correlated with the
absence of hackles and sickle tail-feathers.

Before giving the few recorded cases of forms, which must be ranked as
varieties, being in some degree sterile when crossed, I may remark that
other causes sometimes interfere with varieties freely intercrossing. Thus
they may differ too greatly in size, as with some kinds of dogs and fowls:
for instance, the editor of the 'Journal of Horticulture, &c.,'[215] says
that he can keep Bantams with the larger breeds without much danger of
their crossing, but not with the smaller breeds, such as Games, Hamburgs,
&c. With plants a difference in the period of flowering serves to keep
varieties distinct, as with the various kinds of maize and wheat: thus
Colonel Le Couteur[216] remarks, "the Talavera wheat, from flowering much
earlier than any other kind, is sure to continue pure." In different parts
of {102} the Falkland Islands the cattle are breaking up into herds of
different colours; and those on the higher ground, which are generally
white, usually breed, as I am informed by Admiral Sulivan, three months
earlier than those on the lowlands; and this would manifestly tend to keep