ON THE INCREASED FERTILITY OF UNCROSSED ANIMALS AND PLANTS FROM DOMESTICATION
AND CULTIVATION.

The domesticated races of both animals and plants, when crossed, are, with
extremely few exceptions, quite prolific,--in some cases even more so than the
purely-bred parent-races. The offspring, also, raised from such crosses are
likewise, as we shall see in the following chapter, generally more vigorous
and fertile than their parents. On the other hand, species when crossed, and
their hybrid offspring, are almost invariably in some degree sterile; and here
there seems to exist a broad and insuperable distinction between races and
species. The importance of this subject as bearing on the origin of species is
obvious; and we shall hereafter recur to it.

It is unfortunate how few precise observations have been made on the fertility
of mongrel animals and plants during several successive generations. Dr. Broca
(16/1. 'Journal de Physiolog.' tome 2 1859 page 385.) has remarked that no one
has observed whether, for instance, mongrel dogs, bred inter se, are
indefinitely fertile; yet, if a shade of infertility be detected by careful
observation in the offspring of natural forms when crossed, it is thought that
their specific distinction is proved. But so many breeds of sheep, cattle,
pigs, dogs, and poultry, have been crossed and recrossed in various ways, that
any sterility, if it had existed, would from being injurious almost certainly
have been observed. In investigating the fertility of crossed varieties many
sources of doubt occur. Whenever the least trace of sterility between two
plants, however closely allied, was observed by Kolreuter, and more especially
by Gartner, 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