frequently
originate from crossing is doubtful; that they occasionally thus arise is
probable; but this subject will be more conveniently discussed in a future
chapter on the causes of Variability.

A condensed summary of this and of the three following chapters, together
with some remarks on Hybridism, will be given in the nineteenth chapter.

* * * * *

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CHAPTER XVI.

CAUSES WHICH INTERFERE WITH THE FREE CROSSING OF VARIETIES--INFLUENCE OF
DOMESTICATION ON FERTILITY.

DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN
CROSSED--VARIOUS CAUSES WHICH KEEP VARIETIES DISTINCT, AS THE PERIOD OF
BREEDING AND SEXUAL PREFERENCE--VARIETIES OF WHEAT SAID TO BE STERILE
WHEN CROSSED--VARIETIES OF MAIZE, VERBASCUM, HOLLYHOCK, GOURDS, MELONS,
AND TOBACCO, RENDERED IN SOME DEGREE MUTUALLY STERILE--DOMESTICATION
ELIMINATES THE TENDENCY TO STERILITY NATURAL TO SPECIES WHEN
CROSSED--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 invariability 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[214] 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 {101} almost
certainly have been observed. In investigating the fertility of crossed
varieties many sources