h increased their fertility; and in all cases
this may have occurred indirectly, from the better chance of the more
numerous offspring produced by the more fertile individuals having
survived. But with cats, ferrets, and dogs, and with plants like carrots,
cabbages, and asparagus, which are not valued for their prolificacy,
selection can have played only a subordinate part; and their increased
fertility must be attributed to the more favourable conditions of life
under which they have long existed.

* * * * *

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CHAPTER XVII.

ON THE GOOD EFFECTS OF CROSSING, AND ON THE EVIL EFFECTS OF CLOSE
INTERBREEDING.

DEFINITION OF CLOSE INTERBREEDING--AUGMENTATION OF MORBID
TENDENCIES--GENERAL EVIDENCE ON THE GOOD EFFECTS DERIVED FROM CROSSING,
AND ON THE EVIL EFFECTS FROM CLOSE INTERBREEDING--CATTLE, CLOSELY
INTERBRED; HALF-WILD CATTLE LONG KEPT IN THE SAME
PARKS--SHEEP--FALLOW-DEER--DOGS--RABBITS--PIGS--MAN, ORIGIN OF HIS
ABHORRENCE OF INCESTUOUS MARRIAGES--FOWLS--PIGEONS--HIVE-BEES--PLANTS,
GENERAL CONSIDERATIONS ON THE BENEFITS DERIVED FROM CROSSING--MELONS,
FRUIT-TREES, PEAS, CABBAGES, WHEAT, AND FOREST-TREES--ON THE INCREASED
SIZE OF HYBRID PLANTS, NOT EXCLUSIVELY DUE TO THEIR STERILITY--ON
CERTAIN PLANTS WHICH EITHER NORMALLY OR ABNORMALLY ARE SELF-IMPOTENT,
BUT ARE FERTILE, BOTH ON THE MALE AND FEMALE SIDE, WHEN CROSSED WITH
DISTINCT INDIVIDUALS EITHER OF THE SAME OR ANOTHER SPECIES--CONCLUSION.

The gain in constitutional vigour, derived from an occasional cross between
individuals of the same variety, but belonging to distinct families, or
between distinct varieties, has not been so largely or so frequently
discussed, as have the evil effects of too close interbreeding. But the
former point is the more important of the two, inasmuch as the evidence is
more decisive. The evil results from close interbreeding are difficult to
detect, for they accumulate slowly, and differ much in degree with
different species; whilst the good effects which almost invariably follow a
cross are from the first manifest. It should, however, be clearly
understood that the advantage of close interbreeding, as far as the
retention of character is concerned, is indisputable, and often outweighs
the evil of a slight loss of constitutional vigour. In relation to the
subject of domestication, the whole question is of some importance, as too
close interbreedi