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.
CHAPTER 2.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 OF THE GOOD EFFECTS DERIVED FROM CROSSING, AND ON THE EVIL EFFECTS OF 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.
CHAPTER 2.XVIII.--ON THE ADVANTAGES AND DISADVANTAGES OF CHANGED CONDITIONS OF LIFE: STERILITY FROM VARIOUS CAUSES.
ON THE GOOD DERIVED FROM SLIGHT CHANGES IN THE CONDITIONS OF LIFE--STERILITY FROM CHANGED CONDITIONS, IN ANIMALS, IN THEIR NATIVE COUNTRY AND IN MENAGERIES--MAMMALS, BIRDS, AND INSECTs--LOSS OF SECONDARY SEXUAL CHARACTERS AND OF INSTINCTS--CAUSES OF STERILITY--STERILITY OF DOMESTICATED ANIMALS FROM CHANGED CONDITIONS--SEXUAL INCOMPATIBILITY OF INDIVIDUAL ANIMALS--STERILITY OF PLANTS FROM CHANGED CONDITIONS OF LIFE--CONTABESCENCE OF THE ANTHERS-- MONSTROSITIES AS A CAUSE OF STERILITY--DOUBLE FLOWERS--SEEDLESS FRUIT-- STERILITY FROM THE EXCESSIVE DEVELOPMENT OF THE ORGANS OF VEGETATION--FROM LONG-CONTINUED PROPAGATION BY BUDS--INCIPIENT STERILITY THE PRIMARY CAUSE OF DOUBLE FLOWERS AND SEEDLESS FRUIT.
CHAPTER 2.XIX.--SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS ON HYBRIDISM.
ON THE EFFECTS OF CROSSING--THE INFLUENCE OF DOMESTICATION ON FERTILITY--CLOSE INTERBREEDING--GOOD AND EVIL RESULTS FROM CHANGED CONDITIONS OF LIFE-- VARIETIES WHEN CROSSED NOT INVARIABLY FERTILE--ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND VARIETIES--CONCLUSIONS WITH RESPECT TO HYBRIDISM-- LIGHT THROWN ON HYBRIDISM BY THE ILLEGITIMATE PROGENY OF HETEROSTYLED PLANTS-- STERILITY OF CROSSED SPECIES DUE TO DIFFERENCES CONFINED TO THE REPRODUCTIVE SYSTEM--NOT ACCUMULATED THROUGH NATURAL SELECTION--REASONS WHY DOMESTIC VARIETIES ARE NOT MUTUALLY STERILE--TOO MUCH STRESS HAS BEEN LAID ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND CROSSED VARIETIES-- CONCLUSION.