CHAPTER XII.

GENERAL RESULTS.

Cross-fertilisation proved to be beneficial, and self-fertilisation injurious.--Allied species differ greatly in the means by which cross-fertilisation is favoured and self-fertilisation avoided.--The benefits and evils of the two processes depend on the degree of differentiation in the sexual elements.--The evil effects not due to the combination of morbid tendencies in the parents.--Nature of the conditions to which plants are subjected when growing near together in a state of nature or under culture, and the effects of such conditions.--Theoretical considerations with respect to the interaction of differentiated sexual elements.--Practical lessons.--Genesis of the two sexes.--Close correspondence between the effects of cross-fertilisation and self-fertilisation, and of the legitimate and illegitimate unions of heterostyled plants, in comparison with hybrid unions.

INDEX.

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THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM.