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.