Flowers which are partially or wholly sterile unless visited by insects in the proper manner, such as those of most species of Salvia, of Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less completely to produce seeds if the bees confine their visits to the perforations. The perforated flowers of those species, which are capable of fertilising themselves, will yield only self-fertilised seeds, and the seedlings will in consequence be less vigorous. Therefore all plants must suffer in some degree when bees obtain their nectar in a felonious manner by biting holes through the corolla; and many species, it might be thought, would thus be exterminated. But here, as is so general throughout nature, there is a tendency towards a restored equilibrium. If a plant suffers from being perforated, fewer individuals will be reared, and if its nectar is highly important to the bees, these in their turn will suffer and decrease in number; but, what is much more effective, as soon as the plant becomes somewhat rare so as not to grow in crowded masses, the bees will no longer be stimulated to gnaw holes in the flowers, but will enter them in a legitimate manner. More seed will then be produced, and the seedlings being the product of cross-fertilisation will be vigorous, so that the species will tend to increase in number, to be again checked, as soon as the plant again grows in crowded masses.
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.
The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious. This is shown by the difference in height, weight, constitutional vigour, and fertility of the offspring from crossed and self-fertilised flowers, and in the number of seeds produced by the parent-plants. With respect to the second of these two propositions, namely, that self-fertilisation is generally injurious, we have abundant evidence. The structure of the flowers