Insects visit the flowers of the same species as long as they can.--Cause of this habit.--Means by which bees recognise the flowers of the same species.--Sudden secretion of nectar.--Nectar of certain flowers unattractive to certain insects.--Industry of bees, and the number of flowers visited within a short time.--Perforation of the corolla by bees.--Skill shown in the operation.--Hive-bees profit by the holes made by humble-bees.--Effects of habit.--The motive for perforating flowers to save time.--Flowers growing in crowded masses chiefly perforated. 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. ... THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM. CHAPTER I. INTRODUCTORY REMARKS. Various means which favour or determine the cross-fertilisation of plants. Benefits derived from cross-fertilisation. Self-fertilisation favourable to the propagation of the species. Brief history of the subject. Object of the experiments, and the manner in which they were tried. Statistical value of the measurements. The experiments carried on during several successive generations. Nature of the relationship of the plants in the later generations. Uniformity of the conditions to which the plants were subjected. Some apparent and some real causes of error. Amount of pollen employed. Arrangement of the work. Importance of the conclusions.