Number of species and plants measured.--Tables given.--Preliminary remarks on the offspring of plants crossed by a fresh stock.--Thirteen cases specially considered.--The effects of crossing a self-fertilised plant either by another self-fertilised plant or by an intercrossed plant of the old stock.--Summary of the results.--Preliminary remarks on the crossed and self-fertilised plants of the same stock.--The twenty-six exceptional cases considered, in which the crossed plants did not exceed greatly in height the self-fertilised.--Most of these cases shown not to be real exceptions to the rule that cross-fertilisation is beneficial.--Summary of results.--Relative weights of the crossed and self-fertilised plants. CHAPTER VIII. DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS. Greater constitutional vigour of crossed plants.--The effects of great crowding.--Competition with other kinds of plants.--Self-fertilised plants more liable to premature death.--Crossed plants generally flower before the self-fertilised.--Negative effects of intercrossing flowers on the same plant.--Cases described.--Transmission of the good effects of a cross to later generations.--Effects of crossing plants of closely related parentage.--Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions. CHAPTER IX. THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS. Fertility of plants of crossed and self-fertilised parentage, both lots being fertilised in the same manner.--Fertility of the parent-plants when first crossed and self-fertilised, and of their crossed and self-fertilised offspring when again crossed and self-fertilised.--Comparison of the fertility of flowers fertilised with their own pollen and with that from other flowers on the same plant.--Self-sterile plants.--Causes of self-sterility.--The appearance of highly self-fertile varieties.--Self-fertilisation apparently in some respects beneficial, independently of the assured production of seeds.--Relative weights and rates of germination of seeds from crossed and self-fertilised flowers. CHAPTER X. MEANS OF FERTILISATION. Sterility and fertility of plants when insects are excluded.--The means by which flowers are cross-fertilised.--Structures favourable to self-fertilisation.--Relation between the structure and conspicuousness of flowers, the visits of insects, and the advantages of cross-fertilisation.--The means by which flowers are fertilised with pollen from a distinct plant.--Greater fertilising power of such pollen.--Anemophilous species.--Conversion of anemophilous species into entomophilous.--Origin of nectar.--Anemophilous plants generally have their sexes separated.--Conversion of diclinous into hermaphrodite flowers.--Trees often have their sexes separated. CHAPTER XI. THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.