Species were selected for experiment belonging to widely distinct families, inhabiting various countries. In some few cases several genera belonging to the same family were tried, and these are grouped together; but the families themselves have been arranged not in any natural order, but in that which was the most convenient for my purpose. The experiments have been fully given, as the results appear to me of sufficient value to justify the details. Plants bearing hermaphrodite flowers can be interbred more closely than is possible with bisexual animals, and are therefore well-fitted to throw light on the nature and extent of the good effects of crossing, and on the evil effects of close interbreeding or self-fertilisation. The most important conclusion at which I have arrived is that the mere act of crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution, owing to their progenitors having been subjected during several generations to slightly different conditions, or to what we call in our ignorance spontaneous variation. This conclusion, as we shall hereafter see, is closely connected with various important physiological problems, such as the benefit derived from slight changes in the conditions of life, and this stands in the closest connection with life itself. It throws light on the origin of the two sexes and on their separation or union in the same individual, and lastly on the whole subject of hybridism, which is one of the greatest obstacles to the general acceptance and progress of the great principle of evolution. In order to avoid misapprehension, I beg leave to repeat that throughout this volume a crossed plant, seedling, or seed, means one of crossed PARENTAGE, that is, one derived from a flower fertilised with pollen from a distinct plant of the same species. And that a self-fertilised plant, seedling, or seed, means one of self-fertilised PARENTAGE, that is, one derived from a flower fertilised with pollen from the same flower, or sometimes, when thus stated, from another flower on the same plant. CHAPTER II. CONVOLVULACEAE. Ipomoea purpurea, comparison of the height and fertility of the crossed and self-fertilised plants during ten successive generations. Greater constitutional vigour of the crossed plants. The effects on the offspring of crossing different flowers on the same plant, instead of crossing distinct individuals. The effects of a cross with a fresh stock. The descendants of the self-fertilised plant named Hero. Summary on the growth, vigour, and fertility of the successive crossed and self-fertilised generations. Small amount of pollen in the anthers of the self-fertilised plants of the later generations, and the sterility of their first-produced flowers. Uniform colour of the flowers produced by the self-fertilised plants. The advantage from a cross between two distinct plants depends on their differing in constitution.