Even when individuals of closely allied species mate with one another it is a very rare occurrence that the hybrids so produced ripen their ova and sperms so as to be capable of carrying on the hybrid race, though sometimes they do ripen them and breed. The great naturalist Alfred Wallace, in his most valuable and readable book called "Darwinism," expressed the opinion that the apparent failure of hybrid races to perpetuate themselves by breeding was to a large extent due to the small number of individuals used in experiments on this matter, and the in-and-in breeding which was the consequence. One of the great generalizations established by Darwin is that in-and-in breeding is, as a rule, resisted in all animals and plants, and leads when it occurs to a dying-out of the inbred race by resulting feebleness and infertility. A large part of Darwin's work consisted in demonstrating the devices existing in the natural structure and qualities of plants and animals for securing cross-fertilization among individuals of the same species but of different stock. Both extremes seem to be barred in nature–namely, the inter-breeding of stocks so diverse in structure and quality as to be what we call "distinct species," and again the inter-breeding of individuals of the same immediate parentage or near cousinship. What seems to be favoured by the natural structure and qualities of the plant or the animal is that it shall only breed within a certain group–the species–and shall within that group avoid constant self-fertilization or fertilization by near cousins. Thus we find numerous cases in which, though the same flower has both pollen and ovules, and might fertilize itself, the visits of insects (specially made use of by mechanisms in the flower) carry the pollen of one flower to the ovules of another and to flowers on separate plants growing at a distance. It is necessary to note that there are, nevertheless, self-fertilizing flowers, and also self-fertilizing lower animals, the special conditions of which require and have received careful examination and consideration, upon which I cannot now enter.