From an early period of the life-history of our planet, the preservation of the species had obviously become quite as imperative an end as the preservation of individuals; one is at first inclined to say more imperative, but if we pause long enough to remember that total failure to preserve individuals would be equivalent to immediate extinction of the species, we see that the one requirement is as indispensable as the other. Individuals must be preserved, and the struggle for life is between them; species must be preserved, and in the rivalry those have the best chance in which the offspring are either most redundant in numbers or are best cared for. In plants and animals of all but the higher types, the offspring are spores or seeds, larvæ or spawn, or self-maturing eggs. In the absence of parental care the persistence of the species is ensured by the enormous number of such offspring. A single codfish, in a single season, will lay six million eggs, nearly all of which perish, of course, or else in a few years the ocean could not hold all the codfishes. But the princess in the Arabian tale, who fought with the malignant Jinni, could not for her life pick up all the scattered seeds of the pomegranate; and in like manner of the codfish eggs, one in a million or so escapes and the species is maintained. But in the highest types of animal life in birds and mammals--with their four-chambered hearts, completely arterialized blood, and enhanced consciousness--parental care becomes effective in protecting the offspring, and the excessive production diminishes. With birds, the necessity of maintaining a high temperature for the eggs leads to the building of nests, to a division of labour in the securing of food, to the development of a temporary maternal instinct, and to conjugal alliances which in some birds last for a lifetime. As the eggs become effectively guarded the number diminishes, till instead of millions there are half a dozen. When it comes to her more valuable products, Nature is not such a reckless squanderer after all. So with mammals, for the most part the young are in litters of half a dozen or so; but in Man, with his prolonged and costly infancy parental care reaches its highest development and concentration in rearing children one by one.