Now, the whole significance of these changes, physical and mental, is fundamentally sexual and social. Human life, it may be said, has a twofold aspect. As a mere animal organism, there is the perpetuation of the species, which nature secures by the mere force of the sex impulse. As a human being, he is part of a social structure, cell in the social tissue, to use Leslie Stephen's expressive phrase. And in this direction nature secures what is necessary by the presence of impulses and cravings as imperious as, and even more permanent than, those of mere sex. Of course, in practice these two things operate together. By a process of selection, the anti-social character is weeded out, and the two sets of feelings work together in harmony for the furtherance and the development of the life of the species. The species is perpetuated in the interests of society; society is perpetuated in the interests of the species. Further, it is part of the natural 'plan' that there shall be developed impulses and capacities suitable to each phase of life as it emerges. Thus it has been shown that the lengthening of infancy--that is, the prolongation of the time during which the young human being is dependent upon its parents for support and protection--is nature's method of developing to a greater degree the capacity of the human animal for more complex adjustment. Instead of being launched on the world with a number of instincts practically fully developed, and so capable of attending to its own needs almost as soon as born, man is born with few instincts, and a great capacity for education enabling him to adjust his conduct to the demands of an environment constantly increasing in complexity. In the same way it has been shown that the instinct for play, practically universal throughout the whole of the animal world, is nature's method of preparing the young for the more serious business of nature.[152] It is, therefore, only in line with what is found to be true elsewhere that the changes incident to puberty should receive their rational interpretation in the necessities of social life. That these necessities should be met largely by the play of unreasoning impulse is, again, quite in line with what occurs in other directions. The insistent pressure of social life for thousands of generations secures the emergence of needs of the true nature of which the individual may be ignorant. In no other way, in fact, could the persistence of the species and of human society be secured.