Observe, again, the necessity laid upon us by this law--the necessity of continued evolution to the end. Childhood, beautiful childhood, can not remain--it must quickly pass. If, with the decline of its characteristic faculties, the next higher group characteristic of youth do not increase and become dominant, then the glory of life is already past and deterioration begins. Have we not all seen sad examples of this? Youth, glorious youth, must also pass. If the next higher group of reflective and elaborative faculties do not arise and dominate, then progressive deterioration of character commences here--thenceforward the whole nature becomes coarse, as we so often see in young men, or else shrivels and withers, as we so often see in young women. Finally, manhood, strong and self-relying manhood, must also pass. If the moral and religious sentiments have not been slowly growing and gathering strength all along, and do not now assert their dominance over the whole man, then commences the final and saddest decline of all, and old age becomes the pitiable thing we so often see it. But, if the evolution have been normal throughout; if the highest moral and religious nature have been gathering strength through all, and now dominates all, then the psychic evolution rises to the end--then the course of life is like a wave rising and cresting only at the moment of its dissolution, or, like the course of the sun, if not brightest at least most glorious in its setting. And thus--may we not hope?--the glories of the close of a well-spent life become the pledge and harbinger of an eternal to-morrow? We have thus far illustrated the three laws of succession of organic forms by ontogeny, because this is the type of evolution; but they may be illustrated also by other forms of evolution. Next to the development of the individual, undoubtedly the _progress of society_ furnishes the best illustration of these laws. Commencing with a condition in which each individual performs all necessary social functions, but very imperfectly; in which each individual is his own shoemaker and tailor, and house-builder and farmer, and therefore all persons are socially alike; as society advances, the constituent members begin to diverge, some taking on one social function and some another, until in the highest stages of social organization this diversification or division and subdivision of labor reaches its highest point, and each member of the aggregate can do perfectly but one thing. Thus, the social organism becomes more and more strongly bound together by mutual dependence, and separation becomes mutilation. I do not mean to say that this extreme is desirable, but only that an approach to this is _a_ natural law of social development. _Is not this the law of differentiation?_