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serve their existence. Such is the scientific solution of the problem of evil which has been so long discussed without reach- ing any satisfactory answer. It is not a moral problem at all, but a biological or psychological one, and is exceedingly simple. To live is to suffer, as the pessimists assert, but to the sociologist the problem is how to minimize the amount of suffering and magnify the volume of life. He is on strictly scientific ground. The problem is a practical one, and although the complete abo- lition of pain, like that of friction in machinery, is in the nature of things impossible, still, approaches toward it, in the one case as in the other, may be and are continually made.
These two innate tendencies or impulses of human nature, to escape destructive influences and to seek nutritive substances, constitute the preservative forces of society. They are universal, invariable, and reliable, quite as much so as the physical agencies with which mechanical science deals. Sociology must build upon them as physics builds upon the laws of gravitation, heat, light, or electricity, and only thus can sociology become a science.
Forces of Race Continuance. There is no difference in the principle underlying the preservative and the reproductive for- ces. Independently of the fact above referred to that the latter in the last analysis prove to be only a mode of the former, we see that the law of advantage must secure the one as much as the other. In all the higher forms of animal life, and emphatically in man, the reproductive force is, like the preservative, an appetite, and its strength is as much greater than other appetites as the function is more imperative. It is equally universal, invar- iable, and reliable, and upon it as a true natural force sociology can build with perfect safety.
Under the influence of intellectual development, which, as we saw in the fourth paper, is attended by a corresponding increase in man's sympathetic nature and in his esthetic tastes which shape his ideals, this mainspring of race preservation becomes spiritualized and permeates society in the form of a refining and ennobling influence, which, although far more
