. . . Civilized human societies must be ... in a continuous process of readjustment. Progress is the very law of their being: and if the ruling classes in any society attempt to enforce a policy of standing still, there is bound to be trouble. The only way to avert social revolution, as Turgot declares, is through suitable and well-timed reforms. The surest way to bring on a revolution, on the other hand, is for the ruling classes to attempt to preserve an order of society which no longer works well. ... - Ellwood, The Social Problem, p. 34.

Change is not desirable for its own sake. The heritage of the past is infinitely precious. Some things appear to be settled once for all, or a thousand times for all. Yet change is indispensable if there is to be progress. . . . And men will cease to be men when they are so intimidated by prestige and so bribed or drugged by interest that they will not lift a hand for faith and hope and love - faith in humanity which has martyrs and mothers as well as tyrants and sycophants, hope for humanity which has a future far longer than its past and full of ever-accelerating movement, love of humanity which suffers needless woes and is rich with possibilities as yet unfulfilled. - American Journal of Sociology, Vol. 23, p. 120, Hayes, "The Horrors of Respectability."

In describing the manner in which progress comes about the sociologists adopt to some extent the language and ideas of the biologists. The title of this chapter, and also of the next, are biological terms the meanings of which have permeated sociological thought. The array of technical terminology may seem appalling, but the reader is urged to persevere in the mastery of it.