The biologists recognize two ways in which variation comes about. Let us start with the commonplace observation that individuals differ; there are no two alike, even children of the same parents, puppies of the same litter, plants from the same seed. Continuous variation is simply the accumulation of these slight differences; they are trifling in amount in any one generation but become important when accumulated in the same direction for many generations. Then there is discontinuous variation or mutation, which is a great change coming all at once in a single individual and breeding true in the descendants. Examples of these two methods in social variation will be given presently.

Two other methods remain to be distinguished. Differentiation is the appearance of differences between individuals of the same species or between societies of the same kind when they are dispersed and thus given opportunity to grow apart by continuous variation. In this way varieties become established which are derived from a common ancestral type.

Agglomeration is a method of social variation corresponding to hybridization in physical organisms. When different varieties come into contact with each other they sometimes give rise to new varieties that are different from the parent stocks.

The most useful discussion of causes of variation is by Ross. He makes this distinction between cause and condition:

. . . The appearance of a new situation is considered to be the effect of the precipitating factor. The ferment, the igniting spark, the touching of the electric button, the knocking away of the stay block, the turning of the lever, is looked upon as the cause of what ensues. The factors already present are termed the conditions, not the causes, of the change. . . . Desire is the steam which drives the machinery of society. It is behind all social activities, beneath all groupings and relationships. Its action is essentially statical. If it produces change, that change is incidental. The causes of social transformation are to be sought, not among desires, but in something of a different nature which changes their direction or modifies the framework within which they operate. ... - Ross, Foundations of Sociology, pp. 189, 193.

He avoids the term "evolution " because it -

... is apt to convey the idea that the series of social changes is the mere unfolding of characters pre-formed in the very germ or bud of society. This idea is misleading and should be avoided. It is unsafe to assume that the succession of social changes is predetermined. ... - Ross, Foundations of Sociology, p. 185.

Ross also avoids the use of other biological terms such as those which have been introduced into the foregoing pages of this chapter, and employs instead a classification and terminology of his own. His own summary of it is given herewith to facilitate comparison. The reader who does not relish the biological terminology should read Ross's entire chapter from which this paragraph is taken:

The causes or factors of social change are statico-dynamic processes, transmutations, and stimuli. Statico-dynamic processes are those ordinary functional activities which leave behind them as by-products cumulative effects capable of causing social change. Transmutations are those x gradual unconscious alterations which occur in consequence of the inability of human beings to reproduce accurately the copy their fathers set them. Stimuli, however, which are those factors of change lying outside of the strictly social sphere, furnish most of the impulses toward social transformation. The principal orders of stimuli are the growth of population, the accumulation of wealth, migration, innovation, the cross-fertilization of cultures, the interaction of groups, the conjugation of societies, and alteration of the environment. - Ross, Foundations of Sociology, p. 254.