Taken in the aggregate or average, this human subject is more or less variable; chiefly, no doubt, under a rule of selective conservation of favorable variations.
The selection of favorable variations is perhaps in great measure a selective conservation of ethnic types.

In the life history of any community whose population is made up of a mixture of divers ethnic elements, one or another of several persistent and relatively stable types of body and of temperament rises into dominance at any given point.

The situation, including the institutions in force at any given time, will favor the survival and dominance of one type of character in preference to another; and the type of man so selected to continue and to further elaborate the institutions handed down from the past will in some considerable measure shape these institutions in his own likeness.
But apart from selection as between relatively stable types of character and habits of mind, there is no doubt simultaneously going on a process of selective adaptation of habits of thought within the general range of aptitudes which is characteristic of the dominant ethnic type or types.
There may be a variation in the fundamental character of any population by selection between relatively stable types; but there is also a variation due to adaptation in detail within the range of the type, and to selection between specific habitual views regarding any given social relation or group of relations.
For the present purpose, however, the question as to the nature of the adaptive process--whether it is chiefly a selection between stable types of temperament and character, or chiefly an adaptation of men's habits of thought to changing circumstances--is of less importance than the fact that, by one method or another, institutions change and develop.
Institutions must change with changing circumstances, since they are of the nature of an habitual method of responding to the stimuli which these changing circumstances afford.

The development of these institutions is the development of society.

The institutions are, in substance, prevalent habits of thought with respect to particular relations and particular functions of the individual and of the community; and the scheme of life, which is made up of the aggregate of institutions in force at a given time or at a given point in the development of any society, may, on the psychological side, be broadly characterized as a prevalent spiritual attitude or a prevalent theory of life.