For the preservation of the species there must be reproduction. With mammals, and also with many lower forms of life, this is provided for by the sex instinct. This instinct, when supplemented by parental love, gives rise to that most ancient and unchanging and omnipresent form of society - the family. Of all the crude impulses of the brute which civilization must tame, the sex instinct is the one which causes the most trouble.

Reduced to their lowest terms, which still show large overlapping spheres of influence, the comprehensive and absorbing situations become "play," "food," and "family." In terms of the instinctive habits primitive man may be defined as a playing, feeding, family-bred-and-breeding animal. - Jastrow, Character and Temperament, p. 129.

In the beginning were interests.

The primary interest of every man, as of every animal, is in sheer keeping alive.

... In this group the sex-interest is usually made coordinate with the food-interest, and it is doubtful if there is a third approaching these in importance. I venture to call all the other positive types of bodily interest by the general name the work-interests. Whether this is a good designation or not, I mean by it all the impulses to physical prowess and skill, that vary from the pranks of childhood to systematized trial of skill among athletes. The three species of interest which I call food, sex, and work make up one genus of human interests, to which I give the name the health-interest. By this I mean all the human desires that have their center in exercise and enjoyment of the powers of the body. - Small, General Sociology, pp. 196, 197.

Family life developed two other inborn tendencies which play a large part in society, namely, gregariousness and kindness. Most psychologists class them as instincts, or combinations of instincts.