Conceived of in its lowest terms the unity of the social group may be compared to that of the plant communities. In these communities, the relation between the individual species which compose it seems at first wholly fortuitous and external. Co-operation and community, so far as it exists, consists merely in the fact that within a given geographical area, certain species come together merely because each happens to provide by its presence an environment in which the life of the other is easier, more secure, than if they lived in isolation. It seems to be a fact, however, that this communal life of the associated plants fulfils, as in other forms of life, a typical series of changes which correspond to growth, decay and death. The plant community comes into existence, matures, grows old, and eventually dies. In doing this, however, it provides by its own death an environment in which another form of community finds its natural habitat. Each community thus precedes and prepares the way for its successor. Under such circumstances the succession of the individual communities itself assumes the character of a life-process.
In the case of the animal and human societies we have all these conditions and forces and something more. The individuals associated in an animal community not only provide, each for the other, a physical environment in which all may live, but the members of the community are organically pre-adapted to one another in ways which are not characteristic of the members of a plant community. As a consequence, the relations between the members of the animal community assume a much more organic character. It is, in fact, a characteristic of animal society that the members of a social group are organically adapted to one another and therefore the organization of animal society is almost wholly transmitted by physical inheritance.
In the case of human societies we discover not merely organically inherited adaptation, which characterizes animal societies, but, in addition, a great body of habits and accommodations which are transmitted in the form of social inheritance. Something that corresponds to social tradition exists, to be sure, in animal societies. Animals learn by imitation from one another, and there is evidence that this social tradition varies with changes in environment. In man, however, association is based on something more than habits or instinct. In human society, largely as a result of language, there exists a conscious community of purpose. We have not merely folkways, which by an extension of that term might be attributed to animals, but we have mores and formal standards of conduct.