Whatever social problem we confront, whatever persons come into our field of view, the first questions involved will always be: To what groups do these persons belong? What are the interests of these groups? What sort of means do the groups use to promote their interests? How strong are these groups, as compared with groups that have conflicting interests? These questions go to one tap root of all social interpretation, whether in the case of historical events far in the past, or of the most practical problems of our own neighborhood. 2. The Unity of the Social Group[91] It has long been a cardinal problem in sociology to determine just how to conceive in objective terms so very real and palpable a thing as the continuity and persistence of social groups. Looked at as a physical object society appears to be made up of mobile and independent units. The problem is to understand the nature of the bonds that bind these independent units together and how these connections are maintained and transmitted. 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.