Relations of persons with persons, and of groups with groups, may be either those of isolation or those of contact. The emphasis in this chapter is placed upon _isolation_, in the next chapter upon _contact_ in a comparison of their effects upon personal conduct and group behavior. Absolute isolation of the person from the members of his group is unthinkable. Even biologically, two individuals of the higher animal species are the precondition to a new individual existence. In man, postnatal care by the parent for five or six years is necessary even for the physiological survival of the offspring. Not only biologically but sociologically complete isolation is a contradiction in terms. Sociologists following Aristotle have agreed with him that human nature develops within and decays outside of social relations. Isolation, then, in the social as well as the biological sense is _relative_, not _absolute_. The term "isolation" was first employed in anthropogeography, the study of the relation of man to his physical environment. To natural barriers, as mountains, oceans, and deserts, was attributed an influence upon the location of races and the movements of peoples and the kind and the degree of cultural contact. The nature and the extent of separation of persons and groups was considered by geographers as a reflex of the physical environment. In biology, isolation as a factor in the evolution and the life of the species, is studied from the standpoint of the animal group more than from that of the environment. Consequently, the separation of species from each other is regarded as the outcome not only of a sheer physical impossibility of contact, but even more of other factors as differences in physical structure, in habits of life, and in the instincts of the animal groups. J. Arthur Thomson in his work on "Heredity" presents the following compact and illuminating statement of isolation as a factor in inheritance. The only other directive evolution-factor that biologists are at all agreed about, besides selection, is isolation--a general term for all the varied ways in which the radius of possible intercrossing is narrowed. As expounded by Wagner, Weismann, Romanes, Gulick, and others, isolation takes many forms--spatial, structural, habitudinal, and psychical--and it has various results.