Migrations, with all the incidental collision, conflicts, and fusions of peoples and of cultures which they occasion, have been accounted among the decisive forces in history. Every advance in culture, it has been said, commences with a new period of migration and movement of populations. Present tendencies indicate that while the mobility of individuals has increased, the migration of people has relatively decreased. The consequences, however, of migration and mobility seem, on the whole, to be the same. In both cases the "cake of custom" is broken and the individual is freed for new enterprises and for new associations. One of the consequences of migration is to create a situation in which the same individual-who may or may not be a mixed blood-finds himself striving to live in two diverse cultural groups. The effect is to produce an unstable character-a personality type with characteristic forms of behavior. This is the "marginal man." It is in the mind of the marginal man that the conflicting cultures meet and fuse. It is, therefore, in the mind of the marginal man that the process of civilization is visibly going on, and it is in the mind of the marginal man that the process of civilization may best be studied.