Having thus considered some of the essential features of migration as a whole, it will be well to distinguish further between the four great types of migrations to which reference has been made. One of the earliest, simplest, and most natural of migratory movements is the invasion. This occurs when a rude people, on a low stage of culture, but with much native physical virility, leaves its location, and overruns the territory of a more highly developed state. It is a movement _en masse_, involving the whole, or a large portion, of the tribe. The tribe acts as a unit, and the end sought is the benefit of the tribe as a tribe, not of any individuals. The forces back of it approach the unconscious and irrational, characteristic of wandering, more closely than in any other form of migration. The power of the invasion lies in brute force and numbers. It is a case of a lower civilization temporarily overcoming a higher one—temporarily, because the rude virility which enables the invaders to maintain their own customs for a time succumbs eventually to the enervating influence of a civilization to which it is not trained. Civilization in the end proves itself more permanent than barbarism. This result is often furthered by the fact that the physical stock of the higher race is improved by the infusion of new blood from the very foreigners who are attacking it. This effect upon the physical stock may be very profound and lasting, as an invasion customarily involves large numbers of people. But while the invaders may succeed in checking the progress of civilization for a time, they seldom leave any permanent monuments of themselves, either material or institutional. They are not likely to affect the language, religion, or social customs of the invaded nation to an important degree. The mores are more enduring than the racial stock of the people who possess them.