Human migration in general may be described as a natural function of social development. It has taken place at all times and in the greatest variety of circumstances. It has been tribal, national, class and individual. Its causes have been political, economic, religious, or mere love of adventure. Its causes and results are fundamental for the study of ethnology, of political and social history, and of political economy. Under the form of conquest it makes the grand epochs in history (e.g. the fall of the Roman Empire); under the form of colonization it has transformed the world (e.g. the settlement of America); under free initiative it is the most powerful factor in social adjustment (e.g. the growth of urban population). It must suffice here to indicate the character of the principal movements in the past, and then describe certain aspects of modern migration

The movement of population, however, has continued under the form of immigration/emigration.

taken from the parts of the 1911 Encyclopedia