The fundamental cause of linguistic change and hence of linguistic diversification is the minute deviations occurring in the transmission of language from one generation to another. But other factors contribute to the historical development of languages and determine the spread of a language family over the world’s surface. Population movements naturally play a large part, and movements of peoples in prehistoric times carried the Indo-European languages from a relatively restricted area into most of Europe and into northern India, Persia, and Armenia. The spread of the Indo-European languages resulted, in the main, from the imposition of the languages on the earlier populations of the territories occupied. In the historical period, within Indo-European, the same process can be seen at work in the Western Roman Empire. Latin superseded the earlier, largely Celtic languages of the Iberian Peninsula and of Gaul (France) not through population replacement (the number of Roman soldiers and settlers in the empire was never large) but through the abandonment of these languages by the inhabitants over the generations as they found in Latin the language of commerce, civilization, law, literature, and social prestige.

Conquest does not always lead to the supersession of a language. Greek survived centuries of Turkish rule and indeed remained a focus of national feeling, as has happened elsewhere in history. Much depends on the various circumstances and on the mutual attitudes of those involved; what must be kept quite clear is the difference between movements of peoples and the spread of languages.