The obverse of the fact that intercommunication promotes uniformity in language is that lack of communication brings about language differentiation. The less the intercommunication between groups, the more will the languages of the groups differ, however uniform they may be within the groups themselves. The most important factor in differentiation of language is local differentiation. In some European countries every village speaks its own dialect. In passing from one village to another the dialects may be mutually intelligible, but by the time one has passed from the first village in the chain to the last, one may find that the dialect of the first and last are utterly unintelligible to each other. A real break in language, as opposed to dialect variations, occurs where there is a considerable barrier between groups, such as a mountain range, a river, a tribal or political boundary. The more impenetrable the barriers between two groups the more will the languages differ, and the less mutually intelligible will they be. Looking back over the history of language the student of linguistics infers that those languages which bear striking or significant similarities are related. Thus Spanish, Italian, French, Portuguese, and Roumanian are traceable directly back to the Latin. This does not mean that all over the areas occupied by the speakers of these languages Latin was originally spoken. But the Romans in their conquests, both military and cultural, were able to make their own language predominant. The variations which make French and Roumanian, say, mutually unintelligible, are due to the fact that Latin was for the natives in these conquered territories assimilated to their own languages. So that, in the familiar example, the Latin "homo" becomes "uomo" in Italian, "homme" in French, "hombre" in Spanish, and "om" in Roumanian. Similarly related but mutually unintelligible languages among the American Indians have been traced to three great source-languages. The history of European languages offers an interesting example of differentiation. English and German, for example, are both traceable back to West-Germanic; from that in turn to a hypothecated primitive West-Germanic. All the European languages are traceable back to a hypothecated Primitive Indo-European.[1] The theory held by most students of this subject is that the groups possessing this single uniform language spread over a wider and wider area, gradually became separated from each other by geographical barriers and tribal affiliations, and gradually (and on the part of individual speakers unconsciously) modified their speech so that slight differences accumulated, and resulted finally in widely different and mutually unintelligible languages.