The first of these alternatives is an almost inevitable source of trouble and danger to the state in which the circumstances exist. The danger is twofold. It may arise from the desire of those within the frontier to attain to the position enjoyed by their kinsmen outside it, or from the sympathy of the latter with those of their race who are in subjection to aliens. Such considerations are not the mere abstractions of political philosophy, but are plain, practical questions which have called for practical solution from men to whom the very idea of political philosophy may never have been presented. They appear in actual history in a concrete form, and are therefore hard to recognize amid the numerous factors which are involved in the complicity of human action. #ETHNIC FRONTIERS.# The very use of such modern terms as “scientific” or “ethnic” frontier in reference to policies of ancient date has a tendency to create the idea that the person using them is mistakenly attributing to the past conceptions which are essentially of modern date. But are such conceptions of modern date? Is it not rather the form in which they are expressed which is of recent creation? The very creation of such a form of expression indicates, indeed, that the conception itself has become clearer in course of time; it rarely, if ever, indicates the actual birth of the conception. It is quite possible to grossly exaggerate the difference between the nature of the problems presented to the rulers of empire in the sixth century before Christ, and those which call for solution at the present time. It is not difficult to illustrate from both ancient and modern history:— (1) The dangers of a frontier which is not ethnic; (2) The appreciation of this practical fact by those who have had to face the circumstances. The circumstances present themselves in a complex form, consequently the traceable motive which induces those in power to deal with them is never simple. It is necessary therefore for the student of history to abstract from the complex motive that part with which he wishes specially to deal; and that must be done in the present as in every other instance in which human motive is in question. The problem of the ethnic frontier was one which presented itself at every stage of the making of the Roman Empire. It was perhaps most strikingly illustrated in the relations with the Gauls and Germans.