It is usually assumed that, while human conditions alter throughout the ages, natural surroundings remain sensibly the same. This may be true with regard to people whose history is only affected by the streams which cross their land and the hills and mountains which protect them by natural barriers. When dealing with a country like Belgium however, widely open on all sides, we cannot be content with such wide generalizations. We must ask ourselves if some important physical features have not been altered by the work of man and if some natural obstacles, which have since disappeared, did not affect the earlier stage of Belgian history. The traveller who crosses the country to-day from Ostend to Arlon will at once recognize its main features: first a low-lying plain, between the sea and Brussels, then a district of smooth hills, as far as Namur, and finally, beyond the Meuse, the deeply cut valleys and high plateaux of the Ardennes, reaching an average of 1,500 feet above sea-level. In this last region only will the aspect of the country suggest to him the idea of some natural obstacle to free communications, though it could in no way appear forbidding when compared to the mountains of Scotland and Wales. But at the time of the Roman conquest (57 _B.C._), Belgium, that is to say the country peopled by various tribes designated by Julius Caesar under the name of "Belgae," was very different from what it is to-day. The flat coast, unprotected against the incursions of the sea, was bordered by wide marshes, while all the southern part of the country was covered by a thick forest, the "Silva Carbonaria," which merged in the wild plateaux of the Ardennes and formed, at the time, a serious obstacle to any incursion coming from the north or the east. These physical conditions must have favoured the guerrilla warfare waged for four years by the various Celtic tribes against the Roman invader, and it is no doubt partly to them that the old "Belgae" owed their reputation of courage and fortitude. These tribes, occupying the Scheldt and Meuse valleys, formed the rearguard of the Celtic wave of invasion which, coming from the East, had spread across Western Europe. At the time of the Roman conquest they were already closely pressed by a vanguard of Germanic tribes which had settled in Zeeland and on the left bank of the Rhine, so that even at this early stage of Belgium's history we find the dualistic character of Belgian civilization marked in the division of the country into two Roman provinces, "Belgica Secunda," in the west, and "Germania Inferior," in the east. [_ROMAN INFLUENCES_]