Among the obscure problems in the history of mankind a foremost place must be given to the question of the origin and distribution of the various races of men. Undoubtedly two main factors in the differentiation of these races have been climate and geography. These same physical conditions still perpetuate, if they did not actually originate, the racial distinctions. Even where we may hesitate to adopt a theory as to the initial start of any one of the great branches of mankind, we can hardly fail to recognise that the several nations or tribes comprised within one of these branches are marked off from each other by more or less definite peculiarities, of which some at least may probably be referred to the influence of environment. The landscapes of a country, the form, height, and trend of its mountain-ranges, the position and extent of its plains and valleys, the size and direction of its rivers, the varying nature of its soils and climate, the presence or absence of useful minerals, nearness to or distance from the sea, the shape of the coast-line whether rocky or precipitous, or indented with creeks and harbours--all these and other aspects of the scenery of the land have contributed their share to the moulding of national history and character. For illustrations of this external influence we need go no further than the British Isles, and contrast the aspect and history of central England with those of the Scottish Highlands.[2] Some of these dominant elements in our surroundings remain permanently with little or no appreciable change. The mountains and the plains are now essentially what they were in the infancy of man, though their mantle of vegetation may have been greatly altered. In other cases, geological processes are continually at work, effecting changes which, though individually of small account, become important in the course of centuries by constant repetition. Thus on some parts of our coast-line there has been a great destruction of land, in others the land has gained on the sea. But man himself has become a geological agent, and has in that capacity greatly modified the surface of many of the countries which he has inhabited. The progress of agriculture has led to the draining of mosses, the felling of forests and the transformation of heaths and wastes into arable land. The increase of sheep and cattle has been the means of clothing the hills with pasture, in place of their former rough herbage and copsewood. And man himself becomes involved in the consequences of the changes which he sets in motion. This subject may obviously be discussed independently either from the scientific or from the historical side. Any investigation which deals with the changes that have taken place in the outward aspect of a country since man first set foot upon its surface, and with the sources of information regarding them, must obviously appeal strongly to the lover of science, inasmuch as it brings before him the