in the third sub-division, which was comparative, dealing in this manner with data previously adduced. Varenius did not himself take up the special part of geography, though he defined it as also falling into three sub-divisions, the first of which should deal with the position, boundaries, physical features, and natural products of a country, the second with celestial and atmospheric conditions, and the third with the human element. This last was (as it still is) a popular branch of the subject which, for his own part, he would not have admitted; he held pure geography to be a matter apart from political and social considerations. But here was a geographical framework which, as H. R. Mill has pointed out, “was capable of accommodating itself to new facts, and was indeed far in advance of the knowledge of the period.” Being so, its worth was by no means generally recognized at first, although in 1672 Sir Isaac Newton put forth an annotated English edition for use in connection with lectures of his own. “The method included a recognition of the causes and effects of phenomena, as well as the mere fact of their occurrence, and for the first time the importance of the vertical relief of the land was fairly recognized.” The work found its place in time; the French and Dutch geographers, as well as the English, had it in their own tongues, and it became a standard, not only for the century of its original production, but for the next also. CHAPTER XI. THE NINETEENTH CENTURY: AFRICAN RESEARCH Since the later years of the eighteenth century geographical knowledge has been extended in the manner of a great railway system. The main lines of exploration will provide the subject of this and following chapters; with the ramification of branch lines we can hardly concern ourselves here. Taking one consideration with another, Africa may be termed the most important area of geographical conquest during this latest period of our history. The opening of the interior of that continent was long delayed for geographical reasons which have often been insisted upon. The difficulty of inland communication, the fact that the rivers do not offer uninterrupted highways, the barrier of the tropical forests, the unhealthiness of many parts both of the coast lands and of the interior, which modern science is only now fighting--such are the disabilities against which exploration had to contend, to which must be added the lack of commercial instinct in many of the native peoples, and their unhappy experiences of the early slave-trading, and the labour-recruiting which has in some instances provided its modern counterpart.