Nevertheless, the task of summarising and explaining is one to which the writer of a History of the World must address himself. If he has the disadvantage of limited space, he has the advantage of being able to assume the reader’s knowledge of what has gone before, and to invite the reader’s attention to what will come after. Thus he stands in a better position than does the writer who deals with one country or one epoch only for making each part of history illustrate other parts, for showing how similar social tendencies, similar proclivities of human nature, work similarly under varying conditions and are followed by similar, though never identical, results. He is able to bring out the essential unity of history, expunging from the reader’s mind the conventional and often misleading distinctions that are commonly drawn between the ancient, the mediæval, and the modern time. He can bring the contemporaneous course of events in different countries into a fruitful relation. And in the case of the present work, which dwells more especially on the geographical side of history, he can illustrate from each country in succession the influence of physical environment on the formation of races and the progress of nations, the principles which determine the action of such environment being everywhere similar, though the forms which that action takes are infinitely various. Is there, it may be asked, any central thread in following which the unity of history most plainly appears? Is there any process in tracing which we can feel that we are floating down the main stream of the world’s onward movement? If there be such a process, its study ought to help us to realise the unity of history by connecting the development of the numerous branches of the human family. One such process has already been adverted to and illustrated. It is the gradual and constant increase in man’s power over Nature, whereby he is emancipated more and more from the conditions she imposes on his life, yet is brought into an always closer touch with her by the discovery of new methods of using her gifts. Two other such processes may be briefly examined. One goes on in the sphere of time, and consists in the accumulation from age to age of the strength, the knowledge, and the culture of mankind as a whole. The other goes on in space as well as in time, and may be described as the contraction of the world, relatively to man. [Sidenote: The Great Increase of Population]