Putting together the two processes, the process in time and the process in space, which we have been reviewing, it will be seen that the main line of the development of mankind may be described as the transmission and the expansion of culture--that is to say, of knowledge and intellectual capacity. The stock of knowledge available for use and enjoyment has been steadily increased, and what each people accumulated has been made available for all. With this there has come assimilation, the destruction of weaker types of civilisation, the modification by constant interaction of the stronger types, the creation of a common type tending to absorb all the rest. Assimilation has been most complete in the sphere ruled by natural science--that is to say, in the material sphere, less complete in that ruled by the human sciences (including the sphere of political and social institutions), still less complete in the sphere of religious, moral, and social ideas, and as respects the products of literature and art. Or, in other words, where certainty of knowledge is attainable and utility in practice is incontestable, the process of assimilation has moved fastest and furthest. [Sidenote: Nature & the Unity of Mankind] The process has been a long one, for its beginnings reach back beyond our historical knowledge. So far as it lies within the range of history, it falls into two periods, the earlier of which supplies an instructive illustration of the later one which we know better. The effort which Nature--that is to say, the natural tendencies of man as a social being--has been making towards the unification of mankind during the last few centuries, is her second great effort. The first was in progress from the time when the most ancient records begin down to the sixth and seventh centuries of the Christian era. [Illustration: THE FIRST TRAVELLER ROUND THE GLOBE The great exploit of Ferdinand Magellan, who circumnavigated the globe in 1519-1520, ranks among the events of world importance, and was the culminating achievement of the greatest period of discovery in the world’s history. ] Greek civilisation, which itself had drawn much from Egypt, as well as from Assyria, Phœnicia, and the peoples of Asia Minor, permeated the minds and institutions (except the legal institutions), of the Mediterranean and West European countries, and was propagated by the governing energy of the Romans. In its Romanised form it transformed or absorbed and superseded the less advanced civilisations of all those countries, creating one new type for the whole Roman world. With some local diversities, that type prevailed from the Northumbrian Wall of Hadrian to the Caucasus and the deserts of Arabia. The still independent races on the northern frontier of the Empire received a tincture of it, and would doubtless have been more deeply imbued had the Roman Empire stood longer.