So the historian accepts his limitations as the rules of the game, and sets out to see unity askance. It is his rare chance, if events shift _him_, and set him gazing at a world in which, as now, half his own career is inside the picture; not perhaps very easy to find in a moment--as one might fail to recognize oneself in a group-photograph--but none the less there, and intelligible only in relation to its actual surroundings. Looking back, indeed, over the course of anthropology and prehistoric archaeology, much of which lies in the years since 1870, and nearly all of it since 1815, the first thing which strikes us now is the frequency and delicacy of its response to contemporary thoughts and aspirations. A few of the greatest men have recognized this at the time. I quote from Karl Ernst von Baer, the founder of comparative embryology, and in great matters the master of men as different as Huxley, Spencer, and Francis Balfour. He died in 1876, when political anthropology was still young; but in his great book on Man he 'appeals to the experience of all countries and ages, that if a people has power, and attempts wrongdoing against another, it also does not omit to conceive the other as very worthless and incompetent, and to repeat this conviction often and emphatically' (_Der Mensch_, ii. 235). It is easy for us to dot the _i_ and cross the _t_ here; less easy perhaps to realize that what troubled von Baer was the persistence of British and American ethnologists in the polygenist heresy, which he traced (and rightly) to their reluctance to treat their 'black brother' as if he were their relative at all. Judgement in that ethnological controversy went by default, with the victory of the North in the American Civil War; and in 1871 the lion lay down with the lamb, even in London; inveterate foes in the Ethnological Society and the Anthropological merging their fate in one Anthropological Institute. In 1915 the reluctance of the 'tall fair people who come from the north'--I borrow a phrase from Professor Ridgeway--to fraternize with mere brunettes, beyond Rhine and Danube, comes in its turn before the same tribunal as polygenism in 1862. Our subject, 'Unity in Prehistoric Times', embraces three main topics: (1) the unity of human effort and reason everywhere in Man's struggle with Nature and with his Fellow-man; (2) the special conditions which favoured or hindered unity of prehistoric culture in what has been called elsewhere the 'north-west quadrant' of the Old-World land-mass west of Ararat and the Median hills and north of Sahara, the cradle and nursery of the modern 'western world'; and (3) the convergent lines of advancement within that region, which can be traced through the centuries before Roman policy let Greek culture penetrate almost as deep into peninsular Europe as Alexander's conquests had opened to it the inlands of the Near East.