If the first and most important of these sources is the discovery of the conditions of physical well-being, the second is the discovery of means of communication between the widely separate portions of man's kingdom. The record of the process of bringing the world under the control of the organized government of man is largely the record of the improvement of communications. Side by side with the unending struggle of human reason against cold and hunger and disease we can watch the contest against distance, against ocean and mountain and desert, against storms and seasons. There can be few subjects more fascinating for a historian to study than the record of the migrations of the tribes of men. He might begin, if he wished, with the migrations of animals and describe the westward progress of the many species whose course can be traced by experts along the natural highways of Western Europe. Some of them, so the books tell us, reached the end of their journey while Britain was still joined to the continent. Others arrived too late and were cut off by the straits of Dover. I like to form an imaginary picture, which the austerity of the scientific conscience will, I know, repudiate with horror, of the unhappy congregation, mournfully assembled bag and baggage on the edge of the straits and gazing wistfully across at the white cliffs of England, which they were not privileged to reach--_tendentesque manus ripae ulterioris amore_, 'stretching out their paws in longing for the further bank.'