are _secondary_. They differ from the primary in many respects. They are slower, because the resistance is that of humanity to humanity, and they are violent, because dispossession is the object. They are partial, abortive, followed by the fusion of different populations, or followed by their extermination as the case may be."[297] This passage, written so long ago as 1841, is still applicable to the facts of modern science, and there is only to add to it that the migration of man from a common centre, where life was easy, to all parts of the world, where life has been difficult, must have been undertaken in order to meet some great necessity, and must have become possible by reason of some great force which man alone possessed. The necessity was economic; the force was social development. If the movement has not been geographically ever forward, it has been ethnographically constant.[298] Movement always; sometimes the pressure has come from one direction, sometimes from another; sometimes it has caused compression and at other times expansion; sometimes it has sent humanity to inhabit regions that required generations of victims before it could hold its own. At all times the essential condition of life has been that of constant movement in face of antagonistic forces.[299] In whatever form the movement has come about, movement of a very definite character has taken place over an immense period of time, and sufficient to cover practically the whole earth with descendants from the original human stock. This conclusion is enormously strengthened by the accumulating evidence for the world-wide area covered by the remains of man's earliest weapon, the worked stone implement. It is everywhere. It is practically co-extensive with man's wanderings, and the greatness of the territory it covers marks it off as another of the universal relics of man's primitive life. Of no other weapon or instrument or associated object can this be said. The bow and arrow are unknown to the Australians and other peoples; pottery is unknown to the Bushmen and other peoples; the use of fire in cookery is not found among the South Sea Islanders, and is not claimed for other peoples.[300] We can get behind the development of these and other arts and come upon the ruder people who had not arrived at the stage they represent. But we cannot get behind the worked flint. It must have been the chief material cause of man's success in the migratory movement, and with the social development accompanying it must have made migration not only possible, but the only true method of meeting the earliest economic difficulties. It also provides us with the elements of a chronological basis. Behind palæolithic times there is an immensity of time when man struggled with his economic difficulties and spread out slowly and painfully. During palæolithic times the movement was more rapid and more general. Obstacles were overcome by palæolithic man becoming superior to his