Apart from these conjectures as to particular features in the life of prehistoric man, it will be seen that we have now a broad and firm conception of its evolution. From the ape-level man very slowly mounts to the stage of human savagery. During long ages he seems to have made almost no progress. There is nothing intrinsically progressive in his nature. Let a group of men be isolated at any stage of human evolution, and placed in an unchanging environment, and they will remain stationary for an indefinite period. When Europeans began to traverse the globe in the last few centuries, they picked up here and there little groups of men who had, in their isolation, remained just where their fathers had been when they quitted the main road of advance in the earlier stages of the Old Stone Age. The evolution of man is guided by the same laws as the evolution of any other species. Thus we can understand the long period of stagnation, or of incalculably slow advance. Thus, too, we can understand why, at length, the pace of man toward his unconscious goal is quickened. He is an inhabitant of the northern hemisphere, and the northern hemisphere is shaken by the last of the great geological revolutions. From its first stress emerges the primeval savage of the early part of the Old Stone Age, still bearing the deep imprint of his origin, surpassing his fellow-animals only in the use of crude stone implements. Then the stress of conditions relaxes--the great ice-sheet disappears--and again during a vast period he makes very little progress. The stress returns. The genial country is stripped and impoverished, and the reindeer and mammoth spread to the south of Europe. But once more the adversity has its use, and man, stimulated in his hunt for food, invigorated by the cold, driven into social life, advances to the culmination of the Old Stone Age.