His teeth were large and looked like the teeth of many of our own animals. During the hours of day this primitive human being went about in search of food for himself and his wife and his young. At night, frightened by the noise of the beasts, who were in search of prey, he would creep into a hollow tree or he would hide himself behind a few big boulders, covered with moss and great, big spiders. In summer he was exposed to the scorching rays of the sun. During the winter he froze with cold. When he hurt himself (and hunting animals are for ever breaking their bones or spraining their ankles) he had no one to take care of him. He had learned how to make certain sounds to warn his fellow-beings whenever danger threatened. In this he resembled a dog who barks when a stranger approaches. In many other respects he was far less attractive than a well-bred house pet. Altogether, early man was a miserable creature who lived in a world of fright and hunger, who was surrounded by a thousand enemies and who was for ever haunted by the vision of friends and relatives who had been eaten up by wolves and bears and the terrible sabre-toothed tiger. Of the earliest history of this man we know nothing. He had no tools and he built no homes. He lived and died and left no traces of his existence. We keep track of him through his bones and they tell us that he lived more than two thousand centuries ago. The rest is darkness. Until we reach the time of the famous Stone Age, when man learned the first rudimentary principles of what we call civilization. Of this Stone Age I must tell you in some detail. THE WORLD GROWS COLD Something was the matter with the weather. Early man did not know what "time" meant. He kept no records of birthdays and wedding-anniversaries or the hour of death. He had no idea of days or weeks or years. When the sun arose in the morning he did not say "Behold another day." He said "It is Light" and he used the rays of the early sun to gather food for his family. When it grew dark, he returned to his wife and children, gave them part of the day's catch (some berries and a few birds), stuffed himself full with raw meat and went to sleep. In a very general way he kept track of the seasons. Long experience had taught him that the cold Winter was invariably followed by the mild Spring--that Spring grew into the hot Summer when fruits ripened and the wild ears of corn were ready to be plucked and eaten. The Summer ended when gusts of wind swept the leaves from the trees and when a number of animals crept into their holes to make ready for the long hibernal sleep. [Illustration: THE GLACIAL PERIOD.] It had always been that way. Early man accepted these useful changes of cold and warm but asked no questions. He lived and that was enough to satisfy him. Suddenly, however, something happened that worried him greatly.