But now, something unusual and rather frightening had happened. Something was the matter with the weather. The warm days of summer had come very late. The fruits had not ripened. The tops of the mountains which used to be covered with grass now lay deeply hidden underneath a heavy burden of snow.

Then, one morning, a number of wild people, different from the other creatures who lived in that neighbourhood, came wandering down from the region of the high peaks. They looked lean and appeared to be starving. They uttered sounds which no one could understand. They seemed to say that they were hungry. There was not food enough for both the old inhabitants and the newcomers. When they tried to stay more than a few days there was a terrible battle with claw-like hands and feet and whole families were killed. The others fled back to their mountain slopes and died in the next blizzard.

But the people in the forest were greatly frightened. All the time the days grew shorter and the nights grew colder than they ought to have been.

Finally, in a gap between two high hills, there appeared a tiny speck of greenish ice. Rapidly it increased in size. A gigantic glacier came sliding downhill. Huge stones were being pushed into the valley. With the noise of a dozen thunderstorms torrents of ice and mud and blocks of granite suddenly tumbled among the people of the forest and killed them while they slept. Century old trees were crushed into kindling wood. And then it began to snow.

It snowed for months and months. All the plants died and the animals fled in search of the southern sun. Man hoisted his young upon his back and followed them. But he could not travel as fast as the wilder creatures and he was forced to choose between quick thinking or quick dying. He seems to have preferred the former for he has managed to survive the terrible glacial periods which upon four different occasions threatened to kill every human being on the face of the earth.

In the first place it was necessary that man clothe himself lest he freeze to death. He learned how to dig holes and cover them with branches and leaves and in these traps he caught bears and hyenas, which he then killed with heavy stones and whose skins he used as coats for himself and his family.

Next came the housing problem. This was simple. Many animals were in the habit of sleeping in dark caves. Man now followed their example, drove the animals out of their warm homes and claimed them for his own.

Even so, the climate was too severe for most people and the old and the young died at a terrible rate. Then a genius bethought himself of the use of fire. Once, while out hunting, he had been caught in a forest-fire. He remembered that he had been almost roasted to death by the flames. Thus far fire had been an enemy. Now it became a friend. A dead tree was dragged into the cave and lighted by means of smouldering branches from a burning wood. This turned the cave into a cozy little room.

And then one evening a dead chicken fell into the fire. It was not rescued until it had been well roasted. Man discovered that meat tasted better when cooked and he then and there discarded one of the old habits which he had shared with the other animals and began to prepare his food.

In this way thousands of years passed. Only the people with the cleverest brains survived. They had to struggle day and night against cold and hunger. They were forced to invent tools. They learned how to sharpen stones into axes and how to make hammers. They were obliged to put up large stores of food for the endless days of the winter and they found that clay could be made into bowls and jars and hardened in the rays of the sun. And so the glacial period, which had threatened to destroy the human race, became its greatest teacher because it forced man to use his brain.