CHAPTER I
THE PROBLEM
I stood on the Great Wall of China near the borders of Mongolia. Below me in the valley, standing up square and high, was a wall that had once surrounded a city. Of the city only a few mud houses remained, scarcely enough to lead one's mind back to the time when people and household industry teemed within the protecting wall.
The slope below the Great Wall was cut with gullies of which were fifty feet deep. As far as the eye could see were gullies, gullies, gullies—a gashed and gutted countryside. The little stream that once ran past the city was now a wide waste of coarse sand and gravel which the hillside gullies were bringing down faster than the little stream had been able to carry them away. Hence, the whole valley, once good farm land, had become a desert of sand and gravel, alternately wet and dry, always fruitless. It was even more worthless than the hills.
Beside me was a tree, one lone tree. That tree was locally famous because it was the only tree anywhere in that vicinity; yet its presence proved that once there had been a forest over most of that land—now treeless and waste.
The farmers of a past generation had cleared the forest. They had plowed the sloping land and dotted it with hamlets. Many workers had been busy with flocks and teams, going to and fro among the shocks of grain. Each village was marked by columns of smoke rising from the fires that cooked the simple fare of these sons of Genghis Khan. Year by year the tain has washed away the loosened soil. Now the plow comes not, only the shepherd is here with his sheep and goats—nib-
