Man, whose ancestors crawled out from the salt water, remains still a creature of the sea-margin, his habitat the low plains. A thousand feet is a small fraction of the ocean’s depth; yet, if the ocean were suddenly to rise a thousand feet, man would be largely destroyed. Of Europe would remain only some mountainous islands; the United States would fare not much better. Mexico City, cupped among its mountains, would survive as the largest center of habitation in the world.

Living thus upon the plains, man is upon neutral ground between the mountains and the ocean. The torrents from the hills grow quiet, and let fall their silt. The great slow rivers here and there cut into the banks, and elsewhere build up the flood-plains or thrust forward the long fingers of their deltas into the sea.

Man allies himself now with the mountains, now with air and water. Like the beaver, he builds dams and retaining walls. Like the sheep, he strips the earth and cuts trailways. He protects his habitations against the water, so that through the centuries the level of a city rises foot by foot. In the main, swayed by immediate need and convenience, he remains through the long course of time careless of the struggle, planless.

A father was out walking with his son, and they came to a small stream.

“Why is the water so muddy?” asked the little boy.

“It means the soil is washing away,” said the father. “The government is sending a lot of men out to build dams and stop it.”

But the brown stream was sign and symbol of a great conflict. Its present cycle would not be completed within a hundred million years, before which time man would very likely have run his course and vanished. By then the sky-towering crags would be reduced to gnawed stumps of granite, and a stormless climate, as in the Eocene, would cover most of the earth.

2

On the higher mountains snow was falling. Far beneath the surface the shrunken streams flowed in dim tunnels arched by snow; deep ground-waters fed them. The summer-darting trout were sunk in lassitude, half-hibernating. While the drifts covered the high country, it could suffer little erosion; that was for the time of thaw, when melting snow loosed the torrents.

On the foothills rain was falling. Brown water flowed in the gullies; it ate at the cut-banks; it foamed in the narrows.