It must be remembered that at this time men had still to spread into enormous areas of the land surface of the globe. There were vast mountain wildernesses, forest wildernesses, sandy deserts, and frozen lands. Men still clung closely to water and arable soil in temperate or sub-tropical climates, they lived abundantly only in river valleys, and all their great cities had grown upon large navigable rivers or close to ports upon the sea. Over great areas even of this suitable land flies and mosquitoes, armed with infection, had so far defeated human invasion, and under their protection the virgin forests remained untouched. Indeed, the whole world even in its most crowded districts was filthy with flies and swarming with needless insect life to an extent which is now almost incredible. A population map of the world in 1950 would have followed seashore and river course so closely in its darker shading as to give an impression that _homo sapiens_ was an amphibious animal. His roads and railways lay also along the lower contours, only here and there to pierce some mountain barrier or reach some holiday resort did they clamber above 3000 feet. And across the ocean his traffic passed in definite lines; there were hundreds of thousands of square miles of ocean no ship ever traversed except by mischance. Into the mysteries of the solid globe under his feet he had not yet pierced for five miles, and it was still not forty years since, with a tragic pertinacity, he had clambered to the poles of the earth. The limitless mineral wealth of the Arctic and Antarctic circles was still buried beneath vast accumulations of immemorial ice, and the secret riches of the inner zones of the crust were untapped and indeed unsuspected. The higher mountain regions were known only to a sprinkling of guide-led climbers and the frequenters of a few gaunt hotels, and the vast rainless belts of land that lay across the continental masses, from Gobi to Sahara and along the backbone of America, with their perfect air, their daily baths of blazing sunshine, their nights of cool serenity and glowing stars, and their reservoirs of deep-lying water, were as yet only desolations of fear and death to the common imagination.