Soon the health of mankind began to be affected. Accustomed through long ages to breathe air rich in oxygen and poor in carbon dioxide, men suffered. Only those who lived on high plateaus or on tall mountaintops remained unaffected. The plants of the earth, though nourished and increasing in size beyond those ever seen before, were unable to dispose of the continually increasing flood of carbon dioxide. * * * * * By the middle of the twenty-first century it was generally recognized that a new carboniferous period was about to take place, when the earth's atmosphere would be thick and humid, unbreathable by man, when giant grasses and ferns would form the only vegetation. When the twenty-first century drew to a close the whole human race began to revert to conditions closely approximating savagery. The low-lands were unbearable. Thick jungles of rank growth covered the ground. The air was depressing and enervating. Men could live there, but it was a sickly, fever-ridden existence. The whole population of the earth desired the high lands and as the low country became more unbearable, men forgot their two centuries of peace. They fought destructively, each for a bit of land where he might live and breathe. Then men began to die, men who had persisted in remaining near sea-level. They could not live in the poisonous air. The danger zone crept up as the earth-fissures tirelessly poured out their steady streams of foul gas. Soon men could not live within five hundred feet of sea level. The low-lands went uncultivated, and became jungles of a thickness comparable only to those of the first carboniferous period. Then men died of sheer inanition at a thousand feet. The plateaus and mountaintops were crowded with folk struggling for a foothold and food beyond the invisible menace that crept up, and up-- These things did not take place in one year, or in ten. Not in one generation, but in several. Between the time when the chemists of the International Geophysical Institute announced that the proportion of carbon dioxide in the air had increased from .04 per cent to .1 per cent and the time when at sea-level six per cent of the atmosphere was the deadly gas, more than two hundred years intervened. Coming gradually, as it did, the poisonous effects of the deadly stuff increased with insidious slowness. First the lassitude, then the heaviness of brain, then the weakness of body. Mankind ceased to grow in numbers. After a long period, the race had fallen to a fraction of its former size. There was room in plenty on the mountaintops--but the danger-level continued to creep up.