"Well, every particle of the material which composes this planet, contains radioactivity of some sort; and we long ago discovered a way to release it and use it. One pound of solid granite yields enough energy to—well, a great deal of power."
They had now been flying for two hours, and still no end to that thickly-housed, ever different appearance of the ground. Also, although they saw a great many birds, they noted no animals. Finally, Billie could hold in no longer.
"Are we to understand," she demanded of Estra, "that the whole of this planet is as densely populated as we see it?"
"Just that," replied the Venusian. "Why not? The roof makes our climate uniform from pole to pole, while our buildings are such that, whether on land or on sea, they are equally livable."
"But—Estra!" expostulated the girl. "Venus is nearly as big as the earth. And it looks to be as thickly populated as—as Rhode Island! Why, you must have a colossal population; let me see." And she scribbled away in her memorandum book.
But both Smith and the doctor had already worked it out. They looked up, blinking dazedly.
"Over three hundred billion," murmured the doctor, as though dizzy.
The Venusian checked Smith's correction with, "You dropped one cipher, doctor. There are three and a half trillion of us!"
"Good lord!" whispered Van Emmon, all his antagonism gone for the moment. And again the explorers were silent for a long time.
By and by, however—"We have just seen what it meant, there on Mercury," said the doctor, in a low voice, "for the principle of 'the survival of the fit' to be carried to its logical end; for who is to decide what is fitness, save the fittest? One man, apparently, outlived every one else on the planet, and then he also died.