Earth is the densest of all the planets, only Venus approaching it in this respect. But Venus has no satellite, and the Earth-Moon system forms a double world of a type found nowhere else among the planets. Its mode of formation is a mystery still, but it is known that when Earth was molten the Moon circled at only a fraction of its present distance, and raised gigantic tides in the plastic substance of its companion.

As a result of these internal tides, the crust of the Earth is rich in heavy metals—far richer than that of any other of the planets: They hoard their wealth far down within their unreachable cores, protected by pressures and temperatures that guard them from man’s depredations. So as human civilization spread outward from Earth, the drain on the mother world’s dwindling resources steadily increased.

The light elements existed on the other planets in unlimited amounts, but such essential metals as mercury, lead, uranium, platinum, thorium and tungsten were almost unobtainable. For many of them no substitutes existed; their large-scale synthesis was impractical, despite two centuries of effort—and modern technology could not survive without them.

It was an unfortunate situation, and a very galling one for the independent republics on Mars, Venus and the larger satellites, which had now united to form the Federation. It kept them dependent upon Earth, and prevented their expansion toward the frontiers of the solar system. Though they had searched among the asteroids and moons, among the rubble left over when the worlds were formed, they had found little but worthless rock and ice. They must go cap in hand to the mother planet for almost every gram of a dozen metals that were more precious than gold.

That in itself might not have been serious, had not Earth grown steadily more jealous of its offspring during the two hundred years since the dawn of space travel. It was, thought Sadler, an old, old story, perhaps its classic example being the case of England and the American colonies. It has been truly said that history never repeats itself, but historical situations recur. The men who governed Earth were far more intelligent than George the Third; nevertheless, they were beginning to show the same reactions as that unfortunate monarch.