And Mars clutched at it. A small space rocket was built with all speed. Earth, long eyed with anticipation by believers in the practicality of flight between the worlds, was chosen as the objective. Krai, an acknowledged expert in atmospheric travel and the problems confronting a space traveler, found himself selected to make the voyage. His job was to discover whether the Earth was suitable for Martian settlement, or if it could be made so. Astronomical observation had made this fairly certain, however, and that part of his task was merely one of confirmation. But he was also to ascertain whether there was any intelligent life upon the world. If this proved to be the case and the opportunity presented itself, he was to wipe it out. It was ruthless, of course, but the Martians were facing death themselves and had to be ruthless. They couldn't take a chance on being attacked by Earth's inhabitants before they were accustomed to the new conditions and could fight back without being at a disadvantage. The Martian felt a deep satisfaction as he made his way back to his space ship. There might still remain much to be done, but there was no doubt in his mind about the future now--a bright future and long, new life for Mars.... But Krai might have felt very differently had he seen the hate-filled eyes that pursued his retreating figure. One of the brute-men of the arboreal community had been absent on a private hunting expedition when the Martian began hostilities, only returning in time to see the demon (as Krai appeared to the savage mind) leaving the scene of destruction. The tree-dweller's first thought was of his mate. But prudence dictated that he remain concealed till the retreating form of the demon disappeared among the trees. Then he searched frantically for his mate. Eventually he found her--crushed among the splinters of their tree-house. A terrible, burning hate took hold of the savage. An all-consuming hate overcame his instinctive fear of the demon-thing he had seen. Thoughts of revenge crowded all else from his mind. Then, with quick resolve, the tree-man took to the trees in pursuit of the demon. His hyper-sharp sense of smell soon enabled him to pick up the trail of his quarry. Presently the forest began to thin out, giving way to open plain. Nearing the edge of the trees, the eagle eyes detected movement, then the grotesquely alien figure of the demon-thing came into view. Krai was in the open now. With a spurt of speed, accelerated by white-hot hate, the pursuer reached the edge of the forest--too late. * * * * * The Martian was nearly to his ship. Another moment and he would be safe. The brute-man hated the ground and feared the incredible powers of the demon-thing too much to risk physical combat. There was only one chance.