In the Solar System, the greatest single source of radiation, including the hard rays, was the Sun. It was natural that the siren image should seem stronger in that direction, that it should fade or retreat toward its origin. No one had ever returned from compulsive pursuit of the illusionary woman, though in early days radio contact had been made with ships racing toward the Sun. The heredity block was self-enforcing. Deviously, the race protected itself, or something higher watched over it to assure _human_ continuity. Marlowe wasn't sure which, but it was there. "I think you're on the wrong track," he said. "Shield the ship completely and it won't matter how long the trip takes. The crew can work in safety." Demarest grunted. "Some day we'll have an inertia-free drive and it won't matter how much mass we use. It does now. Our designs are a compromise. Both of us have to work with what's possible, not what we dream of. I'll build my ship; you find the right crew to man it." Marlowe went back to his graphs. Machines could be changed, but the human body clung stubbornly to the old patterns. He couldn't select his crews any younger--but was there perhaps a racial type more resistant to radiation? Where? No place that he knew of. Maybe the biologists could produce one, he thought hopefully, and knew he was fooling himself. Human beings weren't fruit flies; by the time enough generations rolled around for the resistant strain to breed true--and leave a surplus to man the ships--he would be long dead and the problem solved. The best of humanity would be dead, too, wiped out by sterilization. Or the Solar System would be peopled by mutant monstrosities. * * * * * Far away, and not concerned with the problem, Ethan shrugged resignedly. "Guess we'll have to get used to the idea--we just won't see him till he grows up--if we'll still be around." "You've got years and years ahead of you, and not worth a thing the whole time!" Amantha snapped. "Damnation," said Ethan wistfully, "I'd like to dandle him." "Won't be the same when he grows up and comes here," Amantha conceded. "There I go agreein' with you! What's got into me?" "Maybe we can get on the next slow ship. They run them once in a while for people with weak hearts." He considered. "Don't know whether Retired Citizens' Home will let us go, though." "Retired Citizens!" She blew her nose scornfully. "They think we don't know it's just a home for the aged!" She threw away the tissue. "Think they'll let us?" "It won't be them so much that'll stop us. Our hearts ain't too good and we haven't got much space time to use. We shouldn't have gone to Venus." "We had to see Edith and Ed and their kids and we had to come back to Mars so we could be near John and Pearl and Ray. Let's not regret what we've done." She picked at the chair arm. "We've been here a long time, ain't we?" Ethan nodded.