“If it works, our race is saved.” “That’s how they felt about it. We found that raw gryxon was instant death to humans over twenty-five. Those under twenty-five were immune unless they came from stock that matured early--as the Mediterranean peoples. They succumbed before twenty-five. That was what really gave us our answer. We knew that ankylosis--joining of certain spinal vertebrae--occurred earlier in humans that matured earlier--and that age twenty-five was the time for the joinder to occur. So we turned to the serums that had increased human life spans by retarding ankylosis--and developed this one.” They studied Mona’s face again. * * * * * Was it possible that the yellow tinge was stayed--was turned to waxen ivory? The girl’s breast rose and fell--ever so gently. “I concealed our serum,” Rod went on slowly, “from the tetrarchs by burying it in my arm. I had to get a hypo needle before I could use it here.” “Have I had a fever, Rod?” Mona’s grey eyes were open. “Or is this delirium?” Rod Harrow crossed to her swiftly. He cradled her head in his arms. She was smiling again--relaxed. Then he turned to Williamson. “Line ’em up, Bill. With five liters we can inoculate every human on Tetrarch IV. There’ll never be a gryxon glow again. “And as soon as a company has had all men injected, send them to the cave to bring back the reserves. We’ll need their help.” “To celebrate?” Williamson wanted to know. “Not particularly.” Rod was impatient. “We’ll have to do some planet cleaning.” “Planet cleaning!” “Right! We’re ready now to wipe the tetrarchs off our Earth.”