Broderick MacNeil munched pleasantly on a banana-pear that evening, happily unaware that three of his buddies had died of eating that self-same fruit. * * * * * The chemist, Dr. Petrelli, looked at the fruit in his hand, snarled suddenly, and smashed it to the floor. Its skin burst, splattering pulp all over the gray plastic. "It looks," he said in a high, savage voice, "as if that hulking idiot will be the only one left alive when the ship returns!" He turned to look at Smathers, who was peering through a binocular microscope. "Smathers, what makes him different?" "How do I know?" growled Dr. Smathers, still peering. "There's something different about him, that's all." Petrelli forcibly restrained his temper. "Very funny," he snapped. "Not funny at all," Smathers snapped back. "No two human beings are identical--you know that." He lifted his gaze from the eyepiece of the instrument and settled in on the chemist. "He's got AB blood type, for one thing, which none of the volunteers had. Is that what makes him immune to whatever poison is in those things? I don't know. "Were the other three allergic to some protein substance in the fruit, while MacNeil isn't? I don't know. "Do his digestive processes destroy the poison? I don't know. "It's got something to do with his blood, I think, but I can't even be sure of that. The leucocytes are a little high, the red cell count is a little low, the hemoglobin shows a little high on the colorimeter, but none of 'em seems enough to do any harm. "It might be an enzyme that destroys the ability of the cells to utilize oxygen. It might be _anything_!" His eyes narrowed then, as he looked at the chemist. "After all, why haven't you isolated the stuff from the fruit?" "There's no clue as to what to look for," said Petrelli, somewhat less bitingly. "The poison might be present in microscopic amounts. Do you know how much botulin toxin it takes to kill a man? A fraction of a milligram!" Smathers looked as though he were about to quote the minimum dosage, so Petrelli charged on: "If you think anyone could isolate an unknown organic compound out of a--" "Gentlemen! _Please!_" said Dr. Pilar sharply. "I realize that this is a strain, but bickering won't help. What about your latest tests on MacNeil, Dr. Smathers?" "As far as I can tell, he's in fine health. And I can't understand why," said the physician in a restrained voice. Pilar tapped one of the report sheets. "You mean the vitamins?" "I mean the vitamins," said Smathers. "According to Dr. Petrelli, the fruits contain neither A nor B_1. After living solely on them for four weeks now, he should be beginning to show some deficiencies--but he's not. "No signs?" queried Dr. Pilar. "No symptoms?" "No signs--at least no abnormal ones. He's not getting enough protein, but, then, none of us is." He made a bitter face. "But he has plenty of symptoms."