trouble with test animals,” Dr. Pilar said, ruffling his gray beard with a fingertip. “You take a rat, for instance. A rat can live on a diet that would kill a monkey. If there’s no vitamin A in the diet, the monkey dies, but the rat makes his own vitamin A; he doesn’t need to import it, you might say, since he can synthesize it in his own body. But a monkey can’t.

“That’s just one example. There are hundreds that we know of and God alone knows how many that we haven’t found yet.”

Fennister settled his own body more comfortably in the chair and scratched his head thoughtfully. “Then, even after a piece of alien vegetation has passed all the animal tests, you still couldn’t be sure it wouldn’t kill a human?”

“That’s right. That’s why we ask for volunteers. But we haven’t lost a man so far. Sometimes a volunteer will get pretty sick, but if a food passes all the other tests, you can usually depend on its not killing a human being.”

“I gather that this is a pretty unusual case, then?”

Pilar frowned. “As far as I know, yes. But if something kills all the test animals, we don’t ask for humans to try it out. We assume the worst and forget it.” He looked musingly at the wall. “I wonder how many edible plants we’ve by-passed that way?” he asked softly, half to himself.

“What are you going to do next?” the colonel asked. “My men are getting hungry.”

Smathers looked up from the report in alarm, and Pilar had a similar expression on his face.

“For Pete’s sake,” said Smathers, “don’t tell anyone—not anyone—about this, just yet. We don’t want all your men rushing out in the forest to gobble down those things until we are more sure of them. Give us a few more days at least.”

The colonel patted the air with a hand. “Don’t worry. I’ll wait until you give me the go-ahead. But I’ll want to know your plans.”

Pilar pursed his lips for a moment before he spoke. “We’ll check up on MacNeil for another forty-eight hours. We’d like to have him transferred over here, so that we can keep him in isolation. We’ll feed him more of the … uh … what’d he call ’em, Smathers?”

“Banana-pears.”

“We’ll feed him more banana-pears, and keep checking. If he is still in good shape, we’ll ask for volunteers.”

“Good enough,” said the colonel. “I’ll keep in touch.”

On the morning of the third day in isolation, MacNeil rose early, as usual, gulped down his normal assortment of vitamins, added a couple of aspirin tablets, and took a dose of Epsom salts for good measure. Then he yawned and leaned back to wait for breakfast. He was certainly getting enough fresh fruit, that was certain. He’d begun to worry about whether he was getting a balanced diet—he’d heard that a balanced diet was very important—but he figured that the doctors knew what they were doing. Leave it up to them.

He’d been probed and needled and tested plenty in the last couple of days, but he didn’t mind it. It gave him a feeling of confidence to know that the doctors were taking care of him. Maybe he ought to tell them about his various troubles; they all seemed like nice guys. On the other hand, it wouldn’t do to get booted out of the Service. He’d think it over for a while.

He settled back to doze a little while he waited for his breakfast to be served. Sure was nice to be taken care of.

Later on that same day, Dr. Pilar put out a call for volunteers. He still said nothing about MacNeil; he simply asked the colonel to say that it had been eaten successfully by a test animal.

The volunteers ate their banana-pears for lunch, approaching them warily at first, but soon polishing them off with gusto, proclaiming them to have a fine taste.

The next morning, they felt weak