The listeners' minds were working rapidly, trying to put things together. "You mean diggers need to rub angel spit on their bodies in order to mate?" asked Dza.

"Not quite," said Shedemei. "The first time we examined the bodies of angels and diggers, we found a little organ-a gland, actually-near the scrotum. It was identical in both species, even though they have no common ancestor with a similar organ. Very puzzling, of course. But we now know the function of the organ. It continuously secrets tiny amounts of a hormone that suppresses the production of sperm. No, let me be clear. It completely shuts down the production of sperm. While the organ is functioning, males are completely, absolutely sterile."

"What a useful little organ," murmured Oykib. Then, louder, "Why would that evolve?"

"It gets worse," said Zdorab.

"There's a tiny flatworm, a microscopic one, that Hves in all the freshwater rivers of this massif. During the rainy season, when the rivers are in flood, this flatworm burrows into beds of firm clay and lays millions of tiny eggs. They don't develop as long as they remain wet. But when the dry season comes and the water subsides, the eggs develop, forming hard littk coatings that hold in what moisture they have. The embryos are ready to hatch at any time. But they can't, because they can't get rid of their own confining shells. So they hibernate, living off their yolks. They use the yolks so slowly that they can live for twenty or thirty years like that. The next rainy season doesn't cause them to hatch, because water doesn't dissolve the shells. Guess what dissolves them."

"Angel spit," said Oykib.

"Amazing boy," said Shedemei. "My prize student." There was some laughter, but they were all waiting for the story to go on. "No other fluid will do it, because the angels have tiny organelles in the saliva-producing cells of their mouths, which secrete an enzyme that has no function whatsoever within the bodies of the angels-but it dissolves the shells of the flatworm eggs. So when the males bring the clay into their mouths, they're not just softening it to make sculptures. They're also dissolving the shells erf" millions of little flatworms. And it just so happens that the dissolved shells contain precisely the one chemical that suppresses the action of the prophylactic gland near the angels' and diggers' scrota. The fertility chemical breaks down very slowly, and the statues contain useful quantities of it perhaps as long as ten years, certainly for five."

Everybody was getting it now. "So when the diggers rubbed the statues all over their bodies. ..." "Were the angels swallowing some of it?" "How much of the fertility chemical does it take?"

Shedemei raised her hands to damp the questions and comments. "Yes, you've got it. The angel males absorb the fertility enzyme by mouth. It doesn't take much of it to shut down the action of the prophylactic gland, and it doesn't recover and start up again for about two weeks, maybe three. So there's a window there in which reproduction can take place. And the digger males have a special absorbent patch on their lower bellies, near the groin, where the chemical can be absorbed quite directly into the bloodstream. Rubbing the statues on their sweating bellies dissolves some of the day, whereupon the dissolved fertility enzyme is taken into the blood and, just as with the angels, it shuts down the prophylactic gland and the digger males are fertile. But because they actually get a great deal less of the enzyme, the fertility window is only a few days long for them. Doesn't matter, though. Where the angels make their statues once a year and have to score a reproductive hit that one time, the diggers are culturally able to worship the statues any time. In effect, the statues enable them to reproduce whenever they want. They just have to pray first."

"That is the most absurdly complicated, unlikely, ridiculous mechanism I've ever heard of," said Issib.

"Exactly," said Shedemei. "There is no chance that it evolved naturally. Why would the diggers and angels independently evolve identical organs that make them sterile? There's no evolutionary advantage in it. Why didn't the angels simply die out before they ever started making their sculptures in the first place? Why didn't the diggers die out before they ever discovered the virtues of rubbing angel statues all over themselves? And why would a species of flatworm just happen to require a special chemical in angel saliva in order to hatch their eggs? And why did the angels develop a chemical with no use in their bodies except to dissolve the shells of the flatworms?"

"There are a lot of strange things in nature," said Oykib.

"Of course," said Shedemei. "I shouldn't have said there's no chance it evolved naturally. It's just that for me, at least, the coincidence is too great to believe in natural causation. This was done to the diggers and angels."

"But that's not what matters right now," said Zdorab. "Shedya has an answer to that, but what matters is that we have to tell the diggers the truth. They need to go bade to using the statues. And getting new chics."

"Maybe we can get the angels just to give their statues to the diggers," said Padarok. "It's not as if the angels use diem after the women judge the men."

"Maybe," »aid Shedemei. "But it's not just the diggers who are suffering from our interference in their previous social patterns. This relationship, this connection between diggers and angels has been going on for millions of years. Forty million years, to be more precise. And in those countless generations, certain patterns have evolved. The twinning of the angels, for instance. Every pregnancy is double. This isn't accidental. It's only happened twice in all our observations, and never in our own angel village, but when a single birth takes place, the baby is destroyed and the mother is never allowed to mate again. In other words, single births are ruthlessly exduded from angel society. It looks to me as if this is a response to the feet that diggers follow angels wherever they go. The diggers have to follow the angels in order to get the statues. But then the diggers can't help but see the angels as an easy source of meat, especially when the angel infants are at that awkward age when they can't fty well at all, and yet they're too heavy for one adult to carry them alone and still fly. In effect, the twinning allows each generation of angels to have one death and one survivor. Over the years, community cooperation has enabled between two-thirds and three-fourths of the twin-pairs to survive intact. Now, though, in our village all the twins are living to adulthood. And all the damaged, weak, sick, and crippled angels are surviving, where in other villages the diggers cull them out. In short, the angels have evolved a strategy of reproducing far beyond their sustainaWe population in order to survive the depredations of the diggers. When the diggers no longer prey on them, their population balloons out of control"

"It's a delicate balance," Zdorab saki "I found one place where it reached a crisis. The diggers had lost discipline and were eating more than the infants and the strays. They were systematically wiping out the angels in their area. When I got there, only a few beleaguered families of angels survived. But the diggers were already paying the price. They had plenty of old statues, of course.

But no new ones. And so after about five years, their birthrate fell off. Just as it has here. Not so suddenly, of course, since they were still worshipping the statues they had, only the statues contained progressively less and less of the enzyme. Births became more and more rare. With fewer diggers to prey on them, the angel population was going to start recovering soon. When it did, eventually the few surviving diggers would also recover."

Shedemei took over again. "So there's a social balance, you see. The diggers can't harvest too many of the angels for food, because then they lose their own ability to reproduce. It's self-correcting."