I tried to stifle a yawn. I knew all this

Dr. Fletcher looked over at me. "We'll get to the good parts in a minute, Lieutenant. Try to stay awake until then, if you can." I blushed embarrassedly and straightened in my seat.

Dr. Fletcher continued, "If the same processes of evolution have held true on Chtorr, then the planet should have evolved a particularly nasty and competitive food chain-and so far, that's exactly what we've seen.

"Using our own ecology as a model-the only ecology we have to base a model on-we know that the process of evolution is a process of continually adding links to the food chain. Reptiles evolved out of fish to eat fish-and then each other. Mammals evolved out of reptiles to eat reptiles and fish-and each other.

"What comes after mammals? And what comes after that? And after that? And whatever comes after that-presumably, that's what rules Chtorr. Whatever it is, it has to be at the top of its food chain.

"That was the initial hypothesis. I'll give you a minute to think about that. The implications are interesting. Dr. Abbato did his homework on this one."

Dr. Fletcher studied her notes for a moment, then looked back up with a smile. "The first one is that a sentient species has to be at the top of its food chain. It can't be otherwise. Think about it. New forms always arise to feed on the old. What else can they feed on? The high levels have to be predatory. And predators are the life-forms most likely to develop intelligence. You're probably all familiar with Dr. Cohen's famous remark, `Intelligence develops first in predators. After all, how much brains does it take to sneak up on a blade of grass?"'

There were polite chuckles. It was an old joke.

But Dr. Fletcher wasn't interested in laughs. She pushed on. "It's pretty clear that the higher level the predator, the higher its capacity for intelligence. Carrying that one step further, we think that sentience is most likely to develop in a top-level omnivore." Dr. Fletcher allowed herself an impish smile. "We recognize, of course, that we may be biased in that assumption-because we are the only proof of it we have.

"But, we think it is also what we will find with the Chtorran sentient Chtorran species-when we meet it. We expect that they will be nothing less than the most cunning and sophisticated predator of all Chtorran life-forms. And, of course, that implies-given the half-billion-year evolutionary advantage of the Chtorr-that the primary perception that sentience can have of us, specifically our ecology, will be as prey. Food. Another kind of snack. At best, lunch.

"In fact, given that half-billion-year advantage, the rest of the Chtorran ecology can be expected to operate the same way. We are nothing more than fuel for them-and probably not even very efficient fuel, at least not as efficient as they're probably used to, which is probably why they need to burn so much of it-us. As a matter of fact, the Chtorran ecology has demonstrated a voraciousness that is nothing less than stunning. Of course, that also suggests that the Chtorran ecology has to generate a prodigious supply of life-support to fuel its primary species.

"So, given all of that, we have been making the assumptionand so did Dr. Abbato-that the Chtorran species we've seen so far are just the advance guard of a much greater invasion still to come. The assumption is that whatever agency or sentience is responsible for the infestation depends on these creatures for lifesupport-and that we are not going to see the arrival of the next level of infestation until such time as this life-support level is safely and solidly established. As a matter of fact, our whole war effort has been geared not toward eradication-because we don't yet have the resources or the knowledge necessary for that, perhaps someday-but toward destabilizing the interrelationships of the infestation. Finally, that brings us back to Dr. Abbato. And the questions he was left with.

"Dr. Abbato wondered, `If all of these assumptions are indeed true, then what is the purpose of the gastropedes in the Chtorran food chain? What function do they serve?"'

I wondered if she had an answer-and if we'd get to it today. I snuck a glance at my watch.

"This is one of those questions that seems very innocuous, until you get into it-then you find out it's actually a major paradigm-shifter. It's forcing us to rethink everything we know, so pay attention here. You too, Lieutenant-"

She didn't miss a thing! No more front row seats.

"We've been assuming that the worms are at the top of the Chtorran food chain-that is, this particular subset of it. We've not yet found a worm predator. Considering the voraciousness of the worms, I'm not sure we want to see a next step. We still don't know how to cope with this one. But, if the worms actually are at the top of the food chain, then they would also have to be the sentient species-and so far there's no evidence of that. In fact, there's quite a bit of evidence to the contrary. So, we're pretty sure that the worm predator has not yet shown up or established itself. Which brings us back to Dr. Abbato's question. What are the worms?

"As a matter of fact, the worms seem to be something of an anomaly even in their own ecology. For instance, what do the worms feed on?"

They eat people, I answered. But I didn't say it aloud.

"We can't identify a prey species," Dr. Fletcher said. "Yes, we've seen the worms eating millipedes and other Chtorran life-forms-that's to be expected-but for the most part, the worms have been feeding on the host planet ecology: cattle, sheep, horses, dogs, and humans, unfortunately.

"We've analyzed the protein requirements of an average-size worm and measured it against the amount of millipedes and other Chtorran life-forms it would have to consume to generate that amount of protein, and the ratio is simply unworkable. The worms can't eat enough millipedes and shambler bushes and libbits to survive. These Chtorran life-forms are simply not high enough on the chain to be the primary food source for the worms. The worms are not the predators for these Chtorran species, and these species are not the prey for the worms. If this is a food chain as we understand it, then there are links missing from this food chain!

"And that brings us to this very important question: if the worms are supposed to be predators, then where-or what-are the creatures they are supposed to prey on?

"Dr. Abbato has advanced a very interesting hypothesis-albeit an unpleasant one-that we are the intended prey."

Huh? I sat up straight.

Dr. Fletcher had paused to let the murmurs of her audience to die down. She looked out over the room. Abruptly, she pointed at someone behind me. "Yes, you have a question?"

I turned around in my seat to look. It was a tall man in an army uniform. A grim-looking colonel. He had a tight mouth. I wondered, do colonels get special training to master that expression? He asked, "Can you prove that?"

Dr. Fletcher nodded and rubbed the side of her neck thoughtfully. She looked as if she were debating whether to give the long answer or the short one. She glanced up at the rest of us. "The question is, how do we know that the purpose of the worms is to eat people? The answer is, because that's exactly what they're doing."

"That's not the kind of answer I was expecting-" the colonel said.

Dr. Fletcher nodded in agreement. "I know it sounds flip," she said. "I'm sorry, but Dr. Kinsey summed up all animal behavior a long time ago: the only unnatural act is the one you can't do. If the worms couldn't eat Terran life-forms, they wouldn't."

I held my tongue-it made sense. Too much sense. The realization was a pain in the gut.

"Dr. Abbato has based a very interesting argument on this fact. He is postulating that this circumstance is not accidental. He is suggesting that the real purpose of the worms is a cleanup of the top level of the Terran ecology. The worms are specifically targeted to eat those humans who have survived the plagues."