"We devised ways and means to detect and destroy anything that endangered us, long before the danger could be manifested. Like here, on this planet ... but you know about that."
"Radiation, wasn't that it?" said Cole.
"Yes," MacMartree said. "The discovery ship took its readings from out there somewhere, out where this place was only a dust mote in the glare of its sun. They drained off the radiation, scattered it into the void, then seeded the place with grass and went away."
"But that's what I don't understand," Phillips objected. "Why must we patrol? When the discoverers found this planet, they destroyed the only thing about it that could be harmful to man ... so why must we be here?"
MacMartree shrugged. "Caution, boy ... call it caution. We are here to see and observe. The discoverers do not accept their readings as infallible, though I suspect that they are. We're here on the one chance in a hundred million that somewhere on this little world, there's a being or an element that might bear enmity toward mankind."
Abner sighed. "And so we patrol ... for a year."
"Yes," MacMartree agreed. "For a year. And after the year, another patrol, and another year, and so on through a hundred patrols and years, until the place is classified safe for colonization."
"I think my species is cowardly," Cole said, a trifle hotly.
"Cautious," MacMartree corrected gently. "Only cautious. It's as it should be ... they have set up rules of caution, and we've never suffered for it."