There! The edge of a sensation! Two watchbirds dipped simultaneously. One had received the scent a fraction of a second before the other. He continued down while the other resumed monitoring. _Condition four, there are some living organisms who commit murder without the signs mentioned in condition three._ Through his new information, the watchbird knew by extrapolation that this organism was bent on murder, even though the characteristic chemical and electrical smells were absent. The watchbird, all senses acute, closed in on the organism. He found what he wanted, and dived. Roger Greco leaned against a building, his hands in his pockets. In his left hand was the cool butt of a .45. Greco waited patiently. He wasn't thinking of anything in particular, just relaxing against a building, waiting for a man. Greco didn't know why the man was to be killed. He didn't care. Greco's lack of curiosity was part of his value. The other part was his skill. One bullet, neatly placed in the head of a man he didn't know. It didn't excite him or sicken him. It was a job, just like anything else. You killed a man. So? As Greco's victim stepped out of a building, Greco lifted the .45 out of his pocket. He released the safety and braced the gun with his right hand. He still wasn't thinking of anything as he took aim ... And was knocked off his feet. Greco thought he had been shot. He struggled up again, looked around, and sighted foggily on his victim. Again he was knocked down. This time he lay on the ground, trying to draw a bead. He never thought of stopping, for Greco was a craftsman. With the next blow, everything went black. Permanently, because the watchbird's duty was to protect the object of violence--_at whatever cost to the murderer_. The victim walked to his car. He hadn't noticed anything unusual. Everything had happened in silence. * * * * * Gelsen was feeling pretty good. The watchbirds had been operating perfectly. Crimes of violence had been cut in half, and cut again. Dark alleys were no longer mouths of horror. Parks and playgrounds were not places to shun after dusk. Of course, there were still robberies. Petty thievery flourished, and embezzlement, larceny, forgery and a hundred other crimes. [Illustration] But that wasn't so important. You could regain lost money--never a lost life. Gelsen was ready to admit that he had been wrong about the watchbirds. They _were_ doing a job that humans had been unable to accomplish. The first hint of something wrong came that morning. Macintyre came into his office. He stood silently in front of Gelsen's desk, looking annoyed and a little embarrassed. "What's the matter, Mac?" Gelsen asked. "One of the watchbirds went to work on a slaughterhouse man. Knocked him out." Gelsen thought about it for a moment. Yes, the watchbirds would do that. With their new learning circuits, they had probably defined the killing of animals as murder.