Family Tree by Charles L. Fontenay You don't like Darwin's theory of Evolution? Maybe you're right. Maybe Man's ancestors weren't monkeys after all....   

Brain Teaser by Tom Godwin

How can a ship travel both forward and backward and sideways in two different directions, be going twice as fast as the speed of light—and still be completely motionless?

This is a tricky little thrilling tale of space travel, time travel, and um, something else that would be a spoiler.  

Second Variety by Philip K. Dick.

"Second Variety" occurs in the aftermath of an extensive nuclear war between the Soviet Union (sometimes referred to as Russia) and the United Nations. Early Soviet victories forced the North American government and production to flee to a Moon Base, leaving the majority of their troops behind. To counter the almost complete Soviet victory, U.N. technicians develop robots, nicknamed claws—the basic models are "a churning sphere of blades and metal" that ambush their unsuspecting victims "spinning, creeping, shaking themselves up suddenly from the gray ash and darting toward… [any warm body]." U.N. forces are protected from the claws by a special radiation-emitting wrist tab. Within six years, the sophisticated and independent claws have destroyed the Soviet forces, repairing and redesigning themselves in automated underground factories run without any human oversight.

The U.N. forces receive a message from the Soviets asking for a policy-level officer to go to them for a gravely urgent conference. The U.N. victory was costlier than they had expected. Major Joseph Hendricks is sent to negotiate with the Soviets. En route to the rendezvous, he meets a small boy named David who asks to accompany Hendricks. When they near the Soviet bunker, soldiers immediately kill the boy, revealing him to be an android. The claws' development program has evolved to develop sophisticated robots, indistinguishable from humans, designed to infiltrate and kill. The three Soviets met by Major Hendricks—soldiers Klaus, Rudi, and a young woman named Tasso—reveal that the entire Soviet army and command structure collapsed under the onslaught of the new robots - they are all that are left in the command center.
From salvaged internal metal identification plates, two varieties are identified: I-V, a wounded soldier, and III-V, David. II-V—the "second variety"—remains unknown. The different models are produced independently of each other in different factories. The Soviets also reveal that the U.N. protective tabs are ineffective against the new robots. Hendricks attempts to transmit a warning to his H.Q. bunker, but is unable to do so.

    Wrong Analogy by Joseph Shallit

The Pied Piper who had promised them Paradise and led them across half the universe had changed his tune. But this time it was the Piper who had to pay....

Project Hi-Psi by Frank Riley 

The aliens were conducting an experiment under laboratory conditions. So, how could they guess that their guinea pigs held the ultimate weapon?

A World Called Crimson by Darius John Granger

There was a boy and a girl and a strange new planet; the planet was alive with hideous dangers. But the boy and girl were very young and all Robin wanted to know was: "Who stole my doll?" This is a very, very strange little tale.