An anthology of twelve science fiction stories with biological themes.
Keep Out - short story by Fredric Brown
Student Body - novelette by F. L. Wallace [as by Floyd L. Wallace]
A Sound of Thunder - short story by Ray Bradbury
Invariant - short story by John R. Pierce
The Exterminator - short story by A. Hyatt Verrill
Tomorrow's Children - novelette by Poul Anderson and F. N. Waldrop [as by Poul Anderson]
Mary and Joe - short story by Naomi Mitchison
Sea Change - short story by Thomas N. Scortia
Caught in the Organ Draft - short story by Robert Silverberg
Nine Lives - novelette by Ursula K. Le Guin
Alien Earth - novelette by Edmond Hamilton
Grandpa - novelette by James H. Schmitz
Notes (Caught in the Organ Draft: Biology in Science Fiction) - essay by Isaac Asimov