The idea of mutation was not new when van Vogt wrote Slan, but he transformed the motif into a broader view of the world than before. For instance, in The Island of Dr. Moreau (1896), H. G. Wells portrays animals that were made into grotesque copies of human beings by the experiments of a mad scientist; but these were not truly mutations in the Darwinian sense.

Van Vogt deserves credit for more fully grasping the implications of Charles Darwin's theory of natural selection than had earlier writers. In Slan, mutations appear throughout the world in the twenty-first century. Many of the mutations are hideous failures. Many others are the slan, beings who look like humans except for golden tendrils on the backs of their heads. These tendrils enable slans to communicate telepathically. The mutation is an evolutionary leap, not a matter of isolated freaks of nature or...