Involuntary ideas that keep recurring despite the person's efforts to stop them are called _______

Sensations are organized into meaningful perceptions by

a. perceptual constancies. b. localization of meaning. c. perceptual grouping (Gestalt) principles. d. sensory adaptation.

Since natural selection can only act on characteristics that already exist,

__________ provides the raw material on which natural selection can act. a. retention b. extension c. fixation d. generation e. variation