1. Ferster, C. B., and Skinner, B. F., Schedules of Reinforcement (Appleton. N. Y., 1957).
2. Spence, K. W., Psychol. Rev., 43, 427 (1936). Hull, C. L., Principles of Behaviour (Appleton, NY, 1943).
3. Kimble, G. A., Hilgard and Marquis' Conditioning and Learning, chap. 12 (Appleton, NY, 1961).
4. Terrace, H. S., J. Exp. Anal. Behav., 6, 1 (1963).
5. Reynolds, G. S., J. Exp. Anal. Behav., 4, 57 (1961). Bloomfield, T. M., J. Exp. Anal. Behav., 9, 155 (1966).