In preparing a work such as this, one received so much help from so many that to express one's gratitude to everybody becomes impossible. Nevertheless, at the risk of appearing invidious, I must mention a few.

I am particularly grateful to Mr. Leslie Barden for his photography of many of the illustrations; his work has ensured that the best possible results have been achieved. To Mr. David Bygott I owe the exquisite painting of the inverted display of Prince Rudolph's Bird of Paradise (Paradisornis rudolphi), which adorns the cover.

Dr. Erskine Hill has been most helpful in tracing the exact source and context of some of the quotations. Among the many who have kindly allowed me to quote from their works, I must particularly mention Professor R. E. Ulrich, Professor P. W. Anderson, Pofessor John Hick, Sir John Eccles, and Sir Karl Popper. To the latter I owe especial gratitude for readily agreeing to my using some paragraphs from his pen which are still in the press.

Finally my wife attended, and survived, all the lectures and provided valuable direct internal information as to the reactions of my audience—quite apart from help in many other ways.

I am also grateful to the following publishers for their permission to use illustrations of which they own the copyright:

Figure 1. After Guy Murchie, 1955, The Song of the Sky, p. 202. London: Secker & Warburg; Boston: Houghton Mifflin Company.

Figure 2. After H. R. Byers, 1949. Science 110, pp. 291–4, American Association for the Advancement of Science.

Figure 3. After Jean Brachet, in Scientific American, September 1961, p. 55. Copyright © 1961 by Scientific American, Inc. All rights reserved. San Francisco: W. H. Freeman & Company.

Figure 4. From L. H. Hyman, The Invertebrates, Vol. 2, p. 69. Copyright © 1951 by McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.

Figure 5. Cartoon by Roy Mumme from The Worm Runner's Digest, 7 (1965), p. 85.

Figure 6. After P. Ullyott in Journal of Experimental Biology, 13 (1936), pp. 265–78. The Company of Biologists Ltd.

Figure 7. After V. B. Wigglesworth, 1941, Parasitology, 33, p. 105. New York: Cambridge University Press.

Figure 8. After S. O. Mast, 1911, Light and the Behavior of Organisms. London: John Wiley & Sons.

Figure 9. After Muller, 1925, in Zeitschrift für vergleichende Physiologie, 3, pp. 113–44. Berlin: Springer-Verlag.

Figures 10, 11, 12. From J. C. Eccles, 1970, Facing Reality. New York: Springer-Verlag.

Figure 14. After E. O. Wilson and W. H. Bossert in Recent Progress in Hormone Research, 19 (1963), pp. 673–716. New York: Academic Press.

Figures 15, 17. From R. D. Alexander in W. E. Lanyon and W. N. Tavolga, 1960, Animal Sounds and Communication. Washington, D.C.: American Institute of Biological Sciences.

Figure 16, Plate 3. From R. D. Alexander in T. A. Sebeok, 1968, Animal Communication. Bloomington: Indiana University Press.

Figures 18, 19. From P. Marler in P. R. Bell, 1959. Darwin's Biological Work. New York: Cambridge University Press.

Figure 22. From R. W. Doty and J. F. Bosma in Journal of Neurophysiology, 19 (1956), pp. 44–60. American Physiological Society.

Figure 23. After E. J. Batham and C. F. A. Pantin in Quarterly Journal of Microscopic Science, 92 (1951), pp. 27–54. The Company of Biologists Ltd.

Figures 24, 25. From P. Marler and W. J. Hamilton, 1966, Mechanisms of Animal Behavior. New York: John Wiley & Sons.

Figure 26. After H. and H. Schone in Zeitschrift für Tierpsychologie, 20 (1963), pp. 641–56. Copyright © 1963 by Verlag Paul Parey, Berlin and Hamburg.

Figure 27. After J. Crane in Zoologica, 42 (1957), pp. 69–82. New York Zoological Society.

Figure 28. From N. E. and E. C. Collias in University of California Publications in Zoology, 73 (1964), p. 136. Originally published by the University of California Press; reprinted by permission of the Regents of the University of California.

Figures 29, 30. From N. E. and E. C. Collias in Auk, 79 (1962), pp. 568–95.

Figure 31. After D. Morris in Behaviour, 6 (1954), pp. 274, 276, 277; and suppl. VI (1958), pp. 20, 21. Leiden: E. J. Brill.

Figure 34. Modified after R. A. Hinde, 1970, Animal Behavior, p. 636. Copyright © 1970 by McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.

Figure 35. After E. C. Tolman and C. H. Honzik in University of California Publications in Psychology, 4 (1930), pp. 215–32. Originally published by the University of California Press; reprinted by permission of the Regents of the University of California.

Figure 36, Plate 9. After A. D. Blest in Behaviour, 11 (1957), pp. 257–309. Leiden: E. J. Brill.

Figure 38. After L. R. Aronson in American Museum Novitates, 1486 (1951), pp. 1–22. Courtesy of the American Museum of Natural History.

Figure 39. From B. T. Scheer in Quarterly Review of Biology, 14 (1939), p. 422.

Figure 40. From R. A. Hinde, Animal Behaviour, p. 474. Copyright © 1970 by McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.

Figure 41. From P. Marler and W. J. Hamilton, Mechanisms of Animal Behaviour, 1966, p. 349. New York: John Wiley & Sons.

Figures 43, 44. After R. L. Fantz in A. M. Schrier et al., 1965, Behavior of Non-human Primates, Vol. 2. New York: Academic Press.

Figure 45. After R. Held and A. Hein in Journal of Comparative and Physiological Psychology, 56 (1963), pp. 872–6. Copyright © 1963 by the American Psychological Association. Reprinted by permission.

Figure 46. From G. V. T. Matthews, 1953, Bird Navigation, p. 67. New York: Cambridge University Press.

Figure 47. After G. V. T. Matthews in Journal of Experimental Biology, 30 (1953), p. 251. The Company of Biologists Ltd.

Figure 48. After B. G. Tunmore in Proceedings of the XIIth International Ornithological Congress (Tilgmannin Kirjapaino), 1960. Through the courtesy of Lars von Heartman, Secretary General of the XIIth International Ornithological Congress.

Figure 49. From C. Walcott and M. Michener in Proceedings of the XIVth International Ornithological Congress (Blackwell, 1967), p. 315.

Figure 50. After W. W. Cochran et al. in Living Bird, 6 (1967), pp. 213–25. Cornell Laboratory of Ornithology.

Figures 51, 52. From A. Koestler and J. R. Smythies, 1969, Beyond Reductionism. London: Hutchinson Publishing Group Ltd.

Plate 1. From L. A. Borradaile, 1923, The Animal in Its Environment. New York: Oxford University Press.

Plate 2. From I. Eibl-Eibesfeldt, 1967, Grundriss der vergleichenden Verhaltens-forschung, R. Piper & Co. Verlag.

Plate 4. From R. E. Hutchison, J. Stevenson and W. H. Thorpe in Behaviour, 32 (1968), pp. 150–7, Leiden: E. J. Brill.

Plate 5. After Konrad Z. Lorenz in Scientific American, 199 (1958), pp. 71, 72–3. Copyright © 1958 by Scientific American, Inc. All rights reserved. W. H. Freeman & Company.

Plate 6. From J. H. Crook in Ibis, 105 (1963), p. 239. British Ornithologists Union.

Plates 7, 8. From H. B. Cott, 1940, Adaptive Coloration in Animals. London: Methuen.