1. George Santayana, The Works of George Santayana, Volume II, The Sense of Beauty: Being the Outlines of Æesthetic Theory, edited by William G. Holzberger and Herman J. Saatkamp, Jr. (Cambridge: The MIT Press, 1988), Part II, §13, p. 41.

2. Richard Taylor, editor, quoted in George Seldes, The Great Thoughts (New York: Random House, 1985), p. 373.

3. The first clear explanations of sex both as a means of rapid evolution and as an escape of populations—especially small populations—from the cumulative impact of deleterious mutations were made by the geneticist H. J. Muller (e.g., “Some Genetic Aspects of Sex,” American Naturalist 66 [1932], pp. 118–138; “The Relation of Recombination to Mutational Advance,” Mutation Research 1 [1964], pp. 2–9). There is theoretical and experimental support for his proposals (e.g., Joseph Felsenstein, “The Evolutionary Advantage of Recombination,” Genetics 78 [1974], pp. 737–756; Graham Bell, Sex and Death in Protozoa: The History of an Obsession [Cambridge: Cambridge University Press, 1988]; Lin Chao, Thutrang Than, and Crystal Matthews, “Muller’s Ratchet and the Advantage of Sex in the RNA Virus Φ6,” Evolution 46 [1992], pp. 289–299).

Muller stressed that sexual reproduction was hardly necessary for survival, but that “lack of recomination would greatly handicap a species, in long-term evolutionary advancement, in keeping pace with sexually reproducing competitors.” The idea of sex providing a long-term benefit for the species certainly seems to be an example of group selection, as was explicitly noted, without undue alarm, by one of the founders of modern population genetics, R. A. Fisher (The Genetical Theory of Natural Selection [Oxford: Clarendon Press, 1930]). Fisher was one of the first to suggest that, in other cases, what superficially looks like group selection may in fact be kin selection.

4. D. Crews, “Courtship in Unisexual Lizards: A Model for Brain Evolution,” Scientific American 259 (June 1987), pp. 116–121.

5. Raoul E. Benveniste, “The Contributions of Retroviruses to the Study of Mammalian Evolution,” Chapter 6 in R. I. Maclntyre, editor, Molecular Evolutionary Genetics (New York: Plenum, 1985), pp. 359–417.

6. We have scarcely touched on the complexity and diversity of the sexual machinery, both on the molecular level and the level of individual organisms. Nor have we given a full flavor of the debate on what sex is good for. An excellent short summary is in James L. Gould and Carol Grant Gould, Sexual Selection (New York: W. H. Freeman, 1989). See also the influential book by John Maynard Smith, The Evolution of Sex (Cambridge: Cambridge University Press, 1978); H. O. Halvorson and A. Monroy, editors, The Origin and Evolution of Sex (New York: A. R. Liss, 1985); Lynn Margulis and Dorion Sagan, Origins of Sex (New Haven: Yale University Press, 1986); R. E. Michod and B. R. Levin, The Evolution of Sex (Sunderland, MA: Sinauer, 1988); Alun Anderson, “The Evolution of Sexes,” Science 257 (1992), pp. 324–326; and Bell, op. cit. in Note 3.

7. D. J. Roberts, A. B. Craig, A. R. Berendt, R. Pinches, G. Nash, K. Marsh and C. I. Newbold, “Rapid Switching to Multiple Antigenic and Adhesive Phenotypes in Malaria,” Nature 357 (1992), pp. 689–692.

8. W. D. Hamilton, R. Axelrod, and R. Tanese, “Sexual Reproduction as an Adaptation to Resist Parasites (A Review),” Proceedings of the National Academy of Sciences 87 (1990), pp. 3566–3573.

9. Helen Fisher, “Monogamy, Adultery, and Divorce in Cross-Species Perspective,” in Michael H. Robinson and Lionel Tiger, editors, Man and Beast Revisited (Washington and London: Smithsonian Institution Press, 1991), p. 97.

10. E. A. Armstrong, Bird Display and Bird Behaviour. An Introduction to the Study of Bird Psychology (New York: Dover, 1965), p. 305.

11. W. D. Hamilton and M. Zuk, “Heritable True Fitness and Bright Birds: A Role for Parasites?” Science 218 (1982), pp. 384–387.

12. The same bargain is made in the common, sexually repressive version of the story of the Garden of Eden—in which it is sexual activity between Adam and Eve that excites God’s wrath and makes them mortal.

13. This wonderfully vivid image is Frans de Waal’s, in Peacemaking Among Primates (Cambridge: Harvard University Press, 1989), p. 11.

14. Translated by Edward Kissam and Michael Schmidt (Tempe, AZ: Bilingual Press/Editorial Bilingüe, 1983), p. 47.