OBZHANSKY (1955), in a discussion of evolutionary processes in Mendelian populations, has pointed out that “the work in this field is severely handicapped by lack of reliable methods of comparing the fitness of populations and, of course, by the lack of clarity of the concept of fitness itself.” Fitness is a property that can be attributed to an individual, a genotype, a Mendelian p o p lation, a species or a higher biological category. At any of these different levels of integration of the biological world, the word “fitness” may be used meaning widely different things, but in all instances “fitness must refer to the ability of an organism to leave surviving off spring” ( LERNER 1954). The fitness of an individual is measured by the mean number of offspring in the next generation (PENROSE 1949; KNIGHT and ROBERTSON 1957; REED 1959). For a higher biological category, fitness may be defined in terms of the descendants left after one or many generations. THODAY (1953, 1958) defines fitness as the probability that a “contemporary group of individuals” will leave descendants after a given long period of time, such as los years. This kind of definition, whatever its theoretical interest, does not lend itself to an experimental estimation of fitness. Several attempts have been made to define the fitness of a genotype or of a Mendelian population in quantitative terms, so that it may be experimentally approached. These definitions are given in relation to a certain environment. FISHER (1930) has defined the fitness of a genotype as the “expectation of offspring,” that is the contribution of the individuals carrying that genotype to the next generation, and defined the “Malthusian parameter” as the natural logarithm of that quantity. Related concepts are the “intrademic selective value” of a population as defined by WRIGHT (1955) and the “adaptive value” used by population geneticists in their experimental studies (WRIGHT 1931 ; HALDANE 1932) which involve the relative fitness of different genotypes. The fitness of a genotype or of a Mendelian population has been frequently discussed in terms of the various components of the life cycle, such as fecundity, hatchability, rate of development of immature stages, differential mortality of immature stages, sterility, sexual activity of adults, longevity of adults, etc. It should be noted that all these are components of a single value of fitness and not alternative estimates of it, as some workers seem to imply. Attempts have been made (e.g. WALLACE 1948) to combine these components into the fitness in the ’ Research supported in part by grant NSF GB-19OG from the National Science Foundation * Present address The Rockefeller Institute, New York Clty