Adaptations for placing a maximum number of mature offspring in the next generation fall between the two adaptive extremes of having either high reproductive rates and low survival rates or low reproductive rates and high survival rates. Among the fishes, both extremes have been realized; for example, the ling, Molva molva may produce 20-30 X 106 eggs at a single spawning (Blaxter, 1969), whereas some species of viviparous fishes have only one to four well-developed young at a time. Among the variable reproductive adaptations in teleosts, a remarkable array is found within a single family of viviparous fishes, the Poeciliidae. All but one of the 150 species of poeciliids known (Rosen and Bailey, 1963; Bussing, 1963, 1966; Hubbs and Peden, 1969; Miller, 1975; Miller and Minckley, 1970; Rivas, 1969; Rivas and Fink, 1970; Rosen and Kallman, 1969) are viviparous. Brood sizes within the family range from a few to 315 young at a time (Krumholz, 1948). Production of small numbers of young which are born in an advanced state of development clearly places poeciliids at one end of the array of reproductive adaptations. Although poeciliids seemingly have sacrificed quantity for quality, they have not done so to the same degree nor by the same mechanism; thus a fairly wide range of productivity levels persists within the family. Once internal fertilization evolved, various modifications in the reproductive system were possible. For example, adjust-