which individuals are heterozygous (H). A review by Selander (1976] comparing these param- eters in various populations has been followed by many other studies. In the present volume, J. B. Mitton has used H to evaluate the importance of heterozygosity in natural populations. The degree of polymorphism expressed by P, has been used in several contributions to approach various problems of population genetics. particularly breeding structure and mating systems by Hamrick, Barrett and Shore, Brown, Burdon and Jarosz. as well as Soltis and Soltis, and Wyatt. Stoneburner. and Odrzykoski. New knowledge derived from these investigations has strengthened a point of view already stressed by Darwin: evolution takes place in a complex environment, that can be constantly changing over long periods of time. or can alternate between long periods of relative stability and cycles of rapid change. The most successful plant species become adjusted to these vagaries in several ways, including shifts in heterozygosity. polymorphism and mating systems. The strength of isozyme ana ysis for testing hypotheses is well illustrated by the contribution of the Soltises, who have shown clearly that a previously held hypothesis, predicting self fertilization fortified by polyploid genetic segregations in ferns, must be rejected.