1 The Problem -- 2 The Evolutionary Significance of Genetic Diversity; Ecological, Demographic and Life History Correlates -- The Problem -- Materials and Methods -- The Evidence -- Theoretical Consideration -- The Matching of Evidence and Theoretical Models -- Conclusions and Prospects -- Summary -- Acknowledgements -- References of the Data -- References of the Theoretical Discussion -- Appendix I. The Data Set -- Appendix II. The Biotic Questionnaire -- Appendix III. Transformations of Heterozygosity and Polymorphism Estimates -- 3 Genetic Polymorphism and Neomutationism -- Protein Polymorphism -- Dna Polymorphism -- Implications for Evolutionary Theory -- Acknowledgements -- References -- 4 A Darwinian Theory of Enzyme Polymorphism -- The Model -- The Accuracy of the Numerical Calculations -- Why Is Chaotic Motion Rare in Natural Populations? -- Survival Probability for Populations -- Predictions of the Model at the Genetic Level -- Conclusions -- References -- 5 The Possibility of Stress-Triggered Evolution