The work on Maniola jurtina described in the last two chapters aimed at analysing the evolution of polygenic characters in wild populations. It is necessary also to consider how those controlled by major genes can be used for similar evolutionary studies. This can in fact be done in all instances of genetic polymorphism. A strict definition of that situation had proved a fundamental necessity. It was provided in the following terms: Genetic polymorphism is the occurrence together in the same locality of two or more discontinuous forms of a species in such proportions that the rarest of them cannot be maintained merely by recurrent mutation (Ford, 1940a).