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Recurrent mutations not only provide the material for selection to act upon. They may give rise to primary and secondary effects, the former due to the accumulation of mutant genes, the latter to the selective value of conditions which protect the organism against lethal genes. Among the phenomena which can be accounted for by these phenomena are the disappearance of useless organs, recapitulation and the fact that the heterogametic sex is usually male.
J. B. S. Haldane (Sun,) studied this question.