It is now more than fifty years since Darwin gave the theory of natural selection to the world, and the conception of a gradual evolution has long ago become part of the currency of thought. Evolution for Darwin was brought about by more than one factor. He believed in the inherited effects of the use and disuse of parts, and he also regarded sexual selection as operating at any rate among the higher animals. Yet he looked upon the natural selection of small favourable variations as the principal factor in evolutionary change. Since Darwin's time the trend has been to magnify natural selection at the expense of the other two factors. The doctrine of the inherited effects of use and disuse, v…
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