During this same period the doctrine of evolution suffered much from over-enthusiasm on the part of some of its defenders. Even Wallace overdid the hypothesis of sexual selection, and the kindred hypotheses of concealing and protective coloration. Naturalists sought to explain every coloring of animals and plants as being of some value to them, and therefore the real cause of the existence of the species; not a few carried the idea of value in sexual differences, such as those between the male and female peacock, to a similar extreme. But in spite of the inaccuracies which they published, these enthusiasts did far more good than harm, for they aided greatly in securing popular support for the main theory. It was toward the beginning of this century that evolutionary studies received another great stimulus. Professor Hugo de Vries, a Dutch botanist of considerable note, proposed what he called the “mutation theory” as a substitute for Darwin’s conception of “natural selection.” He began his studies by attempting to produce by careful selection a variety of buttercup which should contain in its flower more than the normal number of petals. He actually achieved the desired increase, but it was far from a stable condition; while some of the flowers possessed eight, nine, or ten petals, and a few as high as thirty-one, many of them possessed the original number, five. When selection was abandoned there appeared at once a general retrogression toward the primitive state, and this fact caused de Vries to conclude that selection alone was not enough to cause the formation of a new species of plant or animal[17]. Instead, he concluded that when a change of permanent value took place in a plant or animal it was something entirely different from the constant variations on which Darwin and his followers relied; it was a discontinuous variation--a ‘sport,’ the florist or gardener would call it--to which de Vries applied the new name mutation. Mutation, he believed, involved a very definite change in the reproductive cells of the organism--a change which had absolutely no relation to the environment. They arose from conditions within the plant and animal, and might or might not affect it favorably. Those mutations which were not beneficial would be eliminated by selection; those which were of value to the creature would probably be preserved. Thus, in de Vries’ mind evolution was a process due primarily to internal causes, its course being merely guided by environment, which selected those mutations capable of surviving.