great evolutionists; the philosopher, Spencer; Cope, the American paleontologist, and Huxley, the English champion of scientific rationalism—these, and a host of others spent their lives in demonstrating the workings of evolution. But unfortunately, the opposition which they encountered forced them to write and work largely along lines of argument and thus much of their work was fruitless so far as the discovery of new principles is concerned.
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
