The conclusion which we may reach, therefore, is that both natural selection and mutation operate in the development of new forms from old. The variations, for which Darwin was at a complete loss to account, are in many cases the mutations emphasized by de Vries and his followers. But to what extent climate, food, habits, and multitudinous other environmental factors, coupled with such internal ones as racial old age, complicate the processes of variation and selection cannot yet be said. De Vries, in his mutation theory, supplied one of the deficiencies of Darwinism, and at the same time led scientists in general to realize that evolution is a far more complex problem than was supposed during the later portion of the last century. Darwin’s primitive mudfish, with its trace of mind, and the process of natural selection, will not by any means account for the multitude of higher vertebrate forms which people, and have peopled the lands and waters of the globe. At the same time the scientific public was awaking to the fact that evolution was an almost inconceivably complex affair, many of the post-Darwinian hypotheses began to show themselves of very doubtful importance. The theory of sexual selection, which Darwin elaborated in the “Descent of Man” began a steady decline. Such selection undoubtedly does take place, but it is not carried on to so great an extent as was once supposed. The idea of the protective value of colors and color arrangement, too, began to be doubted, although at the same time its principles became much better known and therefore more strongly emphasized by some naturalists. Inheritance of directly acquired characters was proved to be an impossibility, and much doubt was thrown upon the hypothesis of use and disuse. Instead of legs disappearing because they are not used, they are now thought to disappear because the evolutionary processes going on within the animal demands their disappearance. What these processes are we do not know, but our frank avowal of ignorance gives us a certain confidence that we shall eventually find out.