In the very beginning, the theory of Evolution may be said to have had three distinct branches, represented by the Nebular Theory in Astronomy, Haeckel's Ontogeny, and the Biology of Lamarck, Darwin, Wallace, and Huxley; and to these should properly be added the Sociological Ethics of Spencer, which was not, however, worked out to a complete system. But Du Prel says of later research: "In the progress of modern science, no principle has proved so fruitful as that of evolution. All branches compete with one another in its use, and have brought about by its aid the most gratifying results. Geology interprets the significance of superimposed, hardened strata of the earth's crust in the sense of a history of the earth's development; Biology, in union with the study of fossils, arranges the living and petrified specimens of plants and animals in their order, and constructs a history of the evolution of organic life; Philology prepares a genealogical tree of languages, and finds in it signs which throw light on prehistoric times, and reveal facts forgotten for thousands of years; Anthropology discovers in the form and expression of human beings rudimentary signs that point to a theory of development from lower forms; and, finally, History reveals the evolution of civilization in far-distant historic times; and in all these branches it becomes apparent that we only then understand phenomena when we have comprehended their Becoming."[95] It is due to the gradual perception of the fact that some such theory as that of Evolution is implied in the very conception of the constancy of nature that there has been a continual decrease of that negative form of criticism which has made much of the gaps in the direct proof. Modern science has so grown to, and by, the theory of Evolution that the overthrow of the latter means nothing more nor less than the destruction of science itself in its highest results. Even those who reject the conclusions of Evolution are found to make use of its methods, and must do so perforce. As the breadth and depth and height of the theory come to be perceived, it is seen that the demand for complete proof is nothing more nor less than a demand for the perfection of all branches of knowledge, the refusal of credit without such proof a refusal to place any confidence in the first principles of scientific theory until it has fully explored the universe and left nothing further to be discovered. But science would have less ground for complaint, if the opponents of Darwinism consistently refused, on the ground of the incompleteness of our knowledge, to form any theory whatever on the subject of man's nature and development, permitting the worth of the evolutional theory to be determined by its future results in application as hypothesis. But the peculiar spectacle is afforded us of a party rejecting a theory supported by numberless facts in all branches, and whose very breaches the direction of discovery continually tends to