The foregoing makes it patent that Nietzsche was a thorough-going and uncompromising biological monist. That is to say, he believed that man, while superior to all other animals because of his greater development, was, after all, merely an animal, like the rest of them; that the struggle for existence went on among human beings exactly as it went on among the lions in the jungle and the protozoa in the sea ooze, and that the law of natural selection ruled all of animated nature--mind and matter--alike. Indeed, it is but just to credit him with being the pioneer among modern monists of this school, for he stated and defended the doctrine of morphological universality at a time when practically all the evolutionists doubted it, and had pretty well proved its truth some years before Haeckel wrote his "Monism" and "The Riddle of the Universe." To understand all of this, it is necessary to go back to Darwin and his first statement of the law of natural selection. Darwin proved, in "The Origin of the Species," that a great many more individuals of any given species of living being are born into the world each year than can possibly survive. Those that are best fitted to meet the condition of existence live on; those that are worst fitted die. The result is that, by the influence of heredity, the survivors beget a new generation in which there is a larger percentage of the fit. One might think that this would cause a greater number to survive, but inasmuch as the food and room on earth are limited, a large number must always die. But all the while the half or third, or whatever the percentage may be, which actually do survive become more and more fit. In consequence, a species, generation after generation, tends to become more and more adapted to meet life's vicissitudes, or, as the biologists say, more and more adapted to its environment.