The refining agencies which finally raise the animal up to the man are essentially the same that on the lowest scales of the pedigree have caused the development of the lower organisms into the higher, namely: favorable individual variations, inheritance, acclimatization, survival of the fittest in the struggle for existence, natural, and especially, sexual selection. These are, if not the exclusive, still the main agencies which finally led the primate of the earthly creation upon the stage and furnished him with his superior faculties. But it is particularly by means of his social life, and of the forces which determine, transmit, increase and ennoble the various impulses and instincts promoting it, that man has become what he is. Through the need and faculty of reciprocal help, through sexual selection--which of course is a very essential factor of social life--there originated language, and reflection, and all the intellectual qualities; and through these again originated the moral qualities, which are most important in constituting the specific worth of man, and which were finally developed into self-consciousness and free moral responsibility. But with the description of this attempt to explain more in detail these specific characteristics of man, we leave the ground of pure natural science and enter the region of philosophy, in which we have to take up the {45} question again (in Book II, Ch. I) at the same point where we here leave it. Sec. 2. _The Followers of Darwin.--Ernst Haeckel._ Darwin's theory soon found an enthusiastic corps of followers--on the continent, and especially in Germany, almost more than in his own country. The outlook into an entirely new explanation of the origin of man, and the probable use of this theory for attacks upon faith in a Creator and Master of the world, called wide-spread attention to it; and the theory opened to natural science itself entirely new impulses and paths, and promised the solution of many problems before which it had hitherto been compelled to stand in silence. To be sure, it threatened likewise to allure the mind from the slow but sure ways of solid study to the entertaining but insecure and aimless paths of imagination and hypothesis. Among all the German followers of Darwin who adopted not only the idea of an origin of species through descent and evolution, but also the explanation of evolution by natural selection, and extended it so as to make the principle of selection of exclusive value, Ernst Haeckel occupies the most prominent rank.