This is the outline of the selection theory, as given by Darwin in 1859, and still retained in all its essentials. It is true, in his work on the origin of man he added as supplemental the sexual to the common natural selection, and made it of special importance for the presentation of the beautiful in nature—for the production of beautiful forms, colors, and tones, and for the development of power and intelligence. And in the same work he said that there are many circumstances of structure which seem to be neither beneficial nor detrimental to the individual, and that to have overlooked this fact was one of his greatest mistakes in his former publications. But for the rest, he maintains the selection theory unchanged, with the single modification that it explains, if not the whole development of the species through descent, at least that which is of most importance in it.
That it was only one step in the course of reasoning to extend the selection theory to the descent of man, was seen by many as soon as Darwin's work on the origin of species was published and began to attract
attention; although not a syllable upon this question was presented in this work. Various persons manifested their presentiment or perception according to their point of view—partly by the most violent opposition to the new doctrine, partly by scientific development or modification of their anthropogonic views, partly also by revelling in imagination in the consequences hostile to religious faith which they thought could be drawn from this doctrine. We remind the reader of the itinerant lectures of Karl Vogt about the ape-pedigree of man, and of the echo they found by assent or dissent in press and public; also of Huxley in England, Karl Snell, Schleiden, Reichenbach, and others; of the materialists, L. Büchuer and Moleschott, and of the publications of Ernst Häckel. Finally, Darwin himself made us fully certain of the importance which from the beginning he had attributed to his theory, by publishing his work on the "Descent of Man," in the year 1870.
In this work he explained the descent of man fully from the before-named principles of the descent, evolution, and selection theories, of which we have given all the essentials in the foregoing presentation. He carefully enumerates everything in the structure of the human body that reminds us of our relationship with the animals—especially those embryonic phenomena and rudimentary organs in man which are still to be found in use and in a more developed state in different animal species, and which led him to imagine our ancestors now with a tail, then with sharp ears, now living in the water, then being hermaphrodites. He reviews the spiritual qualities of man, and finds for them all analogous qualities in the animal world. He finds in his work on
"Expression of the Emotions in Man and Animals," published in 1872, new confirmation of the genealogical relationship of both. He looks over the whole course of the zoölogical system and of palæontological discoveries, and searches for the points where the branches and twigs of the animal pedigree of man must have diverged. To begin with the lowest branches, he thinks the most important divergence took place where the series of vertebrates may have been developed out of the invertebrates. Here he adopts the investigations of A. Kowalewsky, and the deductions of Häckel founded upon them, concerning the larva of the ascidiæ, a genus of marine mollusca of the order tunicata, and sees in a cord, to be found in this larva, most decided relationship to the spine of the lancelet fish or amphioxus, the lowest of all the vertebrates, it being yet doubtful whether it belongs at all to the vertebrates. In the transition that once took place from one species of ascidian larva to a form similar to the lancelet fish, he sees the new branch diverging in the series of vertebrates. Out of the fish he concludes that the amphibia were developed, and out of those the reptilia, out of one of them the marsupialia, and from them the lemurs or half-apes, the representatives of which yet live in Madagascar and the southern part of Asia. From these there branched off on the one side the platyrrhini, or apes with a flat nose, on the new continent; on the other, the catarrhini, or apes with a narrow nose, on the old. Among the ancestor of the last, he searches for the common progenitors, from which again two branches started—on the one hand the ignoble branches of the catarrhine species of apes, always remaining lower in
development, to which also belong the anthropomorphous apes, like the orang outang and gibbon in Asia, the gorilla and chimpanzee in Africa; on the other hand, that branch which represents the ascent of animals to man.
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
question again (in [Book II, Ch. I]) at the same point where we here leave it.
§ 2. The Followers of Darwin.—Ernst Häckel.