In the meantime, the great master, Darwin, had decided to deal with this chief evolutionary problem in a special work. The two volumes of his _Descent of Man_ appeared in 1871. They contained an able discussion of sexual selection, or the selective influence of sexual love and high psychic activities connected therewith, and their significance in regard to the origin of man. As this part of Darwin's work was afterwards attacked with particular virulence, I will say that, in my opinion, it is of the greatest importance, not only for the general theory of evolution, but also for psychology, anthropology, and æsthetics. My own feeble early efforts (1866), not only to establish the descent of man from the nearest related apes, but also to determine more precisely the long series of our earlier and lower vertebrate ancestors, had not at all satisfied me. In particular, I had had to leave unanswered in my _General Morphology_ the very interesting question: from which invertebrate animals the vertebrate stem originally came. A clear and unexpected light was thrown on it some time afterwards by the astounding discoveries of Kowalevsky, which revealed an essential agreement in embryonic development between the lowest vertebrate (Amphioxus) and a lowly tunicate (Ascidia). In the succeeding years, the numerous discoveries in connection with the formation of the germinal layers in different animals so much enlarged our embryological outlook that I was able to prove the complete homology of the two-layered _gastrula_ (a cup-shaped embryonic form) in all the tissue-forming animals (_metazoa_) in my _Monograph on the Sponges_. From this I inferred, in virtue of the biogenetic law, the common descent of all the metazoa from one and the same gastrula-shaped stem-form, the _gastræa_. This hypothetical stem-form, to which man's earliest multicellular ancestors also belong, was afterwards proved by Monticelli's observations to be still in existence. The evolution of these very simple tissue-forming animals from still simpler unicellular forms (_protozoa_) is shown by the corresponding processes that we witness in what is called the segmentation of the ovum or gastrulation, in the development of the two-layered germ from the single cell of the ovum.