The three sub-classes of Mammalia—as we have already seen—stand in such a relation to one another that the Marsupials, both as regards their anatomy and their ontogeny and phylogeny, form the direct transition from the Monotrema to Placental animals (p. 247). Consequently, human ancestors must also have existed among Marsupials. They _originated_ out of the Monotrema—which include the primary Mammalia, or Promammalia—by the division of the cloaca into the rectum and the urogenital sinus, by the formation of a nipple on the mammary gland, and by the partial suppression of the clavicles. The oldest Marsupials at all events existed as early as the Jura period (perhaps even in the Trias); during the Chalk period they passed through a series of stages preparing the way for the origin of Placentalia. The certain proof of our derivation from Marsupials—nearly akin to the still living opossum and kangaroo in their essential inner structure—is furnished by the comparative anatomy and the ontogeny of Mammalia. EIGHTEENTH STAGE: +Semi-apes (Prosimiæ)+. The small group of Semi-apes, as we have already seen, is one of the most important and most interesting orders of Mammalia. It contains the direct primary forms of Genuine Apes, and thus also of Man. Our Semi-ape ancestors probably possessed only a very faint external resemblance to the still living, short-footed Semi-apes (Brachytarsi), especially the Maki, Indri, and Lori (p. 256). They _originated_ (probably at the beginning of the Cenolithic, or Tertiary period) out of Marsupials of Rat-like appearance by the formation of a placenta, the loss of the marsupium and the marsupial bones, and by the higher development of the commissures of the brain. The _certain proof_ that Genuine Apes, and hence also our own race, are the direct descendants of Semi-apes, is to be found in the comparative anatomy and the ontogeny of Placental animals. NINETEENTH STAGE: +Tailed Apes (Menocerca)+. Of the two classes of Genuine Apes which developed out of the Semi-apes, it is only the narrow-nosed, or Catarrhini, which are closely related by blood to Man. Our older ancestors from this group probably resembled the still living Nose-apes and Holy-apes (Semnopithecus), which possess jaws and narrow noses like Man, but have a long tail, and their bodies densely covered with hair (p. 271). The Tailed Apes with narrow noses (Catarrhini Menocerci) _originated_ out of Semi-apes by the transformation of the jaw, and by the claws on their toes becoming changed into nails; this probably took place as early as the older Tertiary period. The _certain proof_ of our derivation from Tailed Catarrhini is to be found in the comparative anatomy and the ontogeny of Apes and of Man. TWENTIETH STAGE: +Man-like Apes (Anthropoides)+.