PROGENITORS OF MAN, 29I 
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 (Prosimiz). 
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 commis- 
sures of the brain, The certain proof that Genuine Apes, 
