older mammals of the Eocene period which we regard as stem-forms of the principal groups of chorion animals (Placentalia): Lemuravida, Condylarthra, Esthonychida, and Ictopsida. These form old Tertiary stem-forms of the primates. The ungulates, the rodents, and the carnivores resemble each other so much in bodily structure that we may bring them all together as a single common stem-group of the placental mammals, the primitive chorion animals (Prochoriata). With great probability we may now connect with this the further monophyletic hypothesis that all chorion or placental animals—from the lowest Prochoriata up to man—arose from a common unknown stem-form in the Cretaceous period, and that this oldest of the chorion animals arose from a marsupial group living in the Jurassic period.
But in fact we now possess among those numerous fossil lemurs that have been found for the first time during the last twenty years all the intermediate forms desired, all the "missing links" that are required by phyletic odontology. The oldest Prosimia of the Tertiary period, the pachylemurs (or Hyopsodines) of the old Eocene, have yet the original forty-four teeth of the placental stem-group; in every half jaw, above and below, three incisors, one canine, four premolars, and three molars. The necrolemurs (or Adapides) with forty teeth followed them; they have lost an incisor on each side above and below. Next come the younger autolemurs (or Stenopides) with thirty-six teeth (one premolar less); they have therefore already the same dental formula as the platyrrhines or American apes. The dentition of the catarrhines has arisen from this through loss of a second premolar. These relations are so clear and go so evidently hand in hand with the formation of the entire skull and the stronger development of the typical primate form that we may say: The general elementary features of the primate genealogical tree from the oldest Eocene lemur up to man lie clearly before our eyes within the Tertiary age; there is no longer any "missing- link." The phyletic unity of the primate stock from the oldest lemur up to man is now an historical fact.
It is quite different, however, when we leave the Tertiary and in the Mesozoic period attempt to discover the oldest ancestral series of the mammals. There we meet everywhere with painful gaps in our paleontological record, and the comparatively few remains of Mesozoic mammals (especially scanty in the chalk) are insufficient to enable us to form any definite conclusions as to the systematic placing of the mammalia in question. However, comparative anatomy and ontogeny compel us to the conclusion that the Cretaceous Placentalia arose from Jurassic marsupials, and these from Triassic monotremes. We may also further suppose that among the unknown Placentalia of the chalk there were found Lemuravida and other Prochoriata; that the Amphitheriidæ of the Jurassic were ancestors of the marsupials, and that the monotreme ancestors of the latter are to be sought among the Pantotheria of the Trias. But paleontology does not at this time offer us
