ON OUR PRESENT KNOWLEDGE OF THE ORIGIN OF MAN. 473 



older mammals of the Eocene period which we regard as stem-forms 

 of the principal groups of chorion animals (Placentalia) : Lemuravida, 

 Condylarthra, JSsthonychida, and Ictopsida. These form old Tertiary 

 stem-forms of the primates. The ungulates, the rodents, and the carni- 

 vores 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. Xext 

 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 rela- 

 tions 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 pale- 

 ontological 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 Amphi- 

 theriidw of the Jurassic were ancestors of the marsupials, and that the 

 monotreme ancestors of the latter are to be sought among the Panto- 

 theria of the Trias. But paleontology does not at this time offer us 



