VTa 



ilia: Classification 



77:) 



but with acquisition of some secondary specialization. This line must 

 have diverged from the same primitive stock which, along other 

 diverging lines, gave rise to marsupials and placentals. The beginnings 

 of mammals were probably in the Triassic. In the latter part of the 

 Jurassic lived mammals of about the size of the smaller rodents, lack- 

 ing any highly specialized features and, so far as can be judged from 

 structure of teeth and skull, probably chiefly insectivorous. These 

 Pantotheria (or Trituberculata), or unknown animals allied to 



Fig. 591. Young chimpanzees frater- 

 nizing with the author. (Photo by R. M. 

 Yerkes.) 



them, may have been the point of departure for the diverging marsupial 

 and placental lines. The monotremes and the pantotheres (and their 

 possible allies) must have had common ancestry in much more ancient 

 times. 



Marsupials and placentals both existed in the Cretaceous Period, 

 but which came first is quite uncertain. However, at the close of the 

 period, marsupials seem to have been somewhat more numerous than 

 placentals. Possible vestiges of marsupial structures in embryos of 

 some modern placentals suggest that placentals had marsupial an- 

 cestors. On the other hand, presence of a weakly developed placenta in 

 a few modern marsupials may be taken to mean that marsupials had 

 placental ancestors. Admitting the doubt as to the beginnings of the 



