140 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



of the Cetacea and Sirenia. The results attained by 

 the study of the paleontology of the other orders may 

 be summarized as follows : 



First. It is probable that the common ancestors of 

 the placental and implacental lines of Mammalia are 

 known to us in some of the types of the Jurassic per- 

 iod. Whether they were marsupial in the sense of 

 possessing an external pouch for the young or not, is 

 immaterial. They were probably marsupial in brain 

 characters, in the structure of their reproductive sys- 

 tem, and in the absence of placenta. To this source 

 the existing polyprotodont marsupials may be traced, 

 through such forms as Myrmecobius. The multi- 

 tuberculate type has a contemporary history, and one 

 distinct from that of the Polyprotodontia, and its an- 

 cestry has not yet been discovered. Their earliest 

 forms (of the Jurassic and Triassic) are already highly 

 specialized. They probably represent the Monotre- 

 mata of their time. 



Second. The immediate didelphian ancestors of the 

 monodelphous Mammalia have not yet been certainly 

 discovered. In the oldest of the latter (of the Puerco 

 epoch) numerous points of approach to the insectivo- 

 rous Jurassic forms occur, especiall}' in the prevalent 

 trituberculy of the molars in both epochs. 



Third. The phyiogeny of the clawed group has 

 been traced back to a common ordinal form which has 

 been called the Bunotheria. Of these the most genera- 

 lized are the Creodonta, from which we may trace the 

 Carnivora, the Insectivora, and the Tillodonta, and 

 probably all other Unguiculata. The Ancylopoda only 

 have undergone the alternation of the carpal and tar- 

 sal bones, which obtains in the diplarthrous Ungulata. 



Fourth. The phyiogeny of the hoofed groups car- 



