I 4 2 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



consist among other points in a rotation of the second 

 row of carpal bones inwards on the first row, in the 

 anthropoid apes and man, similar to that which has 

 occurred among the Ungulata, but it has not become 

 so pronounced. 



As a result we get the general phylogenetic scheme 

 as shown on the following page. 



In this diagram, divisions of greater and lesser rank 

 are mixed, so as to display better some of the relation- 

 ships. Thus all the divisions whose names stand on 

 the right side of the middle vertical line are unguicu- 

 lates ; and those on the left side of the line, excepting 

 Sirenia and Cetacea, are ungulates. The three names 

 in the middle vertical line are those of the suborders 

 of the Taxeopoda. 



A review of the characters of the existing Mam- 

 malia as compared with those of their extinct ances- 

 tors displays a great deal of improvement in many 

 ways, and but few instances of retrogression. The 

 succession in time of the Monotremata, the Marsupia- 

 lia, and the Monodelphia, is a succession of advance 

 in all the characters of the soft parts and of the skele- 

 ton which define them (see table of classification). As 

 to the monotremes themselves, it is more than prob- 

 able that the order has degenerated in some respects 

 in producing the existing types. The history of the 

 Monotremata is not made out, but the earliest forms 

 of which we know the skeleton, Polymastodon (Cope) 

 of the Lower Eocene, is as specialized as the most 

 specialized recent forms. The dentition of the Juras- 

 sic forms, Plagiaulax, etc., is quite specialized also, 

 but not more so than that of the kangaroos. The pre- 

 molars are more specialized, the true molars less spe- 

 cialized than in those animals. The history of Marsu- 



