36 HISTORY OF THE HUMAN BODY 



came respectively the Didelphia, or marsupials, and the Mono- 

 delphia, or placentals. The first of these lines then differ- 



PROTOTHERIA 



EUTHERIA 



FIG. 8. Phylogenetic tree of mammals. 



A branch that terminates in an arrow point still possesses living representatives; 

 one that ends in a short cross bar is extinct. 



entiated into the marsupialian Orders of the present time, dis- 

 tinguished mainly by variation in the dentition, and the second, 



