498 Causes and Course of Organic Evolution 



It is not surprising, therefore, that, over a large part of the 

 world during the Jurassic, and on through the cretaceous 

 period, the remains of forms are found that have retained 

 or possibly reverted to some marsupial details in assuming 

 new morphological adaptations, and alongside them others 

 that have largely lost the former details through dominant 

 reaction of the latter adaptations on the life relations of the 

 animal. According to this view therefore, while in one sense 

 living marsupials are a decadent race that are descended from 

 once greatly more abundant, larger, and w^dely distributed 

 ancestors, in another sense the marsupial group has been a 

 dominant race whose evolving and adaptable mammalian 

 progeny now stocks the earth. 



In attempting to follow the phylogenetic history of the 

 higher mammals it seems difficult to trace successfully the 

 progress of the Cetacea, of whose earlier origin even we are 

 still largely ignorant. These and some other groups will 

 therefore be set aside for the present, as unnecessary for our 

 immediate purpose. 



While the later Jurassic and cretaceous rocks were being 

 deposited, ample tirne and opportunity were given for the 

 evolution of some polyprotodont mammals of America and 

 of Europe into those rather small essentially carnivorous mam- 

 mals that have collectively been called the Creodonta, from 

 which again the later typical Carnivora probably sprang. 

 While they have unquestionably lost many of the marsupial 

 characteristics, such statements as follow clearly indicate 

 the drift of opinion. Thus "the curious resemblance of the 

 molars of many of these forms to those of the marsupials may 

 indicate a genetic relationship between the two groups, but 

 on the other hand the presence of a full set of milk teeth, and 

 the absence of palatal vacuities, or of an inflection of the 

 angle of the mandible, sharply distinguishes them from that 

 order" (i6'4." 608). But the discovery of an additional set 

 of rudimentary teeth which never cut the gum in marsupials 

 does away with one of the suj)posed differences. 



Again regarding the creodont Hycenodon Scott has remarked 

 of its skull that it is "somewhat like that of an opossum," 



