522 



CENOZOIC EUA— AGE OF MAMMALS. 



ing from its foot, it was double the size of an ostrich. This is the first 

 example of extinct Cursores found in North America (Cope). 



Mammals — General Remarks. — 1. We have already seen that the evo- 

 lution of this class may be traced back to the borders of the Palaeozoic. 

 The probable steps are : 1. The Hy pother ia, represented by the Thero- 

 morphs of the Permian ; then the Prototheria, represented by hypo- 

 thetical generalized monotremes of the Triass ; then the Metatheria of 

 Jurassic and Cretaceous ; and, finally, the Eutheria, or typical placentals 

 of the Tertiary. 2. But nothing is more noteworthy than the sudden- 

 ness of the appearance of this last term of the series. We find only 

 Mesozoic types even to the borders of the Tertiary (Laramie), and then 

 without warning there appears the higher type, Eutheria, of the Ter- 

 tiary. This might be explained in Europe, where there is unconform- 

 ity at this horizon, by the gap in the record ; but here in America the 

 record seems almost complete, and yet at the same horizon a great 

 change occurs. It is impossible to explain this unless we admit times 

 of rapid evolution. But even this is not sufficient. We must suppose, 

 also, that these new types appeared here in America by migration 

 about the end of the Cretaceous from some other country, where we 

 hope yet to find the intermediate links. 3. Their ajDpearance was not 

 only sudden but in great numbers and considerable variety. In the 

 very lowest beds of the Lower Tertiary (Puerco beds) Cope finds ninety- 

 three / species, and already all the main divisions, such as Carnivores, 

 Herbivores, Insectivores, and Primates. 4. But, although these main 

 divisions are distinguishable, they are not yet widely separated, as we 

 now know them. At that time there were no typical CarDivores, 

 Herbivores, etc. ; on the contrary, they were all very generalized types 

 — i. e., they approached each other very closely, as shown in the diagram 

 (Fig. 895). As time went on, not only were they separated more and 



more by adaptive modification, but 

 also divided into subordinate branches 

 (not shown in the diagram). In order 

 to indicate the fact that these orders 

 were not yet distinctly specialized, it 

 has been proposed to call them pro- 

 Carnivores, ^ro-Herbivores, pro-Si- 

 miaa, etc. — i. e., progenitors of these 

 now widely distinct orders. They 

 were all probably five-toed Planti- 

 grades, with tuberculated molars 

 (Bunodont), and therefore Omnivores. 

 By tracing the divergent lines of 

 the diagram downward, they meet in the Cretaceous in a hypothet- 

 ical ancestor, which was probably an Insectivore. 5. In the course 



C R E TA\\!,'CE:OUS 



PRIMAL EUTHRIUM 



Fig. 895.— Diagram showing Differentiation 

 of Main Orders of Tertiary Mammals. 



