184 NEW YORK ZOOLOGICAL SOCIETY. 



of the existing orders of mammals, and giving rise to primitive 

 Carnivores known as Creodonts, primitive Ungulates known as 

 Amblypoda and Condylarthra, and still other orders not so easily 

 defined. Only partial traces of types which were ancestral to the 

 Puerco fauna are found in the underlying Laramie beds, which 

 are assigned to the top of the Cretaceous period. This would 

 probably indicate that the Puerco mammals, to a large extent, 

 come in from some other country, probably from the North. 



SECOND RADIATION. 



By the Middle Eocene, the early and generalized types of this 

 Puerco fauna were rapidly dying out, leaving only a few carnivo- 

 rous forms to linger on into the Oligocene. We do not, of course, 

 know the causes of their disappearance, but it is safe to hazard 

 the conjecture that their structural development, especially their 

 limited brain capacity, was inadequate to cope with that of the 

 new and more highly organized animals which suddenly appear 

 in the Lower Eocene. These new types were possibly descended 

 from some side line of the earlier radiation, and were derived 

 from members of the Puerco group, which had lingered on in 

 the original northern home, but no direct lines connecting these 

 two faunae are known. 



Assuming that the Puerco mammals were driven out of more 

 northerly or boreal lands, where they had originally developed, 

 by a declining temperature, it is conceivable that some animals 

 remained behind and adjusted themselves to the changed condi- 

 tions, until a still further increase of cold forced them also to 

 follow the path of their predecessors, southward. 



Some of these Lower Eocene types of this second radiation, 

 which are found in the Wasatch beds of Wyoming, have sent 

 down lines of descendants, which have ultimately culminated in 

 existing animals. At this time first appear the horses, tapirs, 

 rhinoceroses, camels and dogs. Some of these animals, such as 

 the horses and rhinoceroses, are found contemporaneously in 

 Europe ; others, like the camels, are peculiar to this country. 



Being more highly organized and better adapted to their en- 

 vironment, these new types entirely supplanted "the older fauna, 

 and by the Oligocene this transformation was complete, and the 

 older fauna had disappeared. This Wasatch fauna culminated 

 in the Miocene, and then faded gradually away on this continent, 

 until in the Middle Pleistocene they were largely supplanted by 

 new arrivals from Eurasia. 



