184 NEW YORK, ZOOLOGICALs SOCIEIY: 
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 faunz 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. 
