4 o THE INTERNATIONAL MONTHLY, 
Quaternary Pleistocene Sheridan Beds 
f Pliocene Blanco 
Miocene 
Nebraska 
Deep River 
Loup Fork 
John Day 
( Protoceras beds 
Tertiary <! Oligocene White Rivera Oreodon beds 
( Titanotherium beds 
r Uinta 
D • , f Washakie 
Bndger < n 
& \ Bndger 
<[ Wind River 
Wasatch 
Torrejon 
v Puerco 
The oldest North American faunas of Eocene date (the Puerco 
and Torrejon), and those of France of approximately equivalent 
date (Cernaysian formation) are very peculiar and, for the most 
part, utterly unlike those of later times. A number of Mesozoic 
types persist and others which seem to be descended from Meso- 
zoic ancestors abound, while a third element, for which no such 
ancestry can yet be assigned, is apparently composed of immi- 
grants from some unknown region. The next Eocene forma- 
tion in the ascending scale, the Wasatch of North America and 
its equivalent the Suessonian of France, has a very much more 
advanced and differentiated mammalian fauna, and one which, 
evidently, to a great extent, has been recruited by immigration. 
The Torrejon and Wasatch faunas are so unlike that, judged by 
ordinary standards, we should believe them to be separated by a 
great lapse of time. For reasons which cannot well be discussed 
here, there probably was no such great interval, and the sudden- 
ness of the change was almost certainly due to migration. 
From the Wasatch to the present, the evolution of many of 
the mammalian types can be followed in a most satisfactory 
way, but there is a great difference between the various orders in 
this regard, due to the mechanical conditions of fossilization. 
Thus, very small animals with fragile bones are less apt to be 
