THE EVOLUTION OF THE MAMMALIA. 247 
may prove to be the long sought ancestors of the group. The 
most primitive of the Oligocene genera are in all respects, except 
the dentition, suggestively like the contemporaneous dogs. 
Throughout the Miocene they increase in stature and power and 
in the size and effectiveness of the cutting and tearing teeth, and 
were evidently the scourges of the northern hemisphere. In 
Europe they gave way to the true cats in the Pliocene, but in 
North and South America huge and terrible forms persisted until 
nearly the end of the Pleistocene. The true cats must have been 
derived from the same ancestral stock, in some region not yet 
known. They make their appearance in the Miocene of Europe 
and North America and gradually developed to take the place of 
the declining sabre-tooth " tigers." 
Of the seven families into which the terrestrial carnivores are 
divided we have thus a reasonably complete history of six, which, 
when traced back, are seen to converge in a single group of 
creodonts in the lower Eocene. The cats are still obscure, but 
when the record of their ancestry has been recovered, it seems 
likely that it will lead back to the same group. 
Of the marine carnivores, seals, walruses, etc., nothing is yet 
definitely known, but it is highly probable that they also were 
derived from creodonts, but from some family quite distinct from 
that which gave rise to the terrestrial flesh-eaters. 
For us, by far the most important and interesting of mamma- 
lian groups is the order Primates, which includes the lemurs, 
monkeys, apes, and man ; and materials for a history of the order 
are by no means lacking, but their tantalizing incompleteness is 
such as to leave a great deal to conjecture and to prevent the 
attainment of satisfactory results upon which observers may agree. 
The group is a very ancient one, and is already distinct in the 
Torrejon division of the lower Eocene and in beds of correspond- 
ing age in Europe. It seems to have been derived from insec- 
tivorous or creodont ancestry. In the Wasatch and Bridger 
and in the upper Eocene and Oligocene of Europe, representa- 
tives of the order, all of small size, are extremely abundant, and 
many different genera have been described. These early Primates 
