LIFE IN THE PAST 185 
Size, however, is not the most important point in any 
animal. Speed, sagacity, variability, and power of 
adaptation, these are the qualities which the world 
prizes, and these the new mammals possessed. 
The next geological era is the Cenozoic, or period 
of modern life. This is divided into two quite dis- 
tinct sections, the Tertiary and the Quaternary. This 
era began about five million years ago, roughly speak- 
ing, and is still going on. The greater half of it is 
known as the Tertiary. It was during this time that 
the mammals came to their own. At first these crea- 
tures belonged to what the scientist knows as gener-~ 
alized types. They are jacks-of-all-trades. The stu- 
dent of early animal life finds in the little Phenacodus, 
which was scarcely bigger than a good-sized setter 
dog, the beginnings from which many forms have 
subsequently developed. This creature showed points 
of structure which to-day may be seen in such diversi- 
fied animals as the dog, the horse, the rabbit, and the 
monkey. It is not, of course, suggested that Phe- 
nacodus was the immediate ancestor of any of these. 
But there were no animals in those times more like 
these I have mentioned than was Phenacodus, and 
from forms like it in main features all of these other 
animals have since been derived, each species of ani- 
mal having become adapted to one particular kind of 
life. The development of diversified situations on the 
