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 



