OF EVOLUTION. 47 



known living types of mammals which are generi- 

 cally represented in the Eocene period are two gen- 

 era of bats Vespertilio and Vesperugo and the 

 opossum (Didelphys). In the Miocene period the 

 faunal difference is measurably lessened by the 

 elimination of the special orders which belong to 

 the period preceding, and by the introduction of a 

 considerable number of modern genera, such as 

 the porcupine, beaver, squirrel, rabbit, tapir, rhi- 

 noceros, hippopotamus, hog, deer, giraffe, elephant, 

 cat, dog, and hyena. The families, moreover, are 

 very largely identical with existing ones, so that 

 in its entirety the Miocene fauna may in a broad 

 way be looked upon as distinctly modern. 

 The species of this period are, however, all, or 

 nearly all, distinct .from those now living. In the 

 period following, the Pliocene, there is a still 

 further approximation to the modern fauna in 

 the introduction of an additional number of exist- 

 ing types such as the sheep, goats, and oxen, 

 the bear and camel, and among monkeys, the 

 macaques. Indeed, the greater number of the 



