26o 



THE ORIGIN AND EVOLUTION OF LIFE 



the modern horses, tapirs, rhinoceroses, and various types of 

 cloven-footed animals. 



A very general principle of mammalian evolution is illus- 

 trated in Fig. 124 (.4, B), namely, the increase of size character- 

 istic of all the herbivorous mammals, which almost without 

 exception are in the beginning extremely small forms that 

 evolve into massive forms possessing for defense either power- 



FiG. 125. A Primitue Whale from the Eocene of Alabama. 



Zeiiglodon cdoides exhibits a secondary elongate, eel-shaped body form analogous to that 

 of many of the aquatic, free-swimming, surface-dwelling reptiles, aquatic amphibians, 

 and fusiform fishes. Restoration by Gidley and Knight in the American IVIuseum of 

 Natural History. 



ful tusks or horns. The most conspicuous example of very 

 rapid evolution which has taken place prior to the close of 

 Eocene time is that of the great primitive whale Zeuglodon 

 cetoides, discovered in the Upper Eocen,e of Alabama, and now 

 known to have been distributed eastward to the region of the 

 Mediterranean. As described above (p. 241), as an example of 

 reversed adaptation and evolution, this animal had already 

 passed through a prior terrestrial phase and had reached a 

 stage of extreme specialization for marine life. These zeu- 

 glodonts parallel several of the marine groups of reptiles (Figs. 

 76, 87), also certain of the amphibians and fishes (Figs. 60, 44), 



