322 



HISTORICAL GEOLOGY 



mals (Fig. 203), like the odd-toed ones, were descended from a 

 five-toed Eocene ancestor. In their development the first toe 



Fig. 203 

 Evolution of foot of even-toed (Artiodactyl) Mammals illustrated 

 by existing forms. A, Pig; B, Roebuck; C, Camel. (From 

 Norton's "Elements of Geology," by permission of Ginn and 

 Company, Publishers.) 



disappeared, while the middle pair of the remaining four became 



larger and the two side toes became smaller and smaller, having 



disappeared altogether in such 

 a tjrpe as the modern Camel. 

 This sort of evolution in the 

 Camel family has been traced 

 in almost as much detail as in 

 the Horse family. Beside the 

 Camel, other two-toed existing 

 forms are Deer, Cattle, and 

 Sheep. The two-toed Artio- 

 dactyls now predominate, while 



the four-toed forms (at present represented e.g. by Hogs and 



Hippopotami) culminated in the Tertiary. 



Proboscidians (e.g. Elephant). This group of hoofed Mam- 



a b 



Fig. 204 



a, Mastodon tooth; b, Mammoth tooth. 

 Both viewed from the side. 



