ELEPHANTS, HORSES, AND CAMELS 



175 



verse ridges are shown as they occur in the fossil remains of ex- 

 tinct species. Here also the transition from the elongate head of 

 Moeritherium to the high short head of the elephant is illustrated 

 from actual remains of a chronological succession of species. Such 

 a series is not merely interpreted as evi- 

 dence of evolution; it is the actual record 

 of evolutionary change. •"- ^'"^~ 



Adaptive Structure of the Horse. Loco- 

 motion. The evolution of the horse is in a 

 number of ways more extreme than that 

 of the elephants. With respect to speed, 

 the legs are elongated and slender, retain- 

 ing only one functional digit. As in many 

 other animals, the feet no longer rest flatly 

 on the ground, but in no other animal is 

 the elevation to the toes more extreme 

 than here, for not merely the tip of the 

 toe, but the hoof alone, the homologue of 

 claws and nails, comes in contact with the 

 ground. This lifting of the body and 

 elongation of the lower parts of the ap- 

 pendages results in a relative shortness 

 and concentration of the leg muscles, and 

 consequent rapid movement and lengthen- 

 ing of stride. The function of propulsion 

 is largely relegated to the hind limbs, in 

 which elongation of the foot and con- 

 centration of leg muscles is extreme. 



Elongation of the legs is compensated in 

 the horse by lengthening of the neck, so 

 that the head of the animal can reach the 

 ground. 



Teeth. The food hal^its of the horse 

 and other grazing types demand special 

 development of the teeth for chewing harsh 

 vegetation. Grazing habits demand no 

 canines, hence they are reduced. The 

 incisors are important for cropping low 

 vegetation and are elongated. The molars 

 and three premolars are similar. All are 



I I e'yvayyvel 



H den-tvyie- 



@ cernertt 



■ ■natvtr'a-l cavity 



Fig. lOL — The grinding 

 surface of horses' teeth. 

 A, worn surface of milk 

 molar of colt about six 

 months old; B, unworn 

 surface of milk molar 

 before birth; C, pre- 

 molar of horse eight or 

 nine years old ; e, enamel ; 

 i, natural cavity in ce- 

 ment; d, cavity, later 

 fi 1 1 e d with (; e m e n t. 

 (From Lull, after 

 Chubb.) 



