206 INTRODUCTION TO EVOLUTION 



the side digits have a function in the three-toed forms, even the ones in 

 which these digits were short relative to digit III? This question is usually 

 answered in the negative but Simpson (1951) has challenged such a 

 conclusion. He pointed out that when a horse is galloping and lands on 

 its middle toe, this toe is bent strongly upward. As noted above, under 

 such great pressure the side toes of, for example, Merychippus would touch 

 the ground. Simpson has suggested that the side toes may have had "an 

 essential function to act as buffers to stop the bending of the middle toe 

 at this point and to lessen the danger of spraining the elastic ligaments 

 by stretching them too far." If this is correct, why did the side toes 

 disappear in the line leading to Eqiius? Perhaps because other structures 

 took over the function of preventing spraining of the spring mechanism. 

 We have mentioned that in Eqiius the tendons of long flexor muscles are 

 connected to adjacent bones by check ligaments. These assist the other 

 elastic ligaments and form part of the spring mechanism, especially under 

 powerful stress. Perhaps the safety factor provided by these check liga- 

 ments was not present in the three-toed horses. This is a point on which we 

 do not have information, however. We may note in passing that the horse 

 has paid a price for its highly specialized springing foot. Lameness con- 

 nected with injury to the elastic ligaments is common. 



Some species of Pliohippus had tiny side toes, though in other species 

 these were represented only by splint bones, as in Equus. Pliohippus 

 attained the size of a modern pony, some 40 inches (10 hands) high. The 

 trends for increase in the preorbital length of the skull and increase in 

 size and complexity of the molar teeth continued. 



The transition from Pliohippus to Equus, the genus to which modern 

 horses, asses, and zebras belong, was a small one, involving further 

 increase in size and some changes in anatomical details. The first repre- 

 sentatives of Equus appeared in late Pliocene times; during the 

 Pleistocene the genus achieved world-wide distribution. Although North 

 America has provided the stage for the greater part of their evolution, 

 horses became extinct on this continent by the close of the Pleistocene. 

 This extinction is difficult to explain. By the time man later reintroduced 

 horses into America conditions on our western plains were highly favor- 

 able for them. 



EVOLUTION OF PROBOSCIDEANS 



The living representatives of Order Proboscidea are 

 the Asiatic and the African elephants. The two differ somewhat in struc- 



