464 THE CHANGING GENERATIONS 



A great deal is known of the history of the placental mammals. It 

 makes a fascinating story, but one which is far too complex for treatment 

 in a book of this nature. For those who would like to learn something of 

 the knob-skulled, tusked unintatheres, giant horned titanotheres, huge- 

 clawed horselike chalicotheres, club-tailed armored glyptodonts, blood- 

 drinking saber-toothed tigers, and many another group as strange, Scott's 

 History of Land Mammals in the Western Hemisphere (The Macmillan 

 Company, New York, 1937) will make good reading. Here we shall have 

 to content ourselves with an account of the development of the horses, 

 perhaps the most completely known of evolutionary histories. 



THE EVOLUTION OF THE HORSES 



The fossil record of the horses constitutes one of the classic examples of 

 evolution. Not only is the record extraordinarily complete, but the 

 important changes that occurred in teeth and skeleton are easily seen 

 and understood. The horses constitute one of the families of the hoofed 

 mammals, or ungulates, and it will be best to begin their story by a survey 

 of the history of this group. 



The hoofed mammals. During the Paleocene epoch there appeared 

 among the primitive herbivores some which had small hoofs instead of 

 claws on their toes. These were the ancestral ungulates. They all began 

 as five-toed animals. In some of them the third and fourth toes were of 

 equal length and the foot axis passed between the two; from such forms 

 arose the even-toed ungulates, or artiodactyls. In others the middle toe was 

 the longest and lay in the axial line of the foot, and these forms gave rise 

 to the odd-toed ungulates, or perissodactyls, including the horses. 



Ungulates have always been the main food of the larger carnivorous 

 mammals. Under selective pressure from these predators the members of 

 all the ungulate groups tended to increase in size and to attain greater 

 speed in running by rising on the toes instead of planting the whole foot 

 on the ground. As the heel was raised, the side toes were lifted from the 

 ground, as can be seen by placing the hand flat on a table and raising the 

 wrist. With loss of function the lateral toes dwindled and in the end some- 

 times disappeared. Some of the ungulates early developed into large, 

 ponderous animals which no longer depended upon speed for protection. 

 In these toe reduction did not proceed far; the legs became pillarlike, and 

 the spread toes buttressed the stumpy feet, as in the elephants. In other 

 ungulate lines, size increase occurred more slowly, and speed continued 

 to be the main defense; in them development of the running leg was 

 carried further. 



What happened to the feet of the even-toed artiodactyls can be visual- 

 ized by raising the wrist, as before, keeping the two middle fingers on the 

 table. First the thumb lifted and was lost, leaving a four-toed foot; then 



