142 EVOLUTION, GENETICS, AND EUGENICS 



No particular member of this extinct group has been found that fulfils 

 all the requirements of a primitive horse ancestor, so the chances are 

 that the real ancestral condylarthran has not been discovered. 



"The course of their [Equidae] evolution," says Dendy,' "has 

 evidently been determined by the development of extensive, dry, 

 grass-covered, open plains on the American continent. In adap- 

 tation to life on such areas structural modification has proceeded 

 chiefly in two directions. The limbs have become greatly elongated 

 and the foot upUfted from the ground, and thus adapted for rapid 

 flight from pursuing enemies, while the middle digit has become more 

 and more important and the others, together with the ulna and the 

 fibula, have gradually disappeared or become reduced to mere vestiges. 

 At the same time the grazing mechanism has been gradually perfected. 

 The neck and head have become elongated so that the animal is able 

 to reach the ground without bending its legs, and the cheek teeth have 

 acquired complex grinding surfaces and have greatly increased in 

 length to compensate for the increased rate of wear. As in so many 

 other groups, the evolution of these special characters has been 

 accompanied by gradual increase in size. Thus Eohippus, of Lower 

 Eocene times, appears to have been not more than eleven inches high 

 at the shoulder, while existing horses measure about sixty-four inches, 

 and the numerous intermediate genera for the most part show a 

 regular progress in this respect. 



"All these changes have taken place gradually, and a beautiful 

 series of intermediate forms indicating the different stages from Eohip- 

 pus to the modern horse [Equus] have been discovered. The sequence 

 of these stages in geological time exactly fits in with the theory that 

 each one has been derived from the one next below it by more perfect 

 adaptation to the conditions of life. Numerous genera have been 

 described, but it is not necessary to mention more than a few." 



The first indisputably horselike animal appears to have been 

 Hyracotherium, of the Lower Eocene of Europe. Another Lower 

 Eocene form is Eohippus, which lived in North America, probably 

 having migrated across from Asia by the Alaskan land connection 

 which was in existence at that time. Li Eohippus the fore foot had 

 four completely developed hoofed digits and a "thumb" reduced to 

 a splint bone; in the hind foot the great toe had entirely disappeared 

 and the little toe is represented by a vestigial structure or spHnt bone. 



' Arthur Dendy, Outlines of Evolutionary Biology (D. Appleton & Company, 

 [916). 



