266 



THE ORIGIN AND EVOLUTION OF LIFE 



not predetermined by the heredity-chromatin, because the 

 same law of limb proportion prevails in all heavy, slow-mov- 

 ing mammals, whatever their descent; for example, this law 

 holds among the heavy, slow-moving reptiles, the Sauropoda 

 (Fig. 97), as well as among the heavy, slow-moving mammals. 

 The most beautiful adjustment of the proportions of the 

 limb segments to speed is observed in the evolution of the 



horses (Fig. 130). Here we see 

 that the upper segments (hu- 

 merus, femur) are abbreviated, 

 while the lower segments (fore- 

 arm, lower leg, manus, and pes) 

 are elongated. This is precisely 

 the reverse of the conditions 

 obtaining among the slow-mov- 

 ing titanotheres and proboscid- 

 ians (Fig. 131). Among the 

 horses, too, the same law pre- 

 vails and governs the very 

 precise adjustment of the ratios 

 of each of the limb segments, 

 quite irrespective of ancestry. 

 In the swift Hipparion of Amer- 

 ica, for example, the highest 

 phase of equine adaptation to 

 speed, the indices and ratios of the limb segments are very 

 similar to those in the existing prong-horn antelopes {Antiloca- 

 pra) of our western plains. Contemporary with the Hipparion 

 of Pliocene time, adapted to racing over hard, stony ground, 

 is the relatively slow-moving, forest-living horse (Hypohippus) 

 of the river borders of western North America (Fig. 130), in 

 which the limb proportions are quite different. There is reason 



Fig. 129. Horses of Oligocexe Time. 



The horses frequenting the semi-arid 

 plains of Oligocene times present an 

 intermediate stage in the evokition of 

 of cursorial motion — Mcsohippus, with 

 a narrow, three-toed type of foot, 

 elongate, graceful limbs, and teeth with 

 crowns beginning to be adapted to the 

 comminution of silicious grasses in 

 accommodation to the contemporane- 

 ous world-wide evolution of grassy 

 plains. This law of the contemporane- 

 ous evolution of an environment of 

 grassy plains and of swift-moving 

 Herbivora was first clearly enunciated 

 by Kowalevsky in 1873. 



Restorations by Osborn, painted by 

 Charles R. Knight, in the American 

 Museum of Natural History. 



