286 ANNALS NEW YORK ACADEMY OF SCIENCES 



in the collateral descendants of Eotiianops, we observe a relative short- 

 ening of the digits, correlated with increasing body size and straighten- 



Mts. III. 

 ing of the knees, so that drops from .34 through .31 and .30 



in the Middle Eocene genera to .28, .26 and even .20 in the gigantic 



Brontotherium. Again, in the Ehinoceroses, the oldest, smallest and most 



Mts. III. 

 primitive forms have a foot of moderate length ( -=.43-42). 



but by progressive relative shortening and broadening, the ratio drops to 



.24 in Metamynodon. In the Hippopotami, which are probably descended 



from animals proportioned about as in Oraodon (with an index of .38), 



gigantism and aquatic habits have brought about a reduction of the index 



to .26. Similarly in the Toxodonts, the smaller and more primitive forms 



Mts. III. 

 had relative long, slender feet, while in the gigantic Toxodon 



falls to .17. F - 



In those groups in which the most primitive known members had 

 already attained a slender foot, with reduced side toes, gigantism is 

 unable to effect a complete approximation to the graviportal type. Thus, 

 in the bisons, which are undoubtedly descended from slender-footed forms 

 having a metatarso-femoral ratio not less perhaps than .75, the sudden 

 increase in size causes the ratio to fall but slightly (to .65). In the. 

 gigantic Irish elk, whose ancestors probably had a very high metatarso- 

 femoral ratio (perhaps 1.00 or more), this ratio falls to .71. 



The evolution of cursorial forms is also indicated by marked changes 

 in the ratio under consideration. Thus, in the Equidse, it rises from 

 .53 in Eohippus, through .68 in Mesohippus to .99 in Hypohippus, reach- 

 ing the extreme of 1.16 in the slender-limbed Upper Miocene horse, 

 Neohipparion. In the relatively small and slender kiang, the ratio is 

 still 1.00, but in the relatively heavy-bodied Equus scotti of the Pleisto- 

 cene, the ratio is only .84, while in modern horses, we observe even in 

 race horses a falling off of the index to .78, and in the stocky-limbed 

 Hippidion, it drops to .72. Correlated with this fall in the length of 

 the metatarsal III, we observe a straightening of the knee. These fig- 

 ures possibly may mean that the modern Equus caballus, on account of 

 its great size, is somewhat less adapted to extreme cursorial locomotion 

 than was the slender NeoJiipparion which closely paralleled the deer 

 Odocoileus.* 1 On the other hand, race horses seem to have compara- 

 tively long femora (cf. Stillman, p. 80) and cheetahs, hounds and other 



41 J. W. Gidlev : Bull. Amer. Mus. Nat. Hist., Vol. 19, pp. 474-476. 1903. 



