GREGORY, QUADRUPEDAL LOCOMOTION 



283 



from a comparison of the photographs (Fig. 1) of an ambling ele- 

 phant and of a galloping horse, it seems probable that the straightness 

 of the limbs in graviportal animals has been evolved pari passu with the 

 short rectigrade feet and with an ambling even gait, in combination with 

 a long stride of minimal acceleration increment (p. 274). Conversely, 

 the bent or angulate character of the limbs in the horse and other cur- 

 sorial animals is correlated in part with the very long, slender unguli- 

 grade feet and with a bounding galloping or trotting gait, in combination 

 with a long, very rapid stride of maximal acceleration increment. 



Fig. 5. — "Cursorial" adaptations for the run, gallop, etc., in the Neohipparion 



Iii other words, the use and structure of the feet have been the teleo- 

 logical dominants which have determined the diverse modifications in 

 the musculature, proportions and angulation of the proximal segments 

 of the limbs, just as in early stages of aquatic adaptation in reptiles 

 (e. g., Thalattosuchia, ISTothosauria, etc.) the aquatic habits are reflected 

 more clearly in the feet or distal segments rather than in the proximal 

 limb segments. 



MECHANICS OF THE FOOT IN GRAVIPORTAL AND CURSORIAL FORMS 



Comparing the structure and function of the graviportal and cursorial 

 types of feet, we see (Figs. 3 and 4) that, in the elephant, the very mas- 

 sive gastrocnemius and soleus muscles are attached at a wide angle to the 

 massive calcaneum, while the foot itself is very short. In the position 

 shown in Fig. 3 (II) and as compared with conditions in the horse, this 



