KINETOGENESIS. 313 



now turns his feet out when using them as bases of re- 

 sistance to muscular labor, the inner digit has become 

 most robust. The mechanical history of the human 

 great toe is however yet unknown. 



As regards the equal development of the third and 

 fourth digits in the Artiodactyla, as distinguished from 

 the development of the middle digit of the Perisso- 

 dactyla, I have advanced the following hypothesis. I 

 have supposed that the primitive members of this for- 

 mer division sprung from pentadactyl plantigrades who 

 dwelt in swamps and walked on very soft ground. 

 The effect of progression in mud is to spread the toes 

 equally in all directions and on each side of the me- 

 dian line. Such feet remain in the mud-loving hippo- 

 potamus, and to a lesser degree in the true pigs. From 

 such ancestry the cloven-footed Diplarthra derived 

 their characters. The Hyracotheriinse, the ancestors 

 of all Perissodactyla, display on the other hand evi- 

 dence of a life on harder ground, especially in the pos- 

 terior foot, where articulations are already rigidly de- 

 fined, and the third digit is longer than the others. 

 Some of their descendants love swamps, as one or two 

 species of tapirs and rhinoceroses, but others live on 

 the dryest ground, as the Andean tapir and the African 

 rhinoceros. As to the highest members of both even 

 and odd toed groups, the Boyidae and the Equidae, 

 their habitat is in the vast majority of cases the dry 

 land (Figs. 80-81). 



Continued and excessive prehensile strain with 

 weight on the longest digits, must be assigned as the 

 cause of the especial elongation; and disuse as the 

 cause of the loss of the external and shorter digits, of 

 the sloths ; so that there remain but two and three 

 (Cholcepus and Bradypus), and in the climbing ant- 



