162 COPE. [Vot. ITI. 
Observation on five-toed plantigrade Mammals shows that their 
feet are turned neither inwards nor outwards in progression, 
but straight forwards. It is probable that the primitive Mam- 
malia moved in the same manner. This is also to be inferred 
from the fact that they were plantigrade, so that the leverage 
transversely in or out which results from the elevated heel 
of the digitigrade leg was wanting to them. In progression 
of this type the middle digits of course leave the ground last, 
and strike it first. Thus the middle toes have been stimulated 
at the expense of the lateral ones, so that in the Diplarthra, 
either the middle one (Perissodactyla) alone remains, or the 
middle two (Artiodactyla). In the kangaroos, the external 
toes have been chiefly used, so that the fourth and fifth digits 
have been principally developed. In man, who now turns his 
feet out when using them as bases of resistance 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 develop- 
ment of the middle digit of the Perissodactyla, I have advanced 
the following hypothesis. I have supposed that the primitive 
members of this former division sprung from pentadactyl plan- 
tigrades 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 median line. Such feet 
remain in the mud-loving hippopotamus, and to a lesser degree 
in the true pigs. From such ancestry the cloven-footed Diplar- 
thra derived their characters. The Hyracotheriine, the ances- 
tors of all Perissodactyla, display on the other hand evidence 
of a life on harder ground, especially in the posterior foot, where 
articulations are already rigidly defined, 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 Bovide and the Equide, their 
habitat is in the vast majority of cases the dry land (Fig. 12). 
Continued and excessive prehensile strain with weight on 
the longest digits, must be assigned as the cause of the espe- 
cial elongation; and disuse as the cause of the loss of the 
