930 HISTOEICAL GEOLOGY. 



For the Artiodactyl, the theoretical history is the same, excepting that 

 two toes, the third and fourth, were concerned instead of one — the two 

 acting together in dynamical unison. An early Ungulate rising on these two 

 toes in running in order to make thus its greatest speed, the toes and also 

 their metatarsals and metacarpals became equally enlarged and alike elon- 

 gated, while the less-used toes either side, the second and fifth, became a 

 shorter, weaker pair — as illustrated in the Hog ; or, after further change, 

 the dominant pair became still longer, while the shorter was reduced to a 

 rudimentary pair or to hoofs, or became wholly obsolete excepting meta- 

 carpal and metatarsal splint bones, as in the fleeter Artiodactyls. 



Further : the stroke of the foot demanded, for high speed and safety, 

 that there should be little or no rotation of the foot by a movement of the 

 bones of the lower leg, — that is, of the radius and ulna of the front pair and 

 the tibia and fibula of the hind pair, — and consequently the ulna and fibula 

 became reduced sometimes to splint bones, or united by coossification sever- 

 ally to the radius and tibia ; and likewise, in the two-toed Artiodactyl, the 

 corresponding two metatarsals and metacarpals, having no movement between 

 them, became coossified into a "cannon bone." 



There is little that is hypothetical in the above statements, for the suc- 

 cessional lines and the sutures of half-finished coossification are fully illus- 

 trated among the species. Modern surgery finds that bones at joints become 

 coossified by too long confinement in splints without a chance for movement. 

 The variety of four-toed Artiodactyls during the Tertiary was very large ; 

 but at present they are confined to the few of the Suillines, or the Hog 

 family, and the Hippopotamus group. The two-toed species, on the contrary, 

 or the Stags, Deer, Cattle, and the like, are most abundant in recent time. 



The following considerations bear on the character of the changes that 

 went forward among the Mammals.' Of the three divisions (1) the Plant- 

 eaters, (2) the Animal-eaters, (3) the Omnivores, the last-mentioned, — that 

 is, the Quadrumana or Monkeys, — must have early taken to the trees, as 

 their habits indicate. This was an easy method of escaping enemies. Being 

 strong in their fore limbs, they had the trees and the ground, fruit and flesh, 

 within their range. For defense or attack they needed no abnormal growths, 

 such as horns ; and they have been from the first without them. 



The Animal-eaters, in their development, would have divided according 

 to food and habits. Those forced to take the poorest and most abundant 

 and easily got of animal food, the Insectivores, fossorial and skulking species, 

 degenerated, becoming small species, mostly remaining plantigrades, the 

 teeth in some losing their differentiation, in others disappearing altogether. 

 The insects which they ate needed no chewing. Some of them found pro- 

 tection in the substitution of spines for fur (the Hedgehogs), and in the safe 

 but cowardly method of rolling into a ball with spines out in all directions. 



The higher section of the Animal-eaters, or the Carnivores, living on the 

 best of animal food, and generally having to fight for it, and always on the 

 alert, having the fore limbs the stronger pair, and efficient as arms in secur- 



