LIFE IN TERTIARY TIMES 299 



to touch the ground in the American MerycJrippus and 

 Hi-ppotherium, as also in the European Hipparion of the Upper 

 Miocene. Finally, in Protohippns and the Pliohippus, only one 

 functional toe remains. These last reached South America, 

 and then produced Hippidium and the true Horses, which 

 gradually spread over both hemispheres, but died out in 

 South America. 



In all these animals, when the foot could no longer make 

 any rotatory movement in relation to the leg, the muscles 

 attached to the fibula, which determine these rotatory 

 movements, were no longer used, but atrophied, as the 

 doctrine of Lamarck prognosticates, and brought about 

 the gradual atrophy of the fibula to which they were 

 attached. For the same reason the radius of the fore leg which 

 corresponds to the fibula became completely united with 

 the ulna. 



To sum up, the same tendencies are at work in the evolution 

 of the limbs both of Mammals and Reptiles. In both classes 

 the terrestrial animal succeeded in penetrating into other 

 environments open to its activity — the water, from which its 

 ancestors formerly came, and the air, from which its weight 

 would seem to exclude it, and it arrived at progression in both 

 these media by analogous procedures. On land its evolution, 

 apart from a few special adaptations, such as that permitting 

 underground or arboreal life, for instance, was dominated by 

 two needs — to see as far and to run as fast as possible — which 

 induced them to erect themselves on their limbs. In both cases 

 the intervention of the animal's own volition, with a view to 

 attaining a desired end, is evident. The resulting modifications 

 were not linked with any particular diet ; therefore the modi- 

 fication of the teeth do not strictly follow those of the limbs. 



The oldest placental Mammals possessed what is called a 

 complete dentition, that is to say, forty-four teeth — eleven to 

 each half of the jaw (three incisors, one canine, four premolars 

 and three molars). This dentition is seen at the beginning of the 

 series in the herbivores, the insectivores, and the carnivores. It is 

 occasionally reduced, but never increased, except where 

 elements of the molars become dissociated, as in Cetacea. It 

 must therefore be considered as the primitive dentition of 

 the placental Mammals, and its generality leads us to think 

 that all these animals descend from the same initial type, one 



