On the. Ofitpolngy of the Hyopotamldte. 181 



forms an enormous advantage over the other, non-ruminant, Pari- 

 digitata occurring in the same strata. They could live on such 

 matters as twigs, bark of trees, mosses, lichens, on which no other 

 Ungulata can subsist ; such food is found everywhere, requires no 

 cunning and very little struggle to get it. All essential modifications 

 were attained very early, and the chief of these are the confluence of 

 the two middle digits in a complete cannonbone and rumination. 

 Then began the luxury of all sorts of appendages — excrescences on 

 the frontal bones covered with skin, uncovered by skin in the form 

 of prickly simple horns {PiuJa), or double (Dicroceras of »Sansan, 

 Muntjac), then branched and palmated. In other groups these 

 bony cores were covered with horny sheaths, which at first differed 

 but little from agglutinated hairs (Antilocapra americana), theii 

 became more compact, as in the smooth and hard homy sheath 

 of the hollow-horned Euminantia. These secondary characters were 

 all acquired, thanks to abundant time, after the essential characters 

 of the type had been assumed ; if man had come on earth a little 

 later than he did, he certainly -nould have found nearly parallel 

 cases in the group of 8uina, monodactyle (-with cannonbone) hogs 

 with different appendages. As it is, Jie stopped the course of 

 events ; all further improvement is out of the question, or only 

 possible in such groups as the Eodentia, who prey on man's food, 

 being at the same time independent of him. 



It may be asked, How stands the matter in the Imparidigitate 

 Ungulata ? And though I cannot enter fully into the case, I may 

 state that the same course of events is observable in them ; only 

 there could be no inadaptive reduction, as the body could not, 

 under any circumstances, be held in equilibrium upon one single 

 third digit, if this one had not taken the whole distal surface 

 of the carpus and tarsus. But the task in tliis group was much 

 more difficult ; to get one middle digit to perform the work shared 

 in the ancestors by five, and in the immediate progenitor by three, 

 reqmred time. To accompKsh this, two geological periods were 

 needed ; but still, by the incessant tendency to reduction, the work 

 was done, and the monodactyle horse spread over the surface of the 

 globe, superseding all other Imparidigitata, which are evidently 

 rapidly dying out. The only ttvo genera which remain stUl, the 

 Rhinoceros and the Tapir, cannot last long. But this spreading 

 and multiplication of the Equidce was also accompanied by a total 

 change of diet : from an omnivorous animal it became a grass-eater ; 

 and indeed, by its teeth and many other characters, the horse is very 

 analogova to the Kuminaiitia, being, as they are, the culmination- 

 point of the group \)f Imparidigitata. The reduction of the horse- 

 foot, however, is not fully accomplished yet; to attain this, the 

 styliform metatarsals and metacarpals (the second and fourth) have 

 to be lost. 



