138 



THE TWO-TOED ARTIODACTYLA. 



in the Middle Miocene, and a series of very 

 closely allied genera can be traced through 

 the preceding strata until we arrive at the 

 two-toed genera Entelodon and Choerotherium, 

 belonging to the Upper Eocene, and the 

 four-toed genera, Chceropotamus and Hyo- 

 potamus, of the Eocene gypsums of Mont- 

 martre. The pigs accordingly belong to a 

 very old stock, the stages in whose devel- 

 opment we can follow without interruption 

 down to the forms now existing. No fossil 

 forms have yet been found, however, repre- 

 senting the babirussa, the wart-hog, or the 

 river-hogs. 



A quite different series is presented by the 

 American peccaries. " We appear," says 

 Marsh, " to have in the series of genetic 

 forms comprised between the Eohyus of the 

 Lower Eocene and the peccaries of our own 

 time (Helohyus, belonging to the Middle 

 Eocene; Perchoerus, to the Lower Miocene; 

 Thinohyus, to the Upper Miocene), the line 

 of descent terminating in the typical American 

 Suidae of the present day. Extinct genera 

 are already found in the Quaternary period, 

 for example, Platygonus ; but at that time the 

 peccaries extended to the northern boundary 

 of the United States, whence they have since 

 retreated." I insist on the difference in the 

 primitive stocks on the two sides of the ocean 

 since Eocene times. "Whatever may be said 

 of them," Marsh continues, "so much is cer- 

 tain, that no authentic remains of the genera 

 Sus, Porcus, Phacochoerus, or Hippopotamus, 

 which constitute the group of the Suidse in the 

 Old World, have ever been found in America." 



On both sides of the ocean the old 

 bunodont Artiodactyla of Eocene times had 

 four toes on the feet. The reduction of this 

 number has gone on during the further 

 development of the type, but has not been 

 completed ; it has stopped short in the 

 peccaries at the stage indicated. That the 

 peccaries are the forms which approach most 

 closely to the ruminants of all the hog family 

 has already been mentioned. 



In the series of the selenodont ruminants 

 we have to take note of analogous facts. 

 With reference to these also we may without 

 fear of contradiction maintain the proposition, 

 that we find two entirely different stem-lines 

 on the two sides of the ocean. These arise 

 from genera belonging to the Middle and 

 Lower Eocene, in which the characters derived 

 from the dentition and the structure of the 

 feet are still not well pronounced, inasmuch 

 as the cheek-teeth exhibit, so to speak, 

 wavering forms between bunodont and sele- 

 nodont types, the number of the incisors 

 begins to be reduced, and the four-toed feet, 

 by continuous reduction of the lateral toes, 

 become by degrees two-toed. It is only in 

 later epochs that we begin to obtain characters 

 derived from antlers and horns. It may be 

 taken as a general rule that these outgrowths 

 are only late products, and that the original 

 ruminants were without them, as the young 

 animals are still. The fact that horns and 

 antlers do not appear till long after birth 

 is in itself enough to show that these appen- 

 dages are of recent acquisition. 



The transition from these equivocal and 

 variable forms, which Leidy has characterized 

 as "ruminant pigs," is effected principally, as 

 Kowalewsky has shown, by the disposition of 

 the bones of the wrist and ankle (carpus and 

 tarsus), which get arranged in two vertical 

 series in such a manner that each series 

 corresponds to one of the principal toes and 

 so helps to carry the weight of the body, 

 while the forms in which this arrangement 

 does not take place remain unfruitful and 

 cannot be continued in the direct line of the 

 present ruminants. 



In the Middle and Upper Eocene are found 

 a number of these forms, whose teeth already 

 show the half-moon-shaped folds, but which 

 still retain the full number of incisors in the 

 upper jaw, and which, moreover, sometimes 

 have very strong canines. In many of these 

 the lateral toes are greatly reduced, although 

 the wrist and ankle still retain the unfavourable 



