150 GRADUAL MODIFICATION OF ANIMAL FORMS xi 



being quite wanting in early forms of the same group, has 

 been noticed by the same author in many different series of 

 Ungulates. 



During the miocene period the peculiar dental character- 

 istics of the modern ruminants, especially the loss of the upper 

 incisors, were developed, all selenodont artiodactyles henceforth 

 showing it. Of this early race of imperfect ruminants, 

 still retaining many generalised characters, especially in the 

 skull, the cervical vertebrae, fibula, stomach, etc., the chevrotains 

 (Tragulidce) are the survivors, especially the West African 

 HyomoscJius, which has existed almost unchanged since the 

 late miocene of Sansans and Steinheim. Then for the first 

 time the appendages called antlers were introduced, but only 

 in a comparatively rudimentary condition, with long pedicles 

 and few branches, as in the modern Muntjaks. It was 

 not till pliocene and especially pleistocene epochs that the 

 wonderful and luxuriant variety of cervine antlers reached 

 their full development. As offsets of the deer group, the 

 giraffe, the gigantic Siwalik Sivatherium, and the Hellado- 

 therium of Greece may be mentioned, the two latter having 

 become extinct, apparently without descendants. 



Later still, the yet more specialised forms of hollow-horned 

 ruminants appear forms which now dominate the earth, 

 being of all Ungulates the most widely diffused and most 

 numerous in species, in individuals, and in outward variety, 

 though in essential structure all alike. One of their principal 

 characteristics is the modification of their molar teeth in the 

 same way as in the modern horses, to which in some respects 

 they seem to form a parallel group. The difference between 

 the molar tooth of a hollow-horned ruminant and that of a 

 deer consists in the great lengthening of the crown without 

 any change in the pattern of the enamel folds, and in the 

 addition of cement to support these folds. This alteration 

 did not take place suddenly, and the crowns of teeth of the 

 artiodactyles before the time of deer were still shorter than in 

 those animals. 



Among the deer themselves, as Lartet observed, the most 

 ancient have very short-crowned molars, and the depressions 

 on the surface are so shallow that the bottom is always 



