306 TOWARDS THE HUMAN FORM 



The upper incisors were absent. Following them, in the 

 Miocene, is Procervulus, in which the horns were not shed and 

 as a rule were simply bifurcated. During the same period, 

 however, they acquired a circle of "pearls" separating the 

 deciduous part from a long persistent peduncle in Dicroceras 

 of the Miocene. In the Upper Miocene the peduncle 

 became shortened and almost the whole of the horn became 

 deciduous in Cervulus, still extant in India. Thus we come 

 to the Roebuck, which dates from the Upper Miocene. The 

 Giramdse (Helladotherium, Sivatherium) appear at the same 

 time. At this time also the Antelope, in which a horny casing 

 covers a bony axis, with hollow spaces in it, becomes distinct 

 from the Deer, and thus opened the series of Ruminants 

 with hollow horns, in whom canines have disappeared. Hence 

 the Aristotelian idea of a " balance ", as G. Saint-Hilaire 

 would say, between the defensive organs. Arsincetherium of 

 Fayum, which will be discussed later, supplies the gravest 

 objection to such a conception. 



It is not easy to explain how the teeth could become simplified 

 and disappear in those animals which are grouped together 

 in the class Edentata. This is, however, no isolated instance. 

 The Ornithorhynchus and the Echidna replaced the multi- 

 tuberculated teeth of their ancestors in the Secondary period 

 by horny ones, except where they lost them altogether. The 

 teeth of the Sirenia and Cetacea, like those of the Edentata, 

 are simplified, multiple, and at last completely atrophied. 

 A general problem thus arises. In the Eocene of Patagonia 

 Ameghino discovered fossil Mammals whose molars were 

 simplified and had become cylindrical, but which still possessed 

 their complete canine and incisor sets. Lestodon and 

 Megalonyx of the same period still retained one canine. They 

 may be considered the ancestors of the living Sloths, which 

 live in trees and feed exclusively on such leaves as they can 

 pull off with a minimum of effort and which they then only need 

 to masticate. We may here invoke the consequences of disuse. 

 The gigantic Megatherium likewise, in spite of its great size, which 

 sometimes equalled that of the Rhinoceros, had numerous 

 affinities with the Sloth. Instead of climbing trees to get their 

 leaves, they pulled them down ; but they walked only on the 

 outer sides of their feet, as the Sloths are compelled to do when 

 on the ground because of the length of their nails. If they are 



