238 THE AGE OF MAMMALS 



such as palaeotheres, lophiodonts, anoplotheres, xiphodonts, in part highly 

 speciahzed antl yet destined to extinction. 



Doubtless the defective brain, the defective tooth, the defective foot 

 contributed to the downfall of the prophetic modern types, and throughout 

 Oligocene times we are able to concentrate our attention on certain specific 

 organs, or parts of animals, as causes of extinction. 



In general those mammals appear to survive which present the highest 

 adaptive combination of favorable characters in fully formed organs as well 

 as the highest adaptability or capacity to further favorable change of habit or 

 structure. Conversely, inadaptive combinations of characters, such as of the 

 brain, the limbs, the teeth, appear to have been the causes of extinction, 

 partly in connection with changes of environment, partly because inherently 

 or relatively inadaptive. 



Thus in Oligocene still more clearly than in Eocene times we observe 

 that extreme bulk, extreme specialization, and the development of certain 

 dominant characters lead to extinction. Certain types of teeth or certain 

 types of limb and foot structure simultaneously over large parts of the 

 world have been found wanting and thus proved fatal to their possessors. 

 These are the general lines of thought which have been followed by many 

 authors since Darwin first directed our attention to this subject. It is 

 desirable to look into some of these causes more critically. 



Inadaptive foot structure. — As already remarked (p. 15), Kowalevsky 

 observed in his great monograph (1873, p. 152), the extinction in Oligocene 

 and Miocene times of all artiodactyls with inadaptive foot structure and 

 inadaptive grinding teeth, as follows: Upper Eocene, Xiphodon, Anoplo- 

 therium, Diplopus; Oligocene, Ancodus, Anthracotherium, Entelodon. He 

 pointed out that the inadaptation of the feet consisted in a mechanical 

 defect in the small bones of the hand, or manus, while the inadaptation in 

 the grinding teeth consisted in the persistent short, or brachyodont crowns 

 composed of partially rounded cones and imperfectly formed crescents. 

 By his theory the mechanically defective feet were incapable of acquiring 

 the elongation into the cursorial type which saved the lives of the artio- 

 dactyls with adaptively formed front feet. The short-crowned teeth could 

 not survive the change of vegetation from the softer herbage of Eocene 

 times to the harder grasses of late Oligocene and Miocene times. 



The accompanying diagram (Fig. 122) bears out Kowalevsky 's general- 

 ization so far as the teeth are concerned. It exhibits the reduction in 

 number of the mammals with short-crowned bunodont teeth toward the 

 close of the Oligocene, and the reduction and extinction of mammals with 

 bunoselenodont grinding teeth of two divisions, that is, both among the 

 artiodactyl anthracotheres and anoplotheres and among the perissodactyl 

 titanotheres and chalicotheres. The purely crested or lophodont types 

 also appear to have suffered a reduction. The most highly successful 

 dental types appear to have been the selenodont, characterizing all 



