30 THE AGE OF MAMMALS 



pointed-lipped rhinoceros (/?. bicornis), with short-crowned or brachyodont 

 teeth, which browses mainly upon shrubs. The feeding ranges of these 

 animals are not very far apart. They do not subsist upon exactly the 

 same food, thus they do not compete. The grazing "white rhinoceros" is 

 long-headed, or dolichocephalic, the browsing "black rhinoceros" is rela- 

 tively short-headed, or mesaticephalic. The local adaptive radiation of 

 the antelopes of Africa furnishes still more remarkable illustrations of the 

 same kind. 



Among living forms we find long-headed and short-headed, long-footed 

 and short-footed, long-toothed and short-toothed types, living near each 

 other, resorting to the same water courses for drink, and thus liable to be 

 killetl in the same spot. We thus find a clue in the present to the inter- 

 pretation of what frequently occurs among fossilized types, namely: in 

 the same geological deposits are often mingled short-footed, or ambulatory, 

 with long-footed, or cursorial, quadrupeds. The cursorial types subsisted 

 upon grasses and ranged out on the plains, the ambulatory tjqjes, chiefly 

 browsers, frequented the river borders, the thickets, and the hillsides. 

 Among the best illustrations of this kind is the difference between the forest- 

 living horse Hypohippus and the plains-living horse Hipparion of the Upper 

 Miocene. (See pp. 243, 295.) 



Thus local adaptive radiation causes the splitting up of mammals 

 dwelling in the same geographic regions into side branches or series which 

 we call phyla. We may find preserved in the same geologic deposits two, 

 three, four, or even as many as five phyla of mammals belonging to direct 

 and collateral lines. Local adaptive radiation is, therefore, one explana- 

 tion of the next very general principle of divergence, which may be known 

 as the polyphyletic law. 



The polyphyletic law. — In these locally separated phyla, sometimes 

 minute, sometimes conspicuous differences are developed. One of the 

 most frequent distinctions is in adaptations to speed, i.e., in amliulatory 

 and cursorial types; another is in the proportions of the skull, whether 

 brachycephalic or dolichocephalic; a third is in the proportions of the 

 horns, if such are developed. This law is so general in Oligocene and Mio- 

 cene times that if we discover light-limbed types we may anticipate the 

 discovery of their more slow-moving counterparts. Horses, camels, 

 rhinoceroses, the related titanotheres, have one after another proved to 

 break up under this polyphyletic law into grazing and browsing types, 

 slender and heavy types, in the same or neighboring geographic regions. 



It is seen at once that this polyphyly renders the study of phylogeny, or 

 the tracing of successive lines of descent among the mammals, far more 

 interesting and at the same time far more difficult, because the fossil re- 

 mains of members of these different series or phyla are often intermingled 

 and it is difficult to separate them. In other cases the nature of the geo- 

 logic deposition affords a ready key to the separation of these phyla. For 



