38 THE PALEONTOLOGIC RECORD 



region. In the Eocene we discover four or five independent local 

 phyla; again in the Oligocene we discover five or six independent local 

 phyla. The evolution of these animals appears to have been chiefly 

 American. 



In other cases, however, the polyphyletic condition appears to have 

 been through the mingling with local phyla of phyla evolved in other 

 countries. This is illustrated in the case of the Middle Miocene rhi- 

 noceroses of America, which are invaded by rhinoceroses of Eurasiatic 

 or European origin. 



In studying the herbivorous quadrupeds, therefore, we must keep 

 in the imagination constantly the production of local phyla through 

 local radiation and the intermingling of foreign phyla through migra- 

 tion. There are a few very striking and profound differences between 

 quadrupeds which recur so frequently that where we discover one form 

 we may surely anticipate the discovery of the opposite or antithetic 

 form: in other words, there are extremes of structure shown in the 

 proportions of the skull, of the teeth, of the limbs, and groups of 

 quadrupeds are constantly tending through adaptive radiation to reach 

 these extremes. Some of the contrasting extremes are the following: 

 brachyodonty vs. hypsodonty, dolichocephaly vs. brachycephaly, dolicho- 

 pody vs. brachypody. 



For example, a local adaptive radiation observed in the horses is 

 that the forest-living types are brachyodont, or possess short-crowned 

 teeth, while the desert-living horses are hypsodont, typically grazers, 

 with long-crowned teeth. 



Extremes of long-headedness and short-headedness, of long-foot- 

 edness and of short-footedness, comprise a very large part of the mech- 

 anism of adaptive radiation; but we have to do also with long-necked 

 and short-necked types, and with many other chances of proportion 

 which are correlated with different feeding habits. 



