THE PALEONTOLOGIC RECORD 81 
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 Hurasiatic 
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. 
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