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 rhinoceroses 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 migration. 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, dolichopody 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-footedness and of short-footedness, comprise a very large part of the mechanism 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.
