360 THE AMERICAN NATURALIST. [Vor. XXXVI. 
most long-footed series because subsistence upon grasses is 
associated with such conditions of life as are afforded by exten- 
sive open plains, long ranges for food, and rapid flight from 
enemies. Again, as shown elsewhere, dolichocephaly and 
dolichopody, brachycephaly and brachypody are frequently but 
not invariably correlated. (See Osborn, '02a.) 
The value of this law of Zoca/ adaptive radiation is especially 
great as a means of interpretation of the frequent contempora- 
neous existence or association of more primitive (brachyodont) 
with more specialized (hypsodont) types. Among Tertiary 
machzrodont cats it is seen in the contemporary long-limbed 
Dinictis and short-limbed Hoplophoneus. In fact, the associa- 
tion has been so often observed that if we find one phylum, we 
may almost anticipate or predict the discovery of the other. 
The law is made more clear by referring to the above table 
and the following explanation of it: (1) As compared with 
the Palzotheriinz, the Palaplotheriinz are so long-footed that 
Huxley believed that they gave rise to the horse, and he actu- 
ally placed Paloplotherium minus as the ancestor of the horse. 
series. (2) The Telmatotheriinz are large animals also dis- 
tinguished by very long, slender limbs; they independently 
acquired horns, and the species 7. cornutum was believed by 
Osborn to be ancestral to the titanotheres; but as a matter of 
fact, as shown by Earle and Hatcher, this horn is a pure case 
of parallelism, since the titanotheres probably sprang from the 
short-skulled and relatively stout species, Pa/eosyops manteo- 
ceras, a member of the Palcosyopinz. (3) The Triplopodinz 
present the most extreme instance of light-limbed develop- 
ment among the perissodactyl ungulates, since the limbs have 
the proportions of some of the most slender and swift- 
footed ruminant mammals, although these animals are found 
in the same beds with the correlated subfamily Hyrachyinz. 
(4) Among the Lophiodontidz, the Helaletinze bring out with 
additional force the principle that this elongation of limb 
occurred at a very early geological period; because the first 
known member of this phylum appears way down in the Lower 
Eocene, Wasatch, or Suessonien beds, in the genus Heptodon 
. Cope, a remarkably light-limbed form, and it runs right through, 
