202 Professor Marsh's Monograph of the Dinocerata. 
other hand, however closely they. may correspond in different 
groups, do not necessarily indicate affinities, but may have been 
acquired by adaptation to peculiar surroundings, in groups quite 
distinct from each other. 
“These facts lie at the foundation of classification, and it is 
only by keeping the two series of characters separate, that the 
true relationship between different groups of animals can be 
made out, and their genealogy indicated with any probability. 
* * * * * 
MoptricaTION oF THE Uneunatre Foor. 
“In the true ungulate mammals, the modifications of the foot 
have undoubtedly taken place very nearly in the following 
also in some of the early Perissodactyls. pee 
During these two stages of modification, a reduction 12 
r together. 
In the next change that took place, two kinds of rede 
tion began. One leading to the existing perissodactyl foot, an 
n the former, the axis of the foot remained in the middle © 
the third digit, as in the pentadactyl foot. In the latter, 4 
shifted to the outer side of this digit, or between the third ra 
fourth toes. An example of the former is seen in the fore f° 
of Brontotherium, while Oreodon shows the latter type- 
