MAMMALIA, , 573 
crowns are still low and there is little or no cement. The incisors are 
few and rudimentary and the upper canines are absent. The nasals 
bear an unpaired ‘‘horn” of purely epidermic origin and having no 
horn core. In the two-horned species the second and smaller horn is 
carried on the frontals: this species is African. The upper lip is long 
and prehensile and the skin is very thick and hard with little hair. The 
food consists of herbage and leaves of trees. 
Family III].—Equidz.—Little need here be said of this family (see 
Horse). The horses are essentially graminivorous inhabitants of hard 
upland plains. The teeth are hypsodont and the crowns are extremely 
complex, though to be derived from the bilophodont type. Cement 
fills up the spaces between the ridges. The third toe alone remains 
and bears a hoof, the second and fourth metapodials being represented 
by two splint bones. 
The pedigree of the horse can be traced from Condylarthra (Phen- 
acodus). (See page §23.) The fossil ancestors of the horse are hard 
to classify as they are gradational, but the Paleotheritde is a family 
often constituted for Palgeotherium, Anchitherium and other forms, 
which, as a rule, were at about the level of the rhinoceros in the 
structure of their teeth and toes. The earlier types of the Zocene, 
such as Pachynolophus and its allies, form the family Lophiodontide. 
They have still more generalised characters and connect the Perdsso- 
dactyla with Condylarthra. 
Thus this sub-order Perissodactyla forms a remarkable field for the 
study of evolution. One important point we may notice before leaving 
it. The tapirs and rhinoceroses take in many structural points a lower 
level than many forms which have perished. For example, Hipparion 
was a horse-like type of the Pliocene, which certainly comes within the 
range of the Zyuzde, and the question often arises—How is it that 
these lower forms (tapir and rhinoceros) have survived and ‘‘ higher” 
have become extinct? Put more generally, the question becomes— 
How is it that primitive animals still survive contemporaneously with 
the higher types? Leaving. out of count special explanations applying 
to cases like the Australian Ae¢atheria, the general explanation is :— 
(1) Species survive only so long as they are in structural harmony with 
their environment. (2) Environments change rapidly, but ‘ancient ” 
environments exist at the present day as well as ‘modern or up-to- 
date” environments. 
Hence the widely-scattered tapirs of the Miocene are now found only 
in the low-lying swampy forest land for which their structure is suited 5 
in the regions where now the open grassy plains have become predom- 
inant the tapir died out, to be replaced by horse-like types more suited 
to the changed surroundings. The soft ground and the arboreal diet are 
complimentary to the numerous toes and the simple teeth of the tapir, 
whilst the hard level ground and siliceous grass calls forth the limb 
with single axis and the deep, complex, cemented teeth of the horse. 
In response to the environmental factors which have changed, such 
as the presence of large Carnzvora, these primitive types have also 
evolved horns (rhinoceros), or incisor tusks (elephant), or have adopted 
an arboreal or fossorial habit (yrax). 
Bs 
