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. — Equidae. — 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 

 hlls 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- 

 acodtis). (See page 523.) The fossil ancestors of the horse are hard 

 to classify as they are gradational, but the PahwtheriidcB is a family 

 often constituted for PalcEotheriitm., Anchitheriiuji 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 Eocene, 

 such as Pachynolophus and its allies, form the family Lophiodou tides. 

 They have still more generalised characters and connect the Perisso- 

 dadyla with Condylarthra. 



Thus this sub-order Peris sodacty la 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 Equidce, 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 MetatheiHa, the general explanation is : — 

 (l) 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 ; 

 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 Ca7'nivo7^a, these primitive types have also 

 evolved horns (rhinoceros), or incisor tusks (elephant), or have adopted 

 an arboreal or fossorial habit {Hyrax). 



