xxx. 9 EVOLUTION OF HORSES 735 



that dwell on plains. The brain is large, and although the organs of 

 smell are well developed the eyes are also large and the neopallium is 

 extensive. Receptors for touch are well developed in the muzzle, in 

 the skin beneath the hoofs, and elsewhere. Hearing is exceptionally 

 acute. Besides the keen senses common to many herbivores the horse, 

 with its large brain, also has considerable powers of learning and 

 ability to vary and restrain its behaviour. There is an elaborate com- 

 munication system, involving not only sounds but movements of the 

 ears, tail, and lips. In these respects horses and elephants, and perhaps 

 also modern artiodactyls, are probably very different from the small- 

 brained herbivores of the Eocene, though, of course, we can only guess 

 at the behaviour of these. 



Modern horses show considerable genetical diversity (Figs. 487 and 

 488), but none of the 'species' are mutually sterile, though the F 1 

 resulting from the cross may be nearly so, as in the case of the mule, 

 produced from the horse-ass cross. Evidently the population is in pro- 

 cess of divergence. The domestic horse E. cabalhis is not found truly 

 wild, but E. przewalskii of central Asia may be. There are several 

 species of wild asses, such as E. onager of Asia and E. asinus of Africa. 

 Several species of zebra live in Africa, one of them being E. zebra. 



Between the modern Equns and the lower Eocene *Hyracotherium 

 a great number of fossil stages can be recognized (Fig. 489). The chief 

 changes that can be followed may be listed as (1) increase of size, 

 (2) lengthening of the distal portion of the legs, (3) reduction of lateral 

 digits, (4) increase in the relative length of the front part of the skull, 

 (5) increase of depth (hypsodonty) and of the grinding lophs of the 

 molars, (6) approximation of premolars to molar structure, (7) com- 

 pletion of post-orbital bar. No doubt there has been change also in 

 many other characteristics, for instance the brain and behaviour; these 

 are difficult to follow in a fossil series, but study of cranial casts sug- 

 gests that a rapid increase in size and folding of the cerebrum occurred 

 relatively early in the evolution. 



The fossil remains are not usually available in long series of layered 

 beds, such that we can be sure that one population has evolved into 

 the next. However, the dating of the fossils can often be done with 

 considerable accuracy by means of the associated animals, and a series 

 can thus be produced such as would be expected in the progress from 

 *Hyracotherium to Equns. There are, however, many fossils that show 

 special developments, and cannot be fitted into the direct series. 

 These are presumed to be divergent lines: it must be emphasized that 

 this is an arbitrary though probably justified procedure. These 'side- 



