388 UNGULATA 



cerebral organisation. The patience of the Ass, the high spirit of 

 the Horse, the obstinacy of the Mule, have long been proverbial. 

 It is very remarkable that, out of so many species, two only should 

 have shown any aptitude for domestication, and that these two 

 should have been from time immemorial the universal and most 

 useful companions and servants of man, while all the others remain 

 in their native freedom to this day. It is, however, still a question 

 whether this really arises from a different mental constitution 

 causing a natural capacity for entering into relations with man, or 

 whether it may not be owing to their having been brought gradually 

 into this condition by long-continued and persevering efforts when 

 the need of their services was keenly felt. It is quite possible 

 that one reason why most of the attempts to add new species to 

 the list of our domestic animals in modern times have ended in 

 failure is that it does not answer to do so in cases in which existing 

 species supply all the principal purposes to which the new ones 

 might be put. It can hardly be expected that Zebras and Quaggas 

 fresh from their native mountains and plains can be brought into 

 competition as beasts of burden and draught with Horses and Asses, 

 Avhose naturally useful qualities have been augmented by the train- 

 ing of thousands of generations of progenitors. 



Not unfrequently instances occur of domestic Horses being 

 produced with a small additional toe with complete hoof, usually on 

 the inside of the principal toe, and, though far more : rarely, three 

 or more toes may be present. These malformations are often cited 

 as instances of reversion to the condition of some of the earlier 

 forms of equine animals previously mentioned. Such explanations, 

 however plausible they appear at first sight, are nevertheless very 

 doubtful. All the feet of polydactyle horses which Ave have 

 examined bear little resemblance to those of Hipjxirion or Anchi- 

 therium, but look rather as if due to that tendency to reduplication 

 of parts which occurs so frequently as a teratological condition, 

 especially among domestic animals, and, whatever its origin, certainly 

 cannot in many instances, as the cases of entire limbs super- 

 added, or of six digits in man, be attributed to reversion. 



Anatomy. — The anatomical structure of the Horse has been de- 

 scribed in great detail in several works devoted to the subject, which 

 will be mentioned in the bibliography, though these have generally 

 been written from the point of view of the veterinarian rather than 

 of the comparative anatomist. The limits of the present work will 

 only admit of the most salient points being indicated, particularly 

 those in which the Horse differs from the other Ungulata. Unless 

 otherwise specified, it must be understood that all that is stated 

 here, although mostly derived from observation upon the Horse, 

 applies equally well to the other existing members of the group. 



Skeleton. — The skull (Fig. 163) as a whole is greatly elongated,, 



