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, 
whose 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 we have 
examined bear little resemblance to those of Hipparion 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, 
