TJie Evohdion of Ihe Uc/riB. 
118 
(at least, associated only with some very immediate allies, struc- 
turally almost identical) placed in an order apart from all other 
mammals, under the name of Solid.unrjula, Solipedia, or Mono- 
daetyla — the animal with the solid foot, or, rather, with a single 
toe on each extremity. 
To understand the natural place of the horse in the zoological 
system, it will be necessary to take a wide glance at the whole 
group to which it belongs. That it is a vertebrate animal, and 
that it occupies a place in the class Mammalia, no one will 
doubt. In that class it belongs to the great order Unrjulata, 
or hoofed animals, the principal characters of which are the 
following : They are all eminently adapted for a terrestrial life, 
and, in the main, for a vegetable diet. Their molar teeth 
have broad crowns, with tuberculated, or ridged grinding sur- 
faces, and they have a very completely-developed set of milk- 
teeth. Their limbs are adapted for carrying the body in ordi- 
nary terrestrial progression, and are of very little use for any 
other purpose, such as flying, climbing, seizing prey, or carrying 
food to the mouth. They have no collar-bones. Their toes are 
provided with blunt, broad nails, which, in the majority of 
cases, more or less surround and enclose their ends, and are 
called ' hoofs.' The great majority of Ungulate animals belong 
to either one or the other of two great and perfectly distinct 
sections, which differ from each other in very many points in 
their structure, the most obvious cf these being the characters 
of their limbs, from which the names of the groups, or sub-orders, 
are derived. One is called Artiodactyla, or "even-toed;" the 
other, I'erissodadyla, or " odd-toed.'' In the former, the third 1 
and fourth toes of both feet are almost equally developed, and 
flattened on their inner, or contiguous surfaces, so that each is 
not symmetrical in itself; but when the two are placed together, 
they form a figure symmetrically disposed to a line between 
them — the so-called cloven hoof. These two toes are always 
present, and well developed ; the second and fifth may be present 
in varying degrees of development, or may be entirely absent ; 
the first is not present in any known member of the group, even 
the most ancient. 
In the Perissodactyle group, the middle or third toe of both 
fore and hind feet is larger than any of the others, and sym- 
1 The number of toes in mammals never exceeds five. For convenience 
of description, they are designated numerically, from the inner side of the 
limb— I., II., III., IV. and V. — the pollex (thumb) and hallux (jrreat toe) being 
the first of the fore and hind limbs respectively, and the third is the nikldle of 
the complete series. When the number falls short of five, it is always easy to 
determine, by their relations to the bones of the wrist ^or ankle, which of the 
typical series are present and which are missing. 
VOL. I. T. S. — 1 I 
