t appears to be a mere process
of the latter bone, as in the Horses.

In _Equus_, finally, the crowns of the grinding-teeth become longer,
and their patterns are slightly modified; the middle of the shaft of
the ulna usually vanishes, and its proximal and distal ends ankylose
with the radius. The phalanges of the two outer toes in each foot
disappear, their metacarpal and metatarsal bones being left as the
"splints."

The _Hipparion_ has large depressions on the face in front of the
orbits, like those for the "larmiers" of many ruminants; but traces
of these are to be seen in some of the fossil horses from the Sewalik
Hills; and, as Leidy's recent researches show, they are preserved in
_Anchitherium_.

When we consider these facts, and the further circumstance that
the Hipparions, the remains of which have been collected in immense
numbers, were subject, as M. Gaudry and others have pointed out, to
a great range of variation, it appears to me impossible to resist the
conclusion that the types of the _Anchitherium_, of the _Hipparion_,
and of the ancient Horses constitute the lineage of the modern Horses,
the _Hipparion_ being the intermediate stage between the other two,
and answering; to B in my former illustration.

The process by which the _Anchitherium_ has been converted into
_Equus_ is one of specialization, or of more and more complete
deviation from what might be called the average form of an ungulate
mammal. In the Horses, the reduction of some parts of the limbs,
together with the special modification of those which are left, is
carried to a greater extent than in any other hoofed mammals. The
reduction is less and the specialization is less in the _Hipparion_,
and still less in the _Anchitherium_; but yet, as compared with
other mammals, the reduction and specialization of parts in the
_Anchitherium_ remain great.

Is it not probable then, that, just as in the Miocene epoch, we find
an ancestral equine form less modified than _Equus_, so, if we go
back to the Eocene epoch, we shall find some quadruped related to the
_Anchitherium_, as _Hipparion_ is related to _Equus_, and consequently
departing less from the average form?

I think that this desideratum is very nearly, if not quite, supplied
by _Plagiolophus_, remains of which occur abundantly in some parts
of the Upper and Middle Eocene formations. The patterns of
the grinding-teeth of _Plagiolophus_ are similar to those of
_Anchitherium_, and their