the distal end of the fibula is so completely united with the tibia that
it 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 specialisation, 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
specialisation is less in the _Hipparion_, and still less in the
_Anchitherium_; but yet, as compared with other mammals, the reduction
and specialisation 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