ary. The fourth digit
exceeds the second in size, and the second is much the shortest of all.
Its metacarpal bone is considerably curved outward. In the hind-foot
of this genus there are but three digits. The fourth metatarsal is much
larger than the second.

"The larger number of equine mammals now known from the tertiary
deposits of this country, and their regular distributions through the
subdivisions of this formation, afford a good opportunity to ascertain
the probable descent of the modern horse. The American representative of
the latter is the extinct Equus fraternus (Leidy), a species almost, if
not wholly, identical with the Old World Equus caballus (Linnaeus), to
which our recent horse belongs. Huxley has traced successfully the later
genealogy of the horse through European extinct forms, but the line in
America was probably a more direct one, and the record is more complete.
Taking, then, as the extreme of a series, Orohippus agilis (Marsh),
from the eocene, and Equus fraternus (Leidy), from the quaternary,
intermediate forms may be intercalated with considerable certainty
from thirty or more well-marked species that lived in the intervening
periods. The natural line of descent would seem to be through the
following genera: Orohippus, of the eocene; Miohippus and Anchitherium,
of the miocene; Anchippus, Hipparion, Protohippus, Phohippus, of the
pliocene; and Equus, quaternary and recent.

"The most marked changes undergone by the successive equine genera are
as follows: First, increase in size; second, increase in speed, through
concentration of limb bones; third, elongation of head and neck, and
modifications of skull. The eocene Orohippus was the size of a fox.
Miohippus and Anchitherium, from the miocene, were about as large as a
sheep. Hipparion and Pliohippus, of the pliocene, equalled the ass in
height; while the size of the quaternary Equus was fully up to that of a
modern horse.

"The increase of speed was equally well marked, and was a direct
result of the gradual formation of the limbs. The latter were slowly
concentrated by the reduction of their lateral elements and enlargement
of the axial bone, until the force exerted by each limb came to act
directly through its axis in the line of motion. This concentration is
well seen--e.g., in the fore-limb. There was, first, a change in the
scapula and humerus, especially in the latter, which facilitated motion
in one line only; second, an expansion of