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PALAEONTOLOGY

Plate III.

This series of feet represents the evolutionary succession from the Eocene Hypohippus (1) to the modern Equus (6) seen in front and in side view. The top bone is the os calcis, or hock bone, to which the tendon Achilles is attached. The bottom bone is the terminal phalanx which is inserted in the heart of the hoof.

Equus caballus.		Modern horse.
Merychippus sp.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Miocene.
Merychippus insignis (milk molar).
Parahippus pawniensis.
Mesohippus intermedius.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Upper Oligocene (White river formation).
Mesohippus bairdi?	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Oligocene (White river formation).
Mesohippus bairdi.		Oligocene (White river formation).
Orohippus sp.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Middle Eocene (Bridger formation).
Eohippus sp.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Lower Eocene (Wind river formation). (Wasatch formation).
Eohippus sp.		Lower Eocene (Wind river formation). (Wasatch formation).

1

2

3

4

5

6

The stages are as follows:

1. Hypohippus, Lower Eocene.	4. Protohippus, Upper Miocene.
2. Mesohippus, Lower Oligocene.	5. Neohipparion, Upper Miocene.
3. Parahippus, Lower Miocene.	6. Equus, Pleistocene and recent.

The evolution consists first in progressive increase in size; second, in the acceleration of the median digit and retardation of the lateral digits, the latter becoming more and more elevated from the ground until finally in Equus (6) they are the lateral splints, which in the embryonic condition have vestigial cartilages attached representing the last traces of the lateral phalanges.

1

2

3

4

5

6

Fig. 8.—TEN STAGES IN THE EVOLUTION OF THE SECOND UPPER MOLAR TOOTH OF THE RIGHT SIDE, ARRANGED ACCORDING TO GEOLOGICAL LEVEL.

(Nos. 1-9 from “American Equidae.”)

Fig. 7.—LAW OF ACCELERATION AND RETARDATION ILLUSTRATED IN THE EVOLUTION OF THE HIND FEET OF THE HORSE.

(From photos lent by the American Museum of Natural History.)

XX. 584.

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