Page:EB1911 - Volume 20.djvu/639

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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.
Miocene.
Merychippus
insignis

(milk molar).
Parahippus
pawniensis.
Mesohippus
intermedius.
Upper
Oligocene
(White
river
formation).
Mesohippus
bairdi?
Oligocene
(White
river
formation).
Mesohippus
bairdi.
Orohippus
sp.
Middle
Eocene
(Bridger
formation).
Eohippus
sp.
Lower
Eocene
(Wind river
formation).
(Wasatch
formation).
Eohippus
sp.
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 fromAmerican 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.)