ANCESTRY OF HORSES 
was from the plentiful fossils collected by Marsh that Huxley, 
in 1876,"° made that philosophic arrangement of the facts 
which presented so convincing a proof of the truth of the 
development theory, then new and comparatively unsupported. 
Latterly the work of the earlier men has been carried forward 
by Professor Henry F. Osborn and his assistants, prominently 
Drs. Wortman and Matthew; and it is due to them that we are 
now so completely informed as to all the details of the develop- 
ment of the group. Dr. W. D. Matthew, especially, has done 
much to popularize as well as to originate knowledge on this 
subject, and it is from his writings that the following condensed 
statement is mainly derived : — 
Among the Cretaceous condylarths (see page 231) there is 
believed to have existed a small creature, the progenitor of the 
equine line. None such has yet actually been dis- po itive 
covered in the rocks; but the Lower Eocene clay qpecnt 
near London—the basic layer of the geological 
series assigned to the Age of Mammals — has yielded a pro- 
equine skull (Hyracotherium) which might have had such feet, 
judging by the structure of the known remains. About the same 
time, possibly three million years ago, there dwelt in the eastern 
foothills of the Rocky Mountains little animals undoubtedly 
of the horse family, known as Eohippus, whose fore feet had 
four complete toes and the ‘“‘splint” of a fifth; the hind feet 
three toes and the splint of another. A little later (Middle 
Eocene) the rocks yield two other forms (Protorohippus and 
Orohippus) from which the splints of Eohippus have disappeared, 
and the middle toe is somewhat larger than the others —a 
feature still more manifest in the Upper Eocene skeletons 
termed Epihippus. At this time Europe had a number of 
related but not ancestral forms which presently died out; while 
the true line of horse genealogy was continued in America by 
the Mesohippus of the Oligocene, which was as large as a 
sheep and had only three toes on each foot (the fifth digit now 
2A 353 
