EVIDENCES FROM PALAEONTOLOGY 71 
No particular member of this extinct group has been found that fulfils 
all the requirements of a primitive horse ancestor, so the chances are 
that the real ancestral condylarthran has not been discovered.—Ep.] 
“The course of their [Equidae] evolution,” says Dendy,’ “has 
evidently been determined by the development of extensive, dry, 
grass-covered, open plains on the American continent. In adap- 
tation to life on such areas structural modification has proceeded 
chiefly in two directions. The limbs have become greatly elongated 
and the foot uplifted from the ground, and thus adapted for rapid 
flight from pursuing enemies, while the middle digit has become more 
and more important and the others, together with the ulna and the 
fibula, have gradually disappeared or become reduced to mere vestiges. 
At the same time the grazing mechanism has been gradually perfected. 
The neck and head have become elongated so that the animal is able 
to reach the ground without bending its legs, and the cheek teeth have 
acquired complex grinding surfaces and have greatly increased in 
length to compensate for the increased rate of wear. As in so many 
other groups, the evolution of these special characters has been 
accompanied by gradual increase in size. Thus Fohippus, of Lower 
Eocene times, appears to have been not more than eleven inches high 
at the shoulder, while existing horses measure about sixty-four inches, 
and the numerous intermediate genera for the most part show a 
regular progress in this respect. 
“All these changes have taken place gradually, and a beautiful 
series of intermediate forms indicating the different stages from Eohip- 
pus to the modern horse [Equus] have been discovered. ‘The sequence 
of these stages in geological time exactly fits in with the theory that 
each one has been derived from the one next below it by more perfect 
adaptation to the conditions of life. Numerous genera have been 
described, but it is not necessary to mention more than a few.” 
{The first indisputably horselike animal appears to have been 
Hyracotherium,” of the Lower Eocene of Europe. Another Lower 
Eocene form is Eohippus, which lived in North America, probably 
having migrated across from Asia by the Alaskan land connection 
which was in existence at that time. In Eohkippus the fore foot had 
four completely developed hoofed digits and a “thumb” reduced to 
a splint bone; in the hind foot the great toe had entirely disappeared 
and the little toe is represented by a vestigial structure or splint bone. 
t Arthur Dendy, Oullines of Evolutionary Biology (D. Appleton & Company, 
1916), ; 
