1882.J The Horse and his Fossil Ancestry. 469 
the fingers and toes, and the bones best known as those of 
the wrist and ankle, which have been gradually reduced in 
number. Corresponding modifications have taken place in 
the bones of the fore arm and of the leg. The outer bones 
of both, the ulna and the fibula, have undergone a corre- 
sponding degradation or retrograde development. In Anci- 
ihevium we find the ulna in proportion strongly developed. 
The fibula, however, is decidedly weak, and its lower part is 
applied closely to the inner bone, or tibia ; but it is still a 
complete and connected bone. 
In Hipparion the process of -degenerative change is more 
advanced. The ulna is so much reduced in the lower part 
of its central portion, and so closely joined together with the 
fully developed radius or inner bone of the fore arm, that it 
is no longer capable of free motion, though it still appears 
as a complete and connected portion of the skeleton. As 
regards the hind legs, the fibula of Hipparion is interrupted 
in the middle, and consists of an upper and a lower part 
no longer connected together. 
As we approach more recent times, we find the same 
changes proceeding further. The ulna of the true modern 
horse is in its middle portion greatly reduced, and even in- 
terrupted, and is connected with the radius in young animals 
by means of sinews, and in old ones by bony growths. 
Now so far it might still be contended, concerning these 
modifications of the fore arm and of the leg, that they supply 
no absolute proof of “ descent with modification.” But 
sometimes in recent horses and asses we meet with a case 
of a complete and uninterrupted ulna, just as it occurred in 
Hipparion. Herr Nehring, in proof of this assertion, exhi- 
bited to his audience the fore-arm bones of a horse of the 
Cleveland breed and of an adult ass. These specimens 
showed that in Equus caballus> as well as in E. asinus, the 
ulna is sometimes so far developed as to take the form of 
the ulna of Hipparion. Such cases are more common in the 
horses of heavy breeds than in such of slighter frame : they 
are also more common in the ass, which seems to represent 
a more primitive type among the recent Equidse. 
As for the fibula it is so degraded in the recent horse as to 
consist merely of a rudimentary portion above, and a lower 
joint-piece which intimately attached to the tibia. Some- 
times, however, as in the Cleveland horse above mentioned, 
the fibula, though not complete, has the interval between its 
upper and lower remnants much smaller than in the majority 
of horses,- — not larger, indeed, than it is found in Hippa - 
rion. 
