42 THE INTERNATIONAL MONTHLY. 
demonstrated in all cases, it seems highly probable that the 
Condylarthra represent the common stock whence nearly or 
quite all the orders, of hoofed animals were derived. Of these 
ungulate orders, none has a more interesting history than the 
Perissodactyla, an order which is now verging towards extinction, 
and which, at the present time, is represented by three families 
only ; the horses, tapirs, and rhinoceroses. It probably will not 
be many centuries before the two latter families will have disap- 
peared and then the horses (using that term to include all horse- 
like animals, the asses, zebras, etc.,) will be left alone to repre- 
sent the Perissodactyla. The order first appears in the Wasatch 
and rapidly increases and diversifies, becoming in the middle and 
upper Eocene (Bridger and Uinta) the most abundant and impor- 
tant ungulate order. After maintaining this dominant position 
for a time, the Perissodactyla began slowly to decline, as other 
hoofed animals came forward to take their places. This decline is 
exhibited not only in the entire loss of many perissodactyl series, 
but also in the restriction of the geographical range of those which 
persisted. 
Of the various perissodactyl families, none has left so clear and 
full a record of its history in the rocks as have the horses, 
(Equidtf) which was first worked out, in its main outlines, by 
the late Professor Marsh. Disregarding a probable but insuffi- 
ciently known ancestor in the Torrejon, the history begins in the 
Wasatch with some little animals hardly larger than domestic 
cats, which, though far more primitive and less specialized than 
the modern representatives of the family, yet have something 
about them which stamps them immediately and unmistakably as 
ancestral types of the horses. However, the observer would per- 
haps hardly venture to call them such, if he could not trace them 
step by step to their very different descendants of the present 
time. These little creatures differ especially from the modern 
horses, which have but one functional toe on each foot, in having 
five toes in the fore foot and four in the hind foot. The limbs, 
and particularly the feet, are relatively quite short, and the two 
bones of the forearm (ulna and radius) and of the lower leg (tibia 
