2 4 o THE INTERNATIONAL MONTHLY. 
early members of this family arose the sheep, goats, and oxen. 
It is interesting to note that in these latter, as well as in many 
antelopes, the grinding teeth are much elongated, not forming 
roots till a late period, and are covered with cement, just as in the 
horses. Representatives of the ox tribe make their first appear- 
ance in the Pliocene of India, whence they rapidly spread over 
the whole earth, with the exception of South America and Aus- 
tralia. The Pecora did not reach North America, till the later 
Miocene, when the horned types had made their appearance; 
till that time the numerous and diversified Tylopoda had held their 
own against all rivals. Even of the Pecora now inhabiting North 
America only a few, at most, can be regarded as descendants of 
these comparatively early migrants, such as some of the deer and 
the prong-horned antelope. On the other hand, most of the 
deer, the mountain sheep and goats, and the bison, are very late 
immigrants, which did not arrive till Pleistocene times. 
A side branch of the Pecora, given off* at a very early date, is 
that of the Tragulina, the so-called mouse deer, or chevrotains. 
These little creatures never reached North America, and are at 
present confined to Africa and the East Indies, and they might 
be described as ruminants in a state of arrested development. 
We saw above that the camels and llamas had pursued a 
course of evolution which, in many ways, was parallel to that 
taken by the horses. A third parallel is the line followed by the 
Pecora, though, as would be expected from their nearer relation- 
ship, the development of the latter more closely follows the 
camels than the horses. We find in both the same reduction of 
the upper incisors, the elongation of the neck, limbs, and feet ; 
the ulna and fibula are greatly diminished and coossified with the 
radius and tibia respectively ; the digits are reduced to a single 
functional pair in each (nos. iii. and iv. of the original five), 
and their long bones fused together. In all these series the 
modifications are all directed to an increase of speed. 
While the foregoing series represent the principal lines of 
artiodactyl descent, there are many groups which have not been 
mentioned, and which have no representative in the present order 
