THE LIFE OF MAMMALS 
digits (second and fifth) of each foot, getting less and less shock and strain 
as the middle toes were enlarged by constant use, remained undeveloped, 
or even gradually lessened, until all that is left of them in most cases are 
the two little ‘‘false hoofs” which hang behind the pastern. 
In the second line (perissodactyls) the strain came, for some unknown 
reason, more upon the third or central toe, and this by a similar history 
developed at the expense of the side toes, until in its most perfect form, the 
horse, nothing remains of the latter, and the whole weight rests upon the 
hoof-shod tip of the single central digit. At the same time, in both divisions 
the smaller bones (ulna and fibula) of the limbs became reduced and fused 
with the radius and tibia; and the clavicles disappeared as a result of the 
elevation and compacting of the body. 
Coupled with this went on changes in the dentition, also along divergent 
lines. Canines would be an incumbrance to grazing animals, and have 
been reduced or have disappeared except where serviceable as weapons. 
The gap they left has gradually broadened into the ‘‘diastema,”’ or space 
where a horse carries its bits. In the artiodactyls, largely browsers, the 
incisors of the upper jaw have been practically lost, and the premolars and 
molars are unlike and have broad, flat crowns; while the perissodactyl 
(grazers) are furnished with strong, chisel-shaped biting incisors in both 
jaws, and all the cheek teeth are much alike, — long, deep-set, and strong, 
with massive, squarish crowns crossed by curving ridges of dentine, making 
them the perfection of grinders. The stomach is a much more compli- 
cated organ in the artiodactyls than in any other group. It must be con- 
ceded, however, that these and other present distinctions weaken or disap-' 
pear when traced back toward the common ancestry of the two divisions 
which seem to have separated at the very dawn of mammalian history. 
An explanation of why there should be two such lines of development 
rather than one is that the originator of the split-hoofed line walked with 
that twisting motion of the feet still very noticeable in cattle. 
The order Artiodactyla is divisible into two well-marked 
sections : — 
A. Ruminantia — ruminants; horned animals. 
B. Swina — non-ruminants; swine. 
The former include the cud chewers — those which gather 
and swallow their food in haste and then at leisure gulp it up 
and rechew it in small quantities (cuds) and very thoroughly. 
This strange operation,’ like the carrying away of food by 
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