THE FORMS, ORIGIN, AND DEVELOPMENT OF THE TEETH. 87 
roots of the human and other teeth. But this separation is by no 
means regular. 
When the animal becomes herbivorous, a yet greater differentia- 
tion becomes necessary ; for there must be large and flat teeth with 
rough surfaces to grind the food into fine particles. Therefore, 
a varied, involved, and complex dental system prevails among the 
hoofed quadrupeds. ‘The specimens of the teeth of the elephant, 
cow, horse, rhinoceros, ibex, and wappiti deer we have here will 
give a good illustration of their general structure. The coalescence 
has in these teeth been carried very much further than before, and 
it has produced an entirely new combination in the hard parts of 
the tooth. We have, in the first place, the same kind of coales- 
cence whereby several cones make up one tooth. But, in addition 
to that, we have several of these teeth, each with its own germ, 
uniting to form one huge grinder. ‘Thus we see in this horse’s 
tooth no less than five teeth united to make one. This union may 
be described as if my fingers and thumb should get joined to- 
gether just where they meet, and then a substance should flow all 
through the interspaces and around the outside, just as the cemen- 
tum (hereafter to be described) flows into all the interspaces 
of these five teeth, and all over the crown, forming a square 
tooth as in the horse. By this arrangement, instead of the 
enamel being on the outside, as in the human and many other 
teeth, it runs all through this complex structure. The effect of 
this arrangement is that the softer cementum gets worn away 
almost as soon as the tooth is erupted, and continues to wear 
down faster than the other structures; then the dentine wears 
away the next, and the enamel, as the hardest part, wears last, 
which always leaves on the grinding surface of the crown an 
uneven surface, particularly adapted for grinding the food. Look 
at the sharp projections in a deer or rhinoceros tooth. What 
cannot such an arrangement do in the way of mastication! The 
deer and the ibex have teeth made up of but four instead of five 
teeth, as in the horse. 
Not only do we find these modifications in the arrangements of 
the hard parts of the teeth in the herbivora, but there are other 
most important changes. Among horses the canines, which are so 
large in the carnivora, are very small and useless in the stallion, 
and merely rudimentary in the mare. In the ox, sheep, antelope, 
&c., modification has gone still further, for these have neither 
canines nor incisors on the upper jaw; and the canines in the 
lower jaw, if indeed they can be called canines, are more like 
incisors. Where the upper incisors are absent, a rough pad is de- 
veloped for the lower teeth to strike against, as may be seen in any 
cow’s mouth, Instead of huge canines, claws, &c., the herbivora 
are provided with horns, antlers, &c., for defence; or when it is 
