it is very generally asserted by those who advocate
a purely vegetable diet that man's teeth are of the shape and pattern
which we find in fruit-eating or in root-eating animals allied to him.
This is true. The warm-blooded hairy quadrupeds which suckle their
young and are called "mammals" (for which word perhaps "beasts" is the
nearest Anglo-Saxon equivalent) show in different groups and orders a
great variety in their teeth. The birds of to-day have no teeth, the
reptiles, amphibians, and fishes have usually simple conical or
peg-like teeth, which are used simply for holding and tearing. In some
cases the pointed pin-like teeth are broadened out so as to be
button-like, and act as crushing organs for breaking up shell-fish.
The mammals alone have a great variety and elaboration of the teeth.

[Illustration: Fig. 21.--Side view of the skull of a clouded tiger
(_Felis nebulosa_) to show the teeth. _inc. s._ The three incisors.
_can. s._ Upper canine, corner-tooth, or dog-tooth. _can. i._ Lower
canine. _m. s._ The four upper molars or cheek-teeth (called
"grinders" in herbivorous animals). _m. i._ The three lower molars or
cheek-teeth.]

[Illustration: Fig. 22.--View in the horizontal plane of the teeth of
the lower and upper jaw of the same clouded tiger's skull. _inc. i._
Lower incisors. _inc. s._ Upper incisors. _can. i._ and _can. s._
Lower and upper canine. _m._ The cheek-teeth--three only in the lower
jaw, a minute fourth molar present in the upper.]

In shape and size, as well as in number, the teeth of mammals are very
clearly related to the nature of their food in the first place, and
secondly to their use as weapons of attack or of defence. When the
surface of the cheek-teeth is broad, with low and numerous tubercles,
the food of the animal is of a rather soft substance, which yields to
a grinding action. Such substances are fruits, nuts, roots, or leaves,
which are "triturated" and mixed with the saliva during the process
of mastication. Where the vegetable food is coarse grass or tree
twigs, requiring long and thorough grinding, transverse ridges of
enamel are present on the cheek-teeth, as in elephants, cattle, deer,
and rabbits (see Figs. 8, 17, 19). Truly carnivorous animals, which
eat the raw carcases of other animals, have a different shape of
teeth. Not only do they have large and dagger-like canines or
"dog-teeth" as weapons of attack, but the cheek-teeth (very few in
number) present a long, sharp-e