he toothless pad of the upper jaw opposed
to them, and are the instruments by which these animals, with an
upward jerk of the head, "crop" the grass and other herbage on which
they feed, to be afterwards triturated by the grinding cheek teeth. A
vast series of living and of fossil animals, called the
Ruminants--including the giraffes, the antler-bearing forms called
deer, the cavicorn or sheath-horned bovines, ovines and caprines, and
the large series of antelopes of Africa and India--all have precisely
this form of jaw, this number and shape and grouping of the teeth. Now
let me call to mind the lower jaw of a hare or rabbit or rat (Figs. 18
and 19). There we find on each side the group of grinding cheek-teeth,
with transverse ridges on their crowns, and a long, toothless gap
before we arrive at the front teeth. But the front teeth are only two
in number, one on each side, close to each other, very large, and each
with a tremendously long, deeply set root. They meet a similar pair of
teeth in the upper jaw, and give the hare, rabbit, rats, mice,
beavers, and porcupines the power of "gnawing" tough substances.
These animals are hence called Rodents, or gnawers, and the two great
front teeth are called "rodent-teeth." No two arrangements of teeth
could be much more unlike than are the group of eight little
chisel-like teeth of the lower jaw of the Ruminants and the two
enormous gnawing teeth of the Rodents. Apparently the two rodent
incisors, or front teeth, of the lower jaw of the rat correspond to
the two middle incisors of the Ruminant's lower jaw; the other front
teeth of the Ruminant have atrophied, disappeared altogether. The
rodent condition has been developed from that of an ancestor which
had several front teeth and not two large ones only; but we have not
at present found the intermediate steps.

[Illustration: Fig. 19.--View in the horizontal plane of the teeth of
the left half of the lower and the left half of the upper jaw of the
Coypu rat to show the single great gnawing incisor on each side, the
four flat grinding molars and the wide gap between molars and
incisors. Compare with Figs. 17 and 22.]

The reader should compare the teeth of the goat and the large rat here
pictured with the more typical and complete series of the pig, given
in Fig. 10, p. 84. The pig's teeth are the same in number as those of
the ancestral primitive typidentate mammal, and their form is near to
that of the ancestor's teeth.