living which have not been found by
man.

But though the study of fossil quadruped be enlightening, it has its own
special difficulties. One great difficulty arises from the fact that it
is very rare to find a fossil skeleton approaching to a complete state.

Fortunately, however, there is a principle in comparative anatomy which
lessens this difficulty. Every organised being constitutes a complete
and compact system with all its parts in mutual correspondence. None of
its parts can be changed without changing other parts, and consequently
each part, taken separately, indicates the others.

Thus, if the intestines of an animal are made to digest raw flesh, its
jaws must be likewise constructed to devour prey, its claws to seize and
tear it, its teeth to rend it, its limbs to overtake it, its organs of
sense to discern it afar. Again, in order to enable the jaw to seize
with facility, a certain form of condyle is necessary, and the zygomatic
arch must be well developed to give attachment to the masseter muscle.
Again, the muscles of the neck must be powerful, whence results a
special form in the vertebrae and the occiput, where the muscles are
attached. Yet again, in order that the claws may be effective, the
toe-bones must have a certain form, and must have muscles and tendons
distributed in a certain way. In a word, the form of the tooth
necessitates the form of the condyle, of the shoulder-blade, and of the
claws, of the femur, and of all the other bones, and all the other bones
taken separately will give the tooth. In this manner anyone who is
scientifically acquainted with the laws of organic economy may from a
fragment reconstruct the whole animal. The mark of a cloven hoof is
sufficient to tell the form of the teeth and jaws and vertebrae and
leg-bones and thigh-bones and pelvis of the animal. The least fragment
of bone, the smallest apophysis, has a determinative character in
relation to the class, the order, the genus, and species to which it may
belong. This is so true that, if we have only a single extremity of bone
well preserved, we may, with application and a skilful use of analogy
and exact comparison, determine all those points with as much certainty
as if we were in possession of the entire animal. By the application of
these principles we have identified and classified the fossil remains of
more than one hundred and fifty mammalia.

_II.--What the Fossils Teach_

An examination of the fossils on