o
turn into rock; in the majority of cases, an entombed bone is infiltrated
or replaced by various mineral substances so that in time little or
nothing of the original thing would remain, though a mold or a cast would
persist.

But even if an animal of the past possessed hard structures, it must have
satisfied certain limited conditions to have its remains prove serviceable
to students of to-day. A dead mammal must fall upon ground that has just
the right consistency to receive it; if the soil is too soft, its several
parts will be separated and scattered as readily as though it had fallen
upon hard ground where it would be torn to pieces by carnivorous animals.
The dead body must then be covered up by a blanket of silt or sand like
that which would be deposited as the result of a freshet. If a skeleton is
too greatly broken up or scattered, it may be difficult or even impossible
for its discoverer to piece together the various fragments and assemble
them in their original relations. Very few individuals have been so buried
and preserved as to meet the conditions for the formation of an ideal
fossil. To realize how little may be left of even the most abundant of
higher organisms, we have only to recall that less than a century ago
immense herds of bison and wild horses roamed the Western plains, but very
few of their skulls or other bones remain to be enclosed and fossilized in
future strata of rocks. When we appreciate all these difficulties, both
geological and biological, we begin to see clearly why the ancient lines
of descent cannot be known as we know the path and mode of embryonic
transformation. The wonder is not that the palaeontological record is
incomplete, but that there is any coherent and decipherable record at all.
Yet in view of the many and varied obstacles that must be surmounted by
the investigator, and the adverse factors which reduce the available
evidence, the rapidly growing body of palaeontological facts is amply
sufficient for the naturalist to use in formulating definite and
conclusive principles of evolution.

* * * * *

For the purposes of palaeontology, the most essential data of geology are
those which indicate the relative ages of the strata that make up the hard
outer crust of the earth, for only through them can the order of animal
succession be ascertained. It does not matter exactly how old the earth
may be. While it is possible to determine the approxi