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 palæontological 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 palæontological facts is amply
sufficient for the naturalist to use in formulating definite and
conclusive principles of evolution.

* * * * *

For the purposes of palæontology, 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 approximate length of time
required for the construction of sedimentary rocks like those which
natural agencies are producing to-day, there are few definite facts to
guide speculation as to the mode or duration of the process by which the
first hard crystalline surface of the earth was formed. But palæontology
does not care so much about the earliest geological happenings, for it is
concerned with the manifold animal forms that arose and evolved after life
appeared on the globe. Questions as to the way life arose, and as to the
earliest transformations of the materials by which the earth was first
formed are not within the scope of organic evolution, although they relate
to intensely interesting problems for the student of the process of cosmic
evolution.