demonstrated by science. The naturalists of a century ago held a similar
opinion regarding the earth, viewing it as an immutable and unchanged
product of supernatural creation, until Lyell led them to see that the
world is a plastic mass slowly altering in countless ways. It is no more
true that living things have ceased to evolve than that mountains and
rivers and glaciers are fixed in their final forms; they may seem
everlasting and permanent only because a human life is so brief in
comparison with their full histories. Like the development of a continent
as science describes it, the origin of a new species by evolution, its
rise, culmination, and final extinction may demand thousands of years; so
that an onlooker who is himself only a conscious atom of the turbulent
stream of evolving organic life does not live long enough to observe more
than a small fraction of the whole process. Therefore living species seem
unchanged and unchangeable until a conviction that evolution is true, and
a knowledge of the method of science by which this conviction is borne
upon one, guide the student onwards in the further search for the
efficient causes of the process.

The biologist employs the identical methods used by the geologist in
working out the past history of the earth's crust. The latter observes the
forces at work to-day, and compares the new layers of rock now being
formed with the strata of deeper levels; these are so much alike that he
is led to regard the constructive influences of the past as identical with
those he can now watch at work. Similarly the biologist must first learn,
as we have done, the principles of animal construction and development,
and of other classes of zoölogical facts, and then he must turn his
attention from the dead object of laboratory analysis to the workings of
organic machines. The way an organism lives its life in dynamic relations
to the varied conditions of existence, as well as the mutual physiological
relations of the manifold parts of a single organism, reveal certain