f apparently insignificant
things, is full of significance. For him it was an historical record,
the revelation of a cause, the lurking-place of a principle."

According to the theory of evolution every structure of to-day finds its
meaning in some condition of the past. The inside of an animal tells
what it really is, for it bears the record of heredity. The outside of
an animal tells where its ancestors have been, for it bears record of
concessions to environment. Similarity in essential structure is known
as _homology_. By the theory of evolution homology, wherever it is
found, is proof of blood-relationship.

The theory of organic evolution through natural law was first placed on
a stable footing by the observations and inductions of Darwin. It has
therefore been long known as Darwinism, although that term has been
usually associated with the recognition of natural selection as the
great motive power in organic change. Darwinism was at first regarded as
a "working hypothesis." It is now an integral part of biological
science, because all opposing hypotheses have long since ceased to work.
It is as well attested as the theory of gravitation, and its elements
are open to less doubt. All investigations in biology must assume it, as
without it most such investigations would be impossible. Naturalists
could no more go back to the old notion of special creation for each
species and its organs than astronomers could go back to the old notion
of guiding angels as directors of planetary motion. Without the theory
of organic development through natural selection, the biological science
of to-day would be impossible.

_Evolution as a Method of Study._ In a third sense the word evolution is
applied to a method of investigation. It is the study of present
conditions in the light of the past. The preliminary work of science is
the descriptive part. This involves accuracy of observation and
precision of statement, but makes no great demands on the powers of
logical analysis and synthesis. The easy work of science is largely
already done. Those who would continue investigation must study not only
facts and structures, but the laws that govern them. In the words of
John Fiske, "Whether plants or mountains or mollusks or subjunctive
moods or tribal confederacies be the things studied, the scholars who
have studied them most fruitfully were those who have studied them as
phases of development. Their work has directed the current of