modified, and gave rise to new forms.

This very important branch of science that deals with active and
passive migration was founded by Darwin, with the aid of the theory of
evolution; and at the same time he advanced the true explanation of
the remarkable relation or similarity of the living population in any
locality to the fossil forms found in it. Moritz Wagner very ably
developed his idea under the title of "the theory of migration." In my
opinion, this famous traveller has rather over-estimated the value of
his theory of migration when he takes it to be an indispensable
condition of the formation of new species and opposes the theory of
selection. The two theories are not opposed in their main features.
Migration (by which the stem-form of a new species is isolated) is
really only a special case of selection. The striking and interesting
facts of chorology can be explained only by the theory of evolution,
and therefore we must count them among the most important of its
inductive bases.

The same must be said of all the remarkable phenomena which we
perceive in the economy of the living organism. The many and various
relations of plants and animals to each other and to their
environment, which are treated in bionomy (from nomos, law or norm,
and bios, life), the interesting facts of parasitism, domesticity,
care of the young, social habits, etc., can only be explained by the
action of heredity and adaptation. Formerly people saw only the
guidance of a beneficent Providence in these phenomena; to-day we
discover in them admirable proofs of the theory of evolution. It is
impossible to understand them except in the light of this theory and
the struggle for life.