at once as a weighty proof of the truth of evolution. The careful
construction of these genealogical trees is, therefore, not an
amusement, but the chief task of modern classification.

Among the chief phenomena that bear witness to the inductive law of
evolution we have the geographical distribution of the various species
of animals and plants over the surface of the earth, and their
topographical distribution on the summits of mountains and in the
depths of the ocean. The scientific study of these features--the
"science of distribution," or chorology (chora = a place)--has been
pursued with lively interest since the discoveries made by Alexander
von Humboldt. Until Darwin's time the work was confined to the
determination of the facts of the science, and chiefly aimed at
settling the spheres of distribution of the existing large and small
groups of living things. It was impossible at that time to explain the
causes of this remarkable distribution, or the reasons why one group
is found only in one locality and another in a different place, and
why there is this manifold distribution at all. Here, again, the
theory of evolution has given us the solution of the problem. It
furnishes the only possible explanation when it teaches that the
various species and groups of species descend from common stem-forms,
whose ever-branching offspring have gradually spread themselves by
migration over the earth. For each group of species we must admit a
"centre of production," or common home; this is the original habitat
in which the ancestral form was developed, and from which its
descendants spread out in every direction. Several of these
descendants became in their turn the stem-forms for new groups of
species, and these also scattered themselves by active and passive
migration, and so on. As each migrating organism found a different
environment in its new home, and adapted itself to it, it was