"Arguments from Embryology," the first is that, when developing embryos
are traced from their common starting-point, and their divergencies and
re-divergencies are symbolized by a genealogical tree, there is manifest
a general parallelism between the arrangement of its primary, secondary,
and tertiary branches, and the arrangement of the divisions and
subdivisions of Mr. Spencer's classifications. Nor do the minor
deviations from this general parallelism, which look like difficulties,
fail on closer observation to furnish additional evidence; since those
traits of a common ancestry which embryology reveals are, if
modifications have resulted from changed conditions, liable to be
disguised in different ways and degrees, in different lines of
descendants. Mr. Spencer next considers the "Arguments from Morphology."
Apart from those kinships among organisms disclosed by their
developmental changes, the kinships which their adult forms show are
profoundly significant. The unities of type found under such different
externals are inexplicable, except as results of community of descent,
with non-community of modification. Again, each organism analyzed apart
shows, in the likenesses obscured by unlikenesses of its component
parts, a peculiarity which can be ascribed only to the formation of a
more heterogeneous organism out of a more homogeneous one. And, once
more, the existence of rudimentary organs, homologous with organs that
are developed in allied animals or plants, while it admits of no other
rational interpretation, is satisfactorily interpreted by the hypothesis
of evolution. Last of the inductive evidences are the "Arguments from
Distribution." While the facts of distribution in space are
unaccountable as results of designed adaptation of organisms to their
habitats, they are accountable as results of the competition of species,
and the spread of the more fit into the habitats of the less fit,
followed by the changes which new conditions induce. Though the facts of
distribution in time are so fragmentary that no positive conclusion can
be drawn, yet all of them are reconcilable with the hypothesis of
evolution, and some of them yield strong support,--especially the near
relationship existing between the living and extinct types in each great
geographical area. Thus of these four categories of evidence, each
furnishes several arguments which point to the same conclusion. This
coincidence would give to the induction a ver