Such arguments against the hypothesis of the direct creation of species
as these are plainly enough deducible from general considerations; but
there are, in addition, phenomena exhibited by species themselves, and
yet not so much a part of their very essence as to have required
earlier mention, which are in the highest degree perplexing, if we adopt
the popularly accepted hypothesis. Such are the facts of distribution
in space and in time; the singular phenomena brought to light by the
study of development; the structural relations of species upon which
our systems of classification are founded; the great doctrines of
philosophical anatomy, such as that of homology, or of the community of
structural plan exhibited by large groups of species differing very
widely in their habits and functions.

The species of animals which inhabit the sea on opposite sides of the
isthmus of Panama are wholly distinct* the animals and plants which
inhabit islands are commonly distinct from those of the neighbouring
mainlands, and yet have a similarity of aspect.

[footnote] *Recent investigations tend to show that this
statement is not strictly accurate.--1870.

The mammals of the latest tertiary epoch in the Old and New Worlds
belong to the same genera, or family groups, as those which now inhabit
the same great geographical area. The crocodilian reptiles which
existed in the earliest secondary epoch were similar in general
structure to those now living, but exhibit slight differences in their
vertebrae, nasal passages, and one or two other points. The
guinea-pig has teeth which are shed before it is born, and hence can
never subserve the masticatory purpose for which they seem contrived,
and, in like manner, the female dugong has tusks which never cut the