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;[Footnote: Recent investigations tend
to show that this statement is not strictly accurate.--1870.] the animals
and plants which inhabit islands are commonly distinct from those of the
neighbouring mainlands, and yet have a similarity of aspect. 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 gum. All the members of the same great group run through
similar conditions in their development, and all their parts, in the adult
state, are arranged according to the same plan. Man is more like a gorilla
than a gorilla is like a lemur. Such are a few, taken at random, among the
multitudes of similar facts which modern research has established; but when