The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigree or armorial
bearings; and we have to discover and trace the many diverging lines of
descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect
to the nature of long-lost structures. Species and groups of species which
are called aberrant, and which may fancifully be called living fossils,
will aid us in forming a picture of the ancient forms of life. Embryology
will often reveal to us the structure, in some degree obscured, of the
prototypes of each great class.
When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have, within a not very
remote period descended from one parent, and have migrated from some one
birth-place; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be enabled
to trace in an admirable manner the former migrations of the inhabitants of
the whole world. Even at present, by comparing the differences between the
inhabitants of the sea on the opposite sides of a continent, and the nature
of the various inhabitants of that continent in relation to their apparent
means of immigration, some light can be thrown on ancient geography.
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