two or more epochs, then the hypothesis of a total change
is not true of them.

Geology makes huge demands upon time; and we regret to find that it has
exhausted ours,--that what we meant for the briefest and most general
sketch of some geological considerations in favor of Darwin's hypothesis
has so extended as to leave no room for considering "the great facts of
comparative anatomy and zooelogy" with which Darwin's theory "very well
accords," nor for indicating how "it admirably serves for explaining the
unity of composition of all organisms, the existence of representative
and rudimentary organs, and the natural series which genera and species
compose." Suffice it to say that these are the real strongholds of the
new system on its theoretical side; that it goes far towards explaining
both the physiological and the structural gradations and relations
between the two kingdoms, and the arrangement of all their forms in
groups subordinate to groups, all within a few great types; that it
reads the riddle of abortive organs and of morphological conformity, of
which no other theory has ever offered a scientific explanation, and
supplies a ground for harmonizing the two fundamental ideas which
naturalists and philosophers conceive to have ruled the organic world,
though they could not reconcile them, namely: Adaptation to Purpose and
the Conditions of Existence, and Unity of Type. To reconcile these two
undeniable principles is a capital problem in the philosophy of natural
history; and the hypothesis which consistently does so thereby secures a
great advantage.

We all know that the arm and hand of a monkey, the foreleg and foot of
a dog and of a horse, the wing of a bat, and the fin of a porpoise are
fundamentally identical; that the long neck of the giraffe has the same
and no more bones than the short one of the elephant; that the eggs of
Surinam frogs hatch into tadpoles with as good tails for swimming as any
of their kindred, although as tadpoles they never enter the water; that
the Guinea-pig is furnished with incisor teeth which it never uses,
as it sheds them before birth; that embryos of mammals and birds
have branchial slits and arteries running in loops, in imitation or
reminiscence of the arrangement which is permanent in fishes; and that
thousands of animals and plants have rudimentary organs which, at least
in numerous cases, are wholly useless to their possessors, etc., etc.
Upon a derivative theory this mo