descendant of a symmetrical ancestor because its structural plan was like
that of other bony fishes. If this be true, and if in its development a
flounder must review its mode of evolution as a species, the young fish
ought to be symmetrical; and it actually is. The grotesque skate and
hammerhead shark were demonstrated to be derivatives of a simpler type of
shark; their embryos are practically indistinguishable from those of
ordinary dogfish and sharks.

Among the jointed animals a wealth of interesting material is found by the
embryologist. All crabs seemed to be modified lobsterlike creatures; to
confirm this interpretation, based solely upon details of adult structure,
young crabs pass through a stage when to all intents and purposes they are
counterparts of lobsters. Even the twisted hermit crab, which has a
soft-skinned hinder part coiled to fit the curve of the snail shell used as
a protection, is symmetrical and lobster-like when it is a larva.

Among the insects many examples occur that are already familiar to every
one. The egg of a common house-fly hatches into a larva called a maggot;
in this condition the body destined to become the vastly different fly is
composed of soft-skinned segments very much alike and also similar to the
joints of a worm. Comparative anatomy demonstrates that the fly and all
other insects have arisen from wormlike ancestors, whose originally
similar segments later differentiated in various ways to become the
diverse segments of adult insects; the embryonic history of flies of
to-day corroborates these assertions, in so far as every individual fly
actually does become a wormlike larva before it changes into the final and
complete adult insect. The other kinds of insects are equally striking in
their life-histories. All beetles, such as the potato bug and June bug,
develop from grubs which, like the maggots of flies, are similar to worms
in numerous respects. Butterflies and moths pass through a caterpillar