which indicates the
closeness of relationship corresponds with the degree of embryonic
agreement; that is, the cat and dog are much alike and their modes of
development are essentially the same to the latest stages, while the cat
and horse agree only during the earliest and middle stages, and their lines
diverge before those of the cat and dog on the one hand, or those of the
horse and pig on the other.

* * * * *

Like the fundamental principle of comparative anatomy in its sphere, the
Law of Recapitulation, formulated as a summary description of the
foregoing and similar facts, is one that holds true throughout the entire
range of embryology and for every division of the animal series, however
large or small. We have discussed its broader application, and now we may
take up some of the more or less special cases mentioned in the earlier
section of the present chapter, to see how it may work in detail.

The flounder was noted as a variant of the fish theme which seemed to be a
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 differenti