pecial complex organs and corresponding
functions. Thus, the caterpillars of butterflies with their specific
and generic peculiarities, hairs, horns, etc., furnish many examples
of secondary acquired characters which have nothing in common with the
worm, which is the ancestral type of the butterfly represented by the
embryonic period when it is a caterpillar. However, many undoubted
vestiges of the ancestral history are found in the embryos at
different periods of their development. It is certain that insects
descended from worms, and there is no doubt that the larvae of insects,
which are almost worms, represent the ontogenetic repetition of the
phylogeny of insects.

It is also certain that whales, although they have whalebone instead
of teeth, have descended from cetacea provided with teeth, which in
their turn descended from terrestrial mammals. But we find in the
embryo whale a complete denture which is of no use to it, and which
disappears in the course of the embryonic period. This denture is
nothing else than a phylogenetic incident in the ontogeny of the
whale.

In the fins of cetacea, as in the four limbs of other mammals, we find
the same bones, which are derived from the bones of the wings and legs
of their bird ancestors. In birds, the same bones are the phylogenetic
derivatives of the limbs of reptiles.

All these facts demonstrate with certainty the descent of animal
forms, a descent which we can follow in all its details. In certain
ants whose bodies show their close relationship with a slave-keeping
group, but which have become the parasitic hosts of other ants, we
find not only the arched mandibles, shaped for rape, but the undoubted
rudiments of the slave instinct, although this instinct has, perhaps,
not been exercised by them for thousands of years.

These examples suffice to show that the form and functions of a living
organism, as well as its mental faculties, are derived not only from
the most recent direct ancestors of this organism, but that they
partly mount much higher in the genealogical tree.

Our coccyx is a vestige of the tail of animals. It is from them also
that we have inherited anger and jealousy, sexual appetites, fear,
cunning, etc. As long as they remain in use, the oldest inherited
characters normally remain the most tenacious and are preserved the
longest. When they cease to be utilized, or become useless, they still
remain for a long time as rudiments before finally disapp