classes of mammals, birds, reptiles, amphibia, and fishes, possess a deep
underlying theme whose dominant motif is the backbone, which proves their
ultimate unity in ancestry. The greater and lesser branches have reached
different levels, for the fish is clearly simpler in its make-up than the
highly specialized bird. But the great fact is that structural evidences
demonstrating the reality of genealogical affinities are displayed by the
entire series of vertebrates; although they differ much or little in many
or fewer respects they have one and the same ground-plan.

* * * * *

The lower animals devoid of backbones, and therefore called invertebrates,
are not so well-known except to the student of comparative anatomy,
because they are not so often met with, and because they are usually very
small or microscopic; but in many respects their importance to the
evolutionist surpasses that of the vertebrates. Their structural plans are
far more varied, and they range more widely from higher and relatively
complicated organisms to the unitary one-celled animals. A knowledge of
some of them is essential for our present purpose, which is to learn how
sure is the basis for the principles of relationship and how complete is
the structural evidence of evolution.

Worms are represented in the minds of most people by the common earthworm
or sandworm. The body in either case is made up of a series of segments or
joints which agree closely throughout the animal in external appearance
and in internal constitution. A section of the digestive tract, a pair of
nerve centers, two funnel-like tubes for excretion, and similar blood
vessels occur in each portion.

Precisely similar features are displayed by the crustacea, which seem to