her animals is indicated solely by the
fact that they are cells composed of protoplasm. Nevertheless the
principle which states that resemblance means consanguinity still holds
true, for cellular constitution is a unique possession of things of the
living world,--something which demonstrates the common origin of all
living things just as truly as the "cat-_ness_" of our first series of
examples reveals for a smaller group the significance of likeness and the
nature of the basic law of comparative anatomy.

* * * * *

Employing a figure of speech, we have climbed down the animal tree from
the higher regions where the mammals belong. Having reached the very foot
of the trunk we are in a position to review and summarize the evidences
which we have discovered all about us as we have descended. The various
examples we have mentioned and the groups to which they belong clearly
occupy different places in the scale which begins with the protozoa and
extends upward to the most complicated and differentiated animals. _Hydra_
takes its place above the protozoa for obvious structural reasons; worms
belong to a still higher zone, surpassed by the more complex jointed
animals like crustacea and insects. Far above these are the vertebrates,
among which we have already demonstrated the occurrence of different
grades of organization, from the fish up to the higher amphibia and
reptiles, and beyond in two directions to the diverging birds and mammals.
The basic characteristics of every group in a high position may be traced
back to some one or another of the divisions at a lower level, so that the
general sequence of the structural levels from low to high becomes
intelligible as the order of their evolution.

To my mind the rudimentary and vestigial structures of animals are in
themselves proof positive of a natural history of change. The few
illustrations can be reinforced by countless examples offered by every
group of living animals. If such structures have not evolved naturally by
degenerating from more efficient counterparts in ancestors of earlier
times, and if they have been specially created, they are utterly
meaningless and their very existence is unreasonable. If common sense is
to be employed, they demonstrate evolution.

Everywhere throughout the whole series animals place themselves in a
treelike arrangement, for in their respective levels they occur like
leaves at the ends of the lines of d