having constant vibratile processes, or shells of some kind, while in
still other cases like individuals combine to make colonies which are more
or less definite and permanent. Here at the very foot of the organic scale
are found animals which seem to be entirely different from those above.
Upon examination they, like _Hydra_, prove to be the same as regards the
number and kind of functions they perform, but in structural regards their
evolutionary relation to all higher 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.