ly in degree from that of lower organisms, and not in kind or
fundamental nature.

* * * * *

When the operations of human mental life are examined, they include what
are called processes of _reason_ as apparently distinctive elements. The
lower mammalia exhibit a simpler order of "mentality" denoted
_intelligence_, while the nervous processes of still simpler forms are
called _instinctive_ and _reflex_ activities. These are the terms of the
comparative array of psychology which are to be separately examined and
classified, and to be brought into an evolutionary sequence if
common-sense directs us to do so.

Let us begin our comparative study with an example of the simplest animals
that consist of only a single cell, such as the little protozoon
_Amoeba_. We have become familiar with this organism as one that carries
on all of the vital functions within the limits of a single structural
unit; it is a mass of protoplasm enclosing a nucleus, and as a biological
individual it must perform all of the eight tasks that are essential for
life. It does not possess a digestive tract, but it does digest; it does
not have breathing organs, but it does respire; and it is particularly
noteworthy that it must coordinate the different activities of its parts,
and maintain definite relations with the environment, even though its
coordination and sensation are not accomplished by any special parts that
would deserve the name of elementary nervous organs. Its many activities
are simple responses to stimuli that reach it from without, and its
reactions to such stimuli are called reflex processes. Should the light
become too strong, it will slowly crawl to a shady place; should the water
in which it lives become warmer, it responds by displaying greater
activity. It exhibits, in a word, the property of _irritability_--that is,
simply the power of receiving and reacting to stimuli; and being only a
single cell this property is held in common by all of its parts.

We come next to a simple many-celled animal like the polyp _Hydra_, or a
jellyfish. In such an animal the body is composed of numerous cells which
are not all alike either in their make-up or in their functions. Some of
them are concerned primarily with digestion, others with protection, while
still others are exempt from these tasks and as sense-cells they devote
all their energies to the reception of stimuli from without, or, beneath
the outer sh