* * * * *
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.
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