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 sheet of cells of the two-layered body, they conduct impulses from one part of the animal to another, and thus serve as coordinating members of the community. For the first time, then, a nervous system as such is set apart and specialized to devote itself to the two tasks of sensation and coordination that are performed by nervous systems throughout the entire range of organisms higher in the scale. But the activities of _Hydra_, like those of _Amoeba_, are reflex and mechanical,--that is to say, _given similar stimuli and similar physiological states of the animal, the reactions will be the same_. A little water-crustacean like _Daphnia_ may swim against the tentacles of _Hydra_; it is stung to death by the minute cell-batteries which the animal possesses, and then in a mechanical way the tentacles transport the food to the mouth, through which it is passed inward to the digestive cavity. There is nothing that can be called "mentality" throughout these processes, but the series of activities is much more complex than in _Amoeba_ because the whole organism is constructed more elaborately, and because the special and peculiar mechanism directing the activities has advanced to a far higher condition.