Mind is always associated, according to our experience and knowledge (and this question must be studied objectively) with a peculiar tissue which is only to be found in animal organisms. This tissue is called nerve, and is made up of cells and, broadly speaking, prolongations of cells which are called nerve-fibres.
Certain accumulations of nerve-cells called ganglions (ganglia) are to be found scattered throughout the structure of animals. Experiment and observation teach that these ganglia subserve a governing influence over nerve-action; hence, they are called nerve-centres.
Nerve-tissue is found in all animals above and including Hydrozoa, according to Romanes;[24] I am inclined to believe, however, that it is present in animals even lower than Hydrozoa, for I have been able, on more than one occasion, to verify Professor Clark's observations in regard to the protozoan, Stentor polymorphus, which, as he asserts,[25] has a well-developed nervous system. Moreover, I have seen, in my opinion, unquestionable acts of conscious determination enacted by this little creature, as I will point out further along in this chapter.
Nerve-tissue has the peculiar faculty of transmitting impressions made upon it by stimuli. When a nerve is acted on by a stimulus, the impression wave is transmitted along the in-going nerve to the ganglion; here, the stimulus is transferred to the out-going nerve, which, going to the muscle, causes it to contract.
This form of nerve-action is called reflex action, and reflex action is, in the beginning, the germ from which spring volition (choice) and all of the higher psychical attributes.
Again, it is to be observed, as animals become more highly organized, that nerves have the power of discriminating between stimuli, and "it is this power of discriminating between stimuli," as Romanes puts it, "irrespective of their relative mechanical intensities, that constitutes the physiological aspect of choice" (volition). It is also through the faculty of discrimination that the special senses, upon which the entire psychical structure depends, have been evolved.
The fact of this power of discrimination has been so clearly and so beautifully demonstrated by Romanes, that I present his experiment and observations, as detailed by him in his magnificent work, Mental Evolution in Animals:—
"I have observed that if a sea-anemone is placed in an aquarium tank, and allowed to fasten on one side of the tank near the surface of the water, and if a jet of sea-water is made to play continuously and forcibly upon the anemone from above, the result of course is that the animal becomes surrounded with a turmoil of water and air-bubbles. Yet, after a short time, it becomes so accustomed to this turmoil that it will expand its tentacles in search of food, just as it does when placed in calm water. If now one of the expanded tentacles is gently touched with a solid body, all the others close around that body, in just the same way as they would were they expanded in calm water. That is to say, the tentacles are able to discriminate between the stimulus which is applied by the turmoil of the water and that which is supplied by their contact with the solid body, and they respond to the latter stimulus notwithstanding that it is of incomparably less intensity than the former."[26]
When a stimulus passes over a nerve to a ganglion, it leaves upon it an impression which remains for a shorter or longer time as the stimulus is great or small. Now, when a stimulus is again applied to the nerve, the impression wave follows in the footsteps, as it were, of the first impression wave, and the ganglion reflects or transfers it just as before, thus showing that nerve has another peculiar quality—that of memory.
Again, when two or more reflexes are excited by the same stimulus or stimuli, the ganglion learns to associate one with the other, thus showing that it possesses another quality—that of the association of ideas (stimuli and reflexes).