s (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.

[24] Romanes, _Mental Evolution in Animals_, p. 24.

[25] Clark, _Mind in Nature_, p. 64 _et seq._

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,