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. [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_:--