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Fish, to take another example, possess associative memory; they can learn. But they learn very slowly, and learn only the simplest things. The jump from their powers of memory to those of a dog, who can be trained comparatively quickly to carry out complicated tricks, is as great as the further jump from the powers of a dog to those of a man capable of learning a page of print by heart in two or three readings.

The first organs connected with mind to become elaborated are the organs of sense: but such receptor organs are useless to their possessor, however elaborate, unless put into relation with proper effector organs—organs for action, whether locomotor or secretory. So that the first steps are the elaboration of sense-organs, the increase of efficiency of muscles and glands, and, equally essential, the construction of an improved “adjustor system,” whereby the stimulus falling on the sense-organ may be translated into action and into the right kind of action. This adjustor mechanism is the central nervous system. Most of the further history of organisms may be summed up in one phrase—the evolution of adjustor mechanisms.