He learns the trick somewhat more rapidly if punishment for incorrect responses is added to reward for correct responses. Place wires along the floor of the two passages, and switch an electric current into the blind alley, behind {305} the door that has no yellow circle on it. When the rat enters the blind alley and gets a shock, he makes a prompt avoiding reaction, scampering back to the starting point and cowering there for some time; eventually he makes a fresh start, avoids the door that led to the shock and therefore enters the other door, though apparently without paying any attention to the yellow sign, since when, on the next trial, the sign is moved, he avoids the _place_ where he got the shock, without reference to the sign. But in a series of trials he learns to follow the sign. Learning to respond to a signal might be classified under the head of substitute stimulus, since the rat learns to respond to a stimulus, the yellow disk, that at first left him unmoved. But more careful consideration shows this to be, rather, a case of substitute response. The natural reaction of a rat to a door is to enter it, not to look at its surface, but the experiment forces him to make the preliminary response of attending to the appearance of the door before entering it. The response of attending to the surface of the door is substituted for the instinctive response of entering. Otherwise put: the response of finding the marked door and entering that is substituted for the response of entering any door at random. The maze experiment. An animal is placed in an enclosure from which it can reach food by following a more or less complicated path. The rat is the favorite subject for this experiment, but it is a very adaptable type of experiment and can be tried on any animal. Fishes and even crabs have mastered simple mazes, and in fact to learn the way to a goal is probably possible for any species that has any power of learning whatever. The rat, placed in a maze, explores. He sniffs about, goes back and forth, enters every passage, and actually covers every square inch of the maze at least once; and in the course of these explorations {306} hits upon the food box. Replaced at the starting point, he proceeds as before, though with more speed and less dallying in the blind alleys. On successive trials he goes less and less deeply into a blind alley, till finally he passes the entrance to it without even turning his head. Thus eliminating the blind alleys one after another, he comes at length to run by a fixed route from start to finish. [Illustration: Fig. 47.--(From Hicks.) Ground plan of a maze used in experiments on the rat. The central square enclosure is the food box. The dotted line shows the path taken by a rat on Its fourth trial, which occupied 4 minutes and 2 seconds.]