Fig. 60.--How the cat learns the trick of escaping from the cage by unlatching the door. S is the situation of being shut up in a cage, and T is the tendency to get out. R1 is the primary response aroused by this tendency, which response meets with failure, not leading to the end-result of the tendency. Responses are then made to various particular stimuli about the cage, and one of these stimuli, the door-latch, X, gives the response R2 which leads to the end-result. Now the response R2 was in part aroused by T, and its pre-existing weak linkage with T is so strengthened by exercise that T, or we may say S, comes to give the correct response without hesitation.
2. Learning to balance on a bicycle.
When the beginner feels the bicycle tipping to the left, he naturally responds by leaning to the right, and even by turning the wheel to the right. Result unsatisfactory--strained position and further tipping to the left. As the bicyclist is about to fall, he saves himself by a response which he has previously learned in balancing on his feet; he extends his foot to the left, which amounts to a response to the ground on the left as a good base of support. Now let him sometime respond to the ground on his left by turning his wheel that way, and, to his surprise and gratification, he finds the tipping overcome, and his balance well maintained. The response of turning to the left, originally made to the ground on the left (but in part to the tipping), becomes so linked with the tipping as to be the prompt reaction whenever tipping is felt. The diagram of this process would be the same as for the preceding instance.
D. Substitute Response, the Response Being a Higher Motor Unit
1. The brake and clutch combination in driving an automobile.
This may serve as an instance of simultaneous coördination, since the two movements which are combined into a higher unit are executed simultaneously. The beginner in driving an automobile often has considerable trouble in learning to release the "clutch", which, operated by the left foot, ungears the car from the engine, and so permits the car to be stopped without stopping the engine. The foot brake, operated by the right foot, is comparatively easy to master, because the necessity for stopping the car is a perfectly clear and definite stimulus. Now, when the beginner gets a brake-stimulus, he responds promptly with his right foot, but neglects to employ his left foot on the clutch, because he has no effective clutch-stimulus; there is nothing [{411}] in the situation that reminds him of the clutch. Result, engine stalled, ridicule for the driver. Next time, perhaps, he thinks "clutch" when he gets the brake-stimulus, and this thought, being itself a clutch-stimulus, arouses the clutch-response simultaneously with the brake-response. After doing this a number of times, the driver no longer needs the thought of the clutch as a stimulus, for the left foot movement on the clutch has become effectively linked with the brake-stimulus, so that any occasion that arouses the brake-response simultaneously arouses the clutch response.
Fig. 61.--Combining clutch-response with brake-response. At first, the brake-stimulus has only a weak linkage with the clutch-response, and an extra stimulus has to be found to secure the clutch-response. But whenever the clutch-response is made while the brake-stimulus is acting, the weak linkage between these two is exercised, till finally the brake-stimulus is sufficient to give the clutch-response, along with the brake-response.