Responses of 



either one alone. This change may consist in 

 (1) reinforcement, a stronger response than 

 usual; (2) inhibition, a weaker response, or 

 none at all; or (3) modification, a different 

 response than either stimulus alone would 

 evoke. All these phenomena depend upon 

 the particular connections, within the ner- 

 vous system, of the reflex arcs involved, and 

 also upon the time and intensity relations of 

 the nerve impulses traversing the various 

 arcs. Thus the behavior of an individual is 

 determined not by a single stimulus, but by 

 the entire situation, that is, the total pattern 

 of stimuli impinging on the animal at any 

 given moment. 



All learned responses of an animal, the 

 fulfillment of which depends upon previous 

 training or experience, fall into the category 

 of conditioned reflexes. Such conditioned re- 

 flexes are frequently extremely complex and 

 indirect; and the many factors that deter- 

 mine this type of behavior are difficult to 

 investigate experimentally. However, the 

 classical experiments of Pavlov, which were 

 mentioned previously (p. 307), have served 

 to initiate many experiments on the learning 

 process. 



Conditioned responses are superimposed 

 upon the unconditioned reflexes of an ani- 

 mal, such as a dog, by a process of substitut- 

 ing one kind of stimulation for another. In 

 an unconditioned dog, for example, only the 

 actual presence of food or other material 

 in the mouth will elicit the salivation reflex; 

 but if another kind of stimulus is applied 

 each time the animal is fed, soon the foreign 

 stimulus alone will evoke a flow of saliva. In 

 other words, the dog has "learned" to sali- 

 vate at the sound of a dinner bell, or at the 

 smell of a tasty morsel — or whenever any 

 other stimulus, however bizarre, is applied 

 repeatedly, in close association with the 

 original stimulus. 



The capacity to form conditioned reflexes 

 is more highly developed in vertebrates as 

 compared to invertebrate animals, and in 

 birds and mammals as compared to lower 

 vertebrates. Among vertebrates, the condi- 



Higher Animals: The Nervous System - 469 



tioning potentiality is definitely related to 

 the development of the cerebral cortex; and 

 if this part of the brain is destroyed, the ani- 

 mal loses almost all previously acquired 

 learning as well as virtually all capacity for 

 forming new conditioned responses. Appar- 

 ently impulses from all the receptors of the 

 body converge upon the association neurons 

 of the cerebral cortex and tend to be shunted 

 into motor pathways that at the moment 

 are superexcitable because they are simul- 

 taneously engaged in carrying out some well- 

 established reflex. Consequently other stimuli 

 acting simultaneously with an original stimu- 

 lus tend to establish an associative connection 

 with the original stimulus. Furthermore, any 

 repetition of this association tends to facili- 

 tate the new synaptic connections and to 

 establish the "habit" of the new response. 



Birds and mammals stand strikingly above 

 all other animals in their ability to learn. 

 Among mammals, an especially high degree 

 of educability is found in some of the larger 

 carnivores and herbivores, such as dogs, 

 horses, and elephants. These animals are not 

 intelligent because they are domesticated — 

 they are domesticated because of their intel- 

 ligence, that is, their ability to be trained. 

 The highest development of intelligence, 

 however, is found in the primates — monkeys, 

 apes, and man. There can be no doubt that 

 monkeys — even after making all due allow- 

 ance for their enormous advantage in the 

 possession of hands — display distinctly more 

 intelligence than four-footed mammals. Mon- 

 keys quickly learn to employ tools as a means 

 to an end — for example, a stick to secure 

 food that is out of reach. Monkeys also dis- 

 play more initiative and curiosity than other 

 animals, and in these features their behavior 

 approaches that of man. Moreover, the an- 

 thropoid apes — which resemble man most 

 closely in the structure of their bodies and 

 especially of their brains — are more nearly 

 "human" in their behavior than the tailed 

 monkeys. 



Behavior of Man: Language. An older 

 point of view regarded human behavior as 



