138 READINGS IN BIOLOGICAL SCIENCE 



determine by any means now at our disposal, there is nothing distinctive in 

 the human nervous system. In this just as in every other part of their 

 physical structure men fit squarely into the general mammalian patterns. 

 Even the human brain is almost identical with the anthropoid brain. We 

 must grant that the structural and mechanical elements underlying be- 

 havior are the same in men and in animals. Let us see whether the uses to 

 which this equipment is put differ in the two cases. 



All behavior consists of reflexes, combinations of stimulus and reactions 

 made possible by the structural and mechanical features just described. 

 Reflexes are of two types, unconditioned and conditioned. In imcojidi- 

 tioned rejiexes the path of the impulse from receptor to effector is al- 

 ready established when the individual is hatched or born. The link-up of 

 the elements within the reflex arc is hereditary, like any other part of the 

 individual's physical structure. In conditioned reflexes the path of the 

 impulse from receptor to effector is not determined at birth. The link-up 

 of the elements within the reflex arc comes as a result of selection and 

 routing of impulses within the reflex centers coupled with the gradual 

 wearing of paths through the synapses. The unconditioned reflex is the 

 foundation of automatic or instinctive behavior, the conditioned reflex is 

 the foundation of learned behavior. All animals with nervous systems 

 have reflexes of both types, but the relation which the reflexes of each 

 type bear to the total behavior of the individual varies tremendously with 

 the kind of animal. For example, insects owe most of their behavior to un- 

 conditioned reflexes, while men owe most of theirs to conditioned reflexes. 



It used to be believed that animal behavior was controlled by instinct, 

 human behavior by a mysterious and purely human quality called thought. 

 No psychologist holds this view to-day. What we call thought is really 

 an integral part of behavior, for there can be no mental activity without 

 muscular activity of some sort. The muscular activity may be reduced to 

 the point where it can be detected only by the most delicate instruments, 

 but it is there just the same. Thinking is as much a matter of reflex arcs 

 as is the winking of the eye. It is based on a combination of unconditioned 

 and conditioned reflexes and on the selection and routing of stimuli. 



In a comparative study of the mental activities of men and animals, the 

 investigator is handicapped at the outset by the fact that with animals 

 there can be no recourse to the introspective method. If any student could 

 be a white rat or a chimpanzee for a half an hour he could give us a clearer 

 picture of what goes on inside animals' minds than we are likely to get in 

 twenty years of experimental work. As it is, we can only deduce the mental 

 processes of animals from their behavior. If we approach the human mental 

 processes from the same angle, the results are almost identical. 



Let us take first of all the matter of learning, i. e., of establishing condi- 

 tioned reflexes. In experiments at the University of Wisconsin the ability 

 for learning mazes in white rats and in sophomores was tested and com- 



