16 STEREOTROPIC REACTIONS 



surface of the head, a different reaction results; the margins of all 

 four of the paired fins are elevated strongly. The effect of this would 

 be to check the forward movement in the water and at the same time 

 to steer the fish downward to the bottom. This is plainly also an 

 example of a negatively stereotropic reaction. (It is necessary to 

 bear in mind that a stimulus on the upper midline of the head is 

 symmetrically placed with reference to the median plane of the 

 animal but not with reference to the horizontal plane.) 



All of the above reactions occur with great regularity and may be 

 called forth over and over again by appropriate stimuli, but it is 

 important to remember that the character of the response depends 

 not only on the location but also on the nature of the stimulus. A 

 stimulus at a given point may cause a change of direction of locomo- 

 tion in the horizontal plane with a slight rotation of the body around 

 its longitudinal axis, while a stronger stimulus at the same point may 

 cause a change of direction out of the horizontal plane, that is, a 

 movement toward the bottom. 



III. 



The above described reactions occur equally well in animals in 

 which the forebrain has been destroyed. I have made repeatedly 

 transsections of the brain as far back as the optic chiasma without 

 affecting them in the least. Complete destruction of the two laby- 

 rinths is equally without effect. Since these movements occur in 

 the absence of the forebrain it would be illogical to speak of them 

 as "voluntary," or "purposeful," or "instinctive." On the other 

 hand they illustrate beautifully the tropistic conception of animal 

 behavior since they are very evidently reactions of the organism 

 as a whole in response to asymmetrically applied stimuli. The 

 effect of these stimuli is to bring about sudden changes of tonus in 

 those groups of muscles which in their state of resting equilibrium 

 hold the eyes and fins in a position of symmetry. The change of 

 tonus causes an unsymmetrical action of the corresponding muscle 

 groups on the two sides of the body with the result that the new posi- 

 tion induced, terminates the contact with the stimulating object. 



At first sight it might appear that these reactions differ in their 

 nature from the other tropisms because in the latter we are concerned 



