S. S. MAXWELL 17 



with the influence of forces acting along definite lines, while in the 

 contact reaction the stimulus is applied to a single spot or a limited 

 area of the skin. It is hardly necessary to point out that in helio- 

 tropic animals with two eyes the light rays act upon two very limited 

 areas, namely, portions of the retinas, and in the geotropic reactions, 

 gravitation acts upon very limited areas in the internal ear to bring 

 about or maintain orientation. While ordinarily the two eyes or the 

 two ears come into play in the heliotropic and geotropic reactions 

 respectively, experiments show that marked effects are produced 

 by stimuli applied to one eye or one ear alone. Another apparent 

 difference is that contact stimuli may act from moment to moment in 

 different directions. This, however, would be paralled by the effect 

 of an intermittent, moving light upon a heliotropic organism. 



It is of interest to picture the behavior of the organism under the 

 play of two tropistic influences. Instead of a direct response to either, 

 the position or movement which occurs may be the simple resultant 

 of the two, as in the case of barnacle larvae exposed to two lights 

 from different directions.- On the other hand the one stimulus 

 suddenly applied may for the moment inhibit the effect of the other. 

 The free swimming fish, for example, reacts to gravitation by definite 

 compensatory movements and positions of the eyes and fins through 

 which it maintains a definite course and a horizontal position. If 

 now suddenly a foreign body comes in contact with a certain portion 

 of the head, say a point on the left upper surface, a negatively stereo- 

 tropic movement occurs; the fins are thrown into an unsymmetrical 

 position causing the left side of the head to be lowered and the course 

 to be changed to the right. These changes terminate the contact 

 and the stereotropic reaction ceases. But the sudden swing to the 

 right has excited the ampulla of the right horizontal canal and a 

 compensatory movement to the left, i.e., to the original course, is 

 produced while at the same time the rotation around the longitudinal, 

 body axis has stimulated the otoHth organs and the ampulla? of 

 the vertical canals in such way that the horizontal position is again 

 attained. The resulting behavior of the animal would in this way 



^Loeb, J., and Northrop, J. H., Heliotropic animals as photometers on the 

 basis of the vaHdity of the Bunsen-Roscoe law for hehotropic reactions, Proc. 

 Nat. Acad. Sc, 1917, iii, 539. 



