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SCIENCE 



[N. S. Vol. XXVI. No. 662 



affected thereby until perchance the pressure 

 on one side becomes less than the pressure on 

 the other. Or, to draw a parallel from the 

 field of organic behavior itself, it has been 

 determined by many investigators that when, 

 instead of a single source of light, two sources 

 of equal intensity, such as two incandescent 

 lights, are placed symmetrically before photo- 

 tropic organisms, the latter may move toward 

 or away from them along the perpendicular, 

 passing through a point midway between them. 

 When one light is cut out, the organisms may 

 change their direction at once, moving toward 

 or away from the remaining light. According 

 to Jennings, they are not in a condition of 

 stimulation while moving toward or away 

 from the two lights, but only during the 

 period between the removal of one light and 

 their orientation to the light remaining. 

 Jennings denies emphatically the possibility 

 of symmetrical stimulation in such a case. 



Now it has been shown already that the 

 constant galvanic current does produce ob- 

 servable constant effects in organisms which 

 are moving directly toward one pole. It is 

 also well known that certain organisms (e. g., 

 newly hatched barnacle larvas) after exposure 

 to light for a time, during which they may 

 move toward the light, may change the sense 

 of their response, moving in the opposite direc- 

 tion. There is no doubt that we are dealing 

 here with a physiological effect produced by a 

 constant stimulus that occurs commonly in 

 nature, and that this effect conditions a defi- 

 nitely directive response. Yet, though this 

 sort of behavior is cited in another oonnection 

 by Jennings, its significance in the present 

 connection is not considered. 



But let us consider briefly one other class 

 of facts' which receive no consideration in 

 Jennings's book. It is well known that cer- 

 tain phototropic Crustacea and insects, when 

 robbed of the use of one eye by a coat of 

 opaque varnish, perform what have been called 

 circus movements. They move in circles in 

 the presence of light, toward or away from the 

 functioning eye according as they are nega- 

 tively or positively phototropic. These move- 

 ments are just what would be expected from 



a phototropic animal that can receive light 

 stimulation only through its eyes, when one 

 eye is kept constantly in the shade. They are 

 in entire accord with the tropic schema. Now 

 it happens that hemisection of the brain causes 

 the phototropic reaction to disappear in cer- 

 tain Amphipods on which the operation has 

 been performed, although unilateral injury of 

 the brain does not interfere with the photo- 

 tropic response. That these facts are to be 

 explained on the assumption of a reflex of 

 some sort between eyes and locomotor mech- 

 anism, and that one eye is connected with that 

 part of the mechanism which operates one 

 side of the body, while the other eye is sim- 

 ilarly associated with the mechanism for the 

 other side of the body, seems clear. The re- 

 actions of the leg muscles of Ranaira, when 

 that animal is subjected to light stimulation 

 alternately on the two sides of the body, 

 change with the utmost deftniteness, according 

 to the position of the light with respect to the 

 eyes. The response is unquestionably reflex 

 and singularly definite and local. 



To consider just one more case that will 

 bring out still more clearly the difference be- 

 tween Jennings's conception of a stimulus and 

 my own. When the semicircular canals on 

 one side of the head of an animal are removed 

 or injured, or the nerve supplying them is cut, 

 the normal response to gravity will be dis- 

 turbed. In man, sensations of unbalance 

 would result, general sensations or feelings, 

 such as discomfort, even distress. These are 

 obviously psychical facts. So far as the in- 

 jured man is concerned, reflex responses to 

 gravity by way of the semicircular canals have 

 never been noted. He has never suspected 

 any mechanism in his body devoted to the task 

 of keeping him physically upright. Accord- 

 ingly, in the absence of the feeling of discom- 

 fort resulting from operation or injury, he 

 may be said to be in a non-stimulated condi- 

 tion, hut only so far as the facts of conscious- 

 ness are concerned. Some such case is what 

 Jennings appears to have in mind when he in- 

 sists that a stimulus depends essentially on a 

 change in condition. When an organism 

 moves in a straight line toward or away from 



