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SCIENCE 



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



connected with galvanic stimulation, the fol- 

 lowing facts should be noted: (1) that the 

 phenomena of galvanic stimulation present 

 certain important correspondences with re- 

 spect both to vertebrate miiscle and free- 

 swimming unicellular organisms; (2) that 

 galvanic stimulation is of two kinds and pro- 

 duces two different efPects, the constant cur- 

 rent producing a definitely directive orienta- 

 tion effect (galvanotropism) ; (3) that the 

 stimulating effect of the constant current does 

 not cease with the establishment of the per- 

 manent orientation. A further fact should 

 be added, namely, that galvanic stimulation 

 is practically vinknown in nature. 



These facts would appear to lend support 

 to the tropism hypothesis. Professor Jen- 

 nings, however, believes that the very unique- 

 ness of the electric stimulus in producing its 

 peculiar local effects upon the organism, and 

 its practical absence from nature, vitiate its 

 claims to consideration in any attempt to 

 formulate a universal explanation of the be- 

 havior of organisms. 



It is not easy to see how, on such grounds, 

 the interesting phenomena of galvanic stimu- 

 lation are to be so lightly put aside. That a 

 stimulus is unique in any respect is hardly 

 ground for neglecting it. And that it does 

 not occur in nature is for the candid analyst 

 one of its most valuable assets. He thereby 

 gets rid of the selection hypothesis and the 

 mass of unestablished inferences which it has 

 gathered to itself. He is free to examine 

 types of animal behavior which never could 

 have been produced by selection. He comes 

 60 much the nearer the fundamental responses 

 of organized matter to at least one stimulus. 

 And he finds, instead of the varied, haphazard 

 reactions which are the only primary reactions 

 for Jennings, two sorts of reactions, one of 

 which is as definitely directive as any class of 

 reactions in the organic world. 



If organisms, without the aid of selection, 

 respond definitely and directively to one sort 

 of stimulation, whether in or out of nature, 

 does that not at least raise a suspicion that 

 definite directive reactions, wherever they oc- 

 cur, may also be interpreted without such aid ? 



With such a suspicion in mind, we may ex- 

 amine some of the evidence from nature which 

 has been counted for the trial and error 

 schema. 



From galvanic stimuli, then, we may turn 

 to a consideration of the reactions of organ- 

 isms to light. 



In this field numerous investigators have 

 been accustomed to distinguish between two 

 types of reactions which parallel the two types 

 of responses to galvanic stimulation. The first 

 type depends upon rapid changes in the in- 

 tensity of light and has been called Unter- 

 schiedsempfindlichkeit by Professor Loeb. 

 Many years ago, he distinguished this type 

 of reaction from the second or tropic reaction, 

 finding it well exemplified among certain an- 

 nelid worms that dwell in tubes from which 

 the anterior ends of their bodies project. 

 When the intensity of the light falling upon 

 one of these projecting ends is rapidly dim- 

 inished beyond a certain degree, the worm 

 suddenly responds by contracting its longi- 

 tudinal muscles and withdrawing into its tube. 

 It is a significant fact that a corresponding 

 increase in the intensity of the light falling 

 upon an extended worm does not cause a con- 

 traction. Similarly, the unicellular Stentor 

 passes from a brightly illuminated field into 

 shadow without reaction, but reacts when it 

 reaches the edge of the shadow in passing in 

 the reverse direction. 



Whether this difference in the response is 

 directly comparable with certain observed dif- 

 ferences in the effects produced by making 

 and breaking the galvanic current we are not 

 yet in a position to determine. That there is 

 an obvious resemblance, however, between the 

 reactions produced in organisms by a constant 

 current and by continuous exposure to light 

 can not be denied. Numerous animals orient 

 themselves with the utmost definiteness and 

 directness so that they may move toward or 

 away from the source of light. Two cases 

 may be examined, both of which Jennings 

 places in the category of trial and error re- 

 sponses. 



1. Euglena is a chlorophyll-bearing protist, 

 with an asymmetrical body, a long flagellum 

 arising from one end and a spot of pigment 



