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SCIENCE 



[N. S. Vol. XXVII. No. 



pendent contributions to the tropism theory 

 and on expositions of it; if it was an error 

 to take these into consideration, I am guilty 

 of that error. Here we are seeking to discover 

 whether there is divergence of view as to the 

 facts themselves, and nothing would be more 

 gratifying than to find in Professor Loeb an 

 ally instead of an opponent, in this question 

 of the complexity of behavior. We may, if 

 we desire, call the theory which I criticized the 

 popular tropism theory. 



Now, how did this popular tropism theory 

 simplify behavior? There is among investi- 

 gators an extraordinary diversity of opinion as 

 to what a tropism is. Some use the word as a 

 mere name for certain observed facts. In 

 reading, conversation and correspondence I 

 have met the following definitions, each held 

 by well-known investigators : (1) any reaction 

 of a lower organism is a tropism; (2) any 

 reaction to the chemical or physical agents of 

 the environment is a tropism; (3) any move- 

 ment toward or away from a source of stimula- 

 tion is a tropism ; (4) a tropism is any reaction 

 in which the organism turns as directly as pos- 

 sible toward or away from the source of stimu- 

 lation ; (5) a tropism is any turning produced 

 by stimulation; (6) a tropism is any reaction 

 in which orientation to a steadily acting ex- 

 ternal force is the main characteristic. It 

 seems clear that there is no tropism " theory " 

 in any of these views; they merely apply a 

 name to certain facts, leaving the nature of the 

 reaction to be determined by experiment, and 

 permitting different explanations in different 

 cases. I myself at first used the term (or its 

 equivalent "taxis") in some such collective 

 sense, till a paper from the laboratory of one 

 of the feading exponents of the tropism theory 

 set forth with some warmth that the phe- 

 nomena I described had "nothing to do with 

 the tropisms." 



Among those who use the word tropism in a 

 precisely defined sense, implying a theory as 

 to the nature of the reaction,' there is likewise 



• I am uncertain whether Professor Loeb, in his 

 recent paper (Journ. Exp. Zool., 4), wishes to 

 range himself with those for whom thb word 

 tropism implies nothing as to the nature of the 

 reaction. In his note on page 156, he says that 



diversity of view. The theories held by cer- 

 tain investigators give no ground for consider- 

 ing the tropism a simple, elementary phe- 

 nomenon, nor one of wide application to lower 

 organisms; they involve a highly developed 

 sensory apparatus and a complex activity of 

 the nervous system. Against such theories my 

 criticism was not directed. On the other 

 hand, there is a widely prevalent theory of 

 tropisms which if correct really justifies the 

 common view of the elementary simplicity of 

 these phenomena. This is the " local action 

 theory of tropisms," and it was against this 

 that my criticism was directed. 



I wish to emphasize this point, as it gives 

 the key to the entire discussion. I found the 

 fountain head of the commonly held belief in 

 the simplicity and uniformity of the behavior 

 of lower organisms in the " local action 

 theory " — representing the stimulus as pro- 

 ducing its reaction in that part of the body on 

 which it directly falls, so that the organism re- 

 acts as a bundle of independent parts rather 

 than as a unit. I therefore attacked this 

 theory, and no other, in the chapter of my 

 book which deals with this matter. I believe 

 I made it perfectly clear that this was the 

 theory under criticisin; in the title of the 

 chapter the " local action theory of tropisms "^ 

 is specified; I defined precisely what I meant 

 by it; all through the chapter I took pains to 

 specify it, and in my summing up I expressly 

 my statement of the tropism theory on page 94 

 of my original paper is erroneous. The essential 

 point in my characterization of the theory on that 

 page seems to be the statement that " the theory 

 of tropisms says that certain definite things hap- 

 pen in the change of position undergone by organ- 

 isms under the influence of stimuli; that the 

 organisms perform certain acts in certain ways." 

 If this is the point which Loeb holds to be er- 

 roneous, my criticisms of course do not touch his 

 views in the least. Many authors present th« 

 tropism theory as a theory of how reactions occur, 

 and it was as such that I criticized it. If I have 

 anywhere wrongly classified Professor Loeb with 

 these, I regret it, and am delighted to discover 

 my mistake. Any one who holds a theory (or 

 would be a theory?) of tropisms that says noth- 

 ing as to how the reaction occurs will hardly find 

 anything in my discussion to oppose his views. 



