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SCIENCE 



[N. S. Vol. XXVII. No. 696 



perimented with the animals literally day and 

 night for long periods, remarked to me, in dis- 

 cussing this matter, that he never saw any 

 tropisms. "Without going so far as this, my 

 answer to the question asked above was that 

 "the theory of tropisms does not go far in 

 helping us to understand the behavior of the 

 lower organisms." I did not deny the exist- 

 lenee of the phenomena which the theory takes 

 into consideration, but it seemed to me that 

 there are so many other factors, playing such 

 important parts, that the tropism factor is of 

 relatively small importance in a general con- 

 sideration of behavior. In my first paper on 

 this subject' I included the more complex 

 forms of the tropisms in this judgment. The 

 remark of the investigator above mentioned 

 illustrates the fact that there are certainly 

 ■other aspects of behavior so striking and im- 

 portant as to quite mask the existence of 

 tropisms. 



Let us attempt a brief characterization of 

 tropisms, their history, and the part they play 

 in behavior. 



1. The essential point in the tropism, as 

 originally applied by Loeb to reactions to light 

 was, in a word, the idea that the organism in 

 going toward or away from the light is not 

 trying to go somewhere or to reach something, 

 hut is merely taking a certain position or 

 orientation in the light. This recognition 

 that the position is the essential point was a 

 great step in advance, and its application by 

 Loeb to certain features of the behavior of 

 animals was an achievement of the highest 

 importance. 



2. This idea of orientation having proved 

 so helpful in the study of reactions to light, 

 the next step was, very properly, to apply 

 it to other features of behavior, to see if it 

 would not prove equally useful elsewhere. 

 The reactions to chemicals, heat and cold, 

 contact with solids, electricity, light, gravity, 

 etc., were all brought under this point of view ; 

 attempts to show that the position is the 

 essential point in each of these have for a 



» " Contributions to the Study of the Behavior 

 of the Lower Organisms," Carnegie Institution, 

 Publication 16, pp. 89-107. 



long time been made with energy and per- 

 sistence. Our knowledge has grown till we 

 are in a position to estimate the results. In 

 the main it appears that to most of the be- 

 havior the orientation idea has little appli- 

 cability. To that immense province of 

 behavior comprised in the reactions to chem- 

 icals of all sorts (including food reactions, 

 respiratory reactions, etc.) it has shown it- 

 self quite inapplicable. The case is the same 

 with the reactions to heat and cold. With 

 regard to the reactions to solid bodies nearly 

 the same may be said, though there are some 

 special cases in which the idea of orientation 

 is applicable. The reaction to the electric 

 current furnishes a typical orientation. In 

 some of the reactions to gravity and to water 

 currents the orientation idea is helpful. Tet 

 the recent work of Lyon and others shows 

 that even in these the movement in a cer- 

 tain direction is an essential part of the reac- 

 tions ; they are essentially compensatory move- 

 ments, and the taking of a certain orienta- 

 tion is by no means the only important point. 

 In certain reactions to light the orientation 

 idea has been most helpful, yet in an im- 

 mense proportion of the reactions of organ- 

 isms to this agent it does not show itself 

 the essential point. The orientation theory 

 is of greatest service in such cases as the 

 going of insects toward a bright light, yet 

 even here such work as that of Holmes on 

 Eanatra shows that the orientation is not the 

 only point; the approach to the light seems 

 after all essential, since if a certain orienta- 

 tion does not bring the animal to the light, 

 it learns by experience to take a different 

 orientation which does have this effect. The 

 work of Cole, of Eadl and others, shows that 

 in the lower organisms we have the begin- 

 nings of reactions to ohjects perceived 

 visually; the animal is not merely oriented 

 by the strongest light, but goes toward such 

 objects, whether bright or dark, as might be 

 said to be " of interest "" to it at the given 



" The expression " of interest " of course has 

 some objective equivalent, but to try to use it 

 would be to substitute an unintelligible conjectura 

 for an expression which at least conveys an idea 



