THE THEORY OF TROPISMS. 95 



theory of tropisms assumes. Certainly a reaction of the organism as a 

 unit, in response to a localized stimulus, is a phenomenon of a higher 

 and more complex order than would be a simple contraction or other 

 direct change in the motor organs at the point stimulated. 



In the higher Metazoa the reaction to a slight mechanical stimulus 

 at one side is usually a turning either toward or away from the source 

 of stimulus. So long as we do not analyze the process further, this 

 result might be interpreted either as due to the direct response, by con- 

 traction, of the muscles primarily affected (thus in accordance with the 

 tropism theory), or as a response of the organism as a whole, depend- 

 ent, perhaps, on an alteration in its physiological condition brought 

 about by the stimulus. The former interpretation is doubtless much 

 the simpler. But we find in the unicellular organisms that this first 

 interpretation is impossible, and that we are forced to the less simple 

 and definite conclusion that the organism reacts as a whole. Does it 

 not then become probable that in the higher animals the very simple, 

 almost mechanical, explanation is likewise incorrect ; that we have in 

 them a phenomenon at least as complex as that found in the unicellular 

 animals? In other words, should we conclude that the reactions in 

 the higher Metazoa are simpler and less unified than in the Protozoa? 



Fortunately, however, we are not forced to base our conclusions on 

 general considerations. These reactions have been minutely studied in 

 very few of the bilateral Metazoa, but Pearl (1903) has given us a 

 thorough analysis of the reactions of a flatworm (Planaria). This 

 cannot be taken up in detail here, but we may quote Pearl's con- 

 clusion in regard to the positive reaction. This consists in a turning 

 toward the point stimulated, on a superficial view a very simple reac- 

 tion, one especially well fitted for explanation on the theory of direct 

 action of the agent on the motor organs of the region stimulated. Pearl 

 concludes, after exhaustive study, that the processes in the reaction are 

 as follows : 



A light stimulus, when the organism is in a certain definite tonic condition, 

 sets off a reaction involving (i) an equal bilateral contraction of the circular 

 musculature, producing the extension of the body ; (2) a contraction of the 

 longitudinal musculature of the side stimulated, producing the turning toward 

 the stimulus (this is the definitive part of the reaction); and (3) contraction of 

 the dorsal longitudinal musculature, producing the raising of the anterior end. 

 In this reaction the sides do not act independently, but there is a delicately 

 balanced and finely coordinated reaction of the organism as a whole, depending 

 for its existence on an entirely normal physiological condition. (/. c., p. 619.) 



Further studies carried on under the direction of the writer, and 

 soon to be published, will show that in certain other bilateral Metazoa 

 it is equally impossible to explain the simple turning toward a stimulus 

 as a direct reaction of the motor organs of the part stimulated. 



