THE THEORY OF TROPISMS. 



TO WHAT EXTENT DOES THE THEORY OF TROPISMS THROW 

 LIGHT ON THE BEHAVIOR OF LOWER ORGANISMS? 



The writer has been engaged for a number of years in a study, as 

 exact and detailed as possible, of the behavior and reactions of a num- 

 ber of lower organisms. While the results obtained have not, as a 

 rule, agreed with the view that the behavior of these organisms is 

 determined largely in accordance with the prevailing theory of tropisms 

 or taxis, he has not discussed their relation to this theory in detail. 

 This was because of the possibility that the reactions which he had 

 studied were exceptional, and that further investigation might show 

 after all that the behavior of the lower organisms is largely in accord- 

 ance with the tropism schema. 



At the present time the writer feels that the work which he has 

 done, or which has been done by those associated with him, is of suffi- 

 cient extent to justify the pointing out of certain general relations. 

 The reactions of ciliate infusoria, which have long been used as the 

 types of illustration for the tropisms, have been examined in much 

 detail, and less extensive studies have been made on the Bacteria 

 (Jennings & Crosby, 1901), the Flagellata, and the Rotifera. The 

 reactions of a flat worm have been studied in much detail (Pearl, 

 1903), and researches are nearly ready for publication, by investigators 

 associated with the author, on the behavior of Hydra and of the leech, 

 and still other studies are under way. Thorough studies, directed to 

 the observation of the exact movements of organisms under stimuli, 

 have recently been given us by other observers also. It seems, there- 

 fore, worth while to bring out, in a preliminary way at least, the 

 relation of the observations made to the prevailing theories of animal 

 behavior. In the present paper this will be limited to a consideration 

 of the theory of tropisms, since this is the theory most widely held. 



The great apparent value of the theory of tropisms or taxis lies in 

 the fact that it seems to reduce to very simple factors a large number 

 of the most striking activities of organisms, namely, those involved in 

 going toward or away from sources of stimuli of almost any character. 

 It is a schema, in accordance with which almost any movements of the 

 organism (not purely random) might be supposed to take place. 



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