100 THE BEHAVIOR OF LOWER ORGANISMS. 



suggest that the explanation on the tropism schema is incorrect. These 

 details become intelligible as soon as we understand the real method 

 of reaction as set forth in the second of these contributions. The 

 assumption that the reaction is a typical tropism, when only the fact of 

 orientation is known, is as likely to fall to the ground in other cases 

 as in those just mentioned. 



The reaction of bacteria to light, as shown by Bacterium photo- 

 metricum, described by Engelmann (1882), is a typical example of a 

 reaction through a motor reflex not fitting the tropism schema at all. 



To sum up, it is clearly shown in certain cases that the reaction to 

 light takes place in a way that is not consistent with the theory of 

 tropisms, and this is true in some cases where a pronounced orienta- 

 tion exists. In many cases of orientation, where it is supposed that 

 the theory of tropisms holds, this is an assumption, for the observations 

 which would decide the matter are lacking. 



REACTIONS TO GRAVITY. 



In no case have the exact movements of unicellular organisms in 

 response to gravity been worked out in the manner in which this has been 

 done for the reactions to mechanical stimuli, chemicals, heat, light, and 

 electricity. We are, therefore, without the requisite data for deciding 

 whether these reactions agree with the theory of tropisms or do not.* 



In the higher organisms in which the positive and negative reactions 

 to gravity have been observed (starfish, holothurians, flies, insect larva?, 

 etc.), the conditions are so complex that, so far as I am able to see, 

 observations which are crucial for deciding as to the mechanism of the 

 reactions have not been made and perhaps can not be made. 



REACTIONS TO ELECTRICITY. 



As we have seen in the third section of this paper, the reactions of 

 the rotifer to the continuous electric current do not take place in 

 accordance with the theory of tropisms. Anuraea shows a striking 

 orientation to the electric current, swimming directly to the cathode. 

 Yet this orientation is brought about in a way that is quite inconsistent 

 with the tropism schema. The reaction takes place through a " motor 

 reflex," the direction of turning is determined by an internal factor, 

 and not by the way in which the current strikes the organism. The 

 reaction can only be interpreted, therefore, as a reaction of the organism 

 as a whole. 



*In a forthcoming paper by the author, based on work done since the above 

 was written, it will be shown that the reactions of Paramecium to gravity take 

 place in the same way as the reactions to most other stimuli, so that they do not 

 agree with the theory of tropisms. 



