TROPISMS AND THE BEGINNINGS OF SENSE 225 



that the Paramecium has an idea of nice or nasty. But it is 

 very plain that the protozoa are repulsed by the presence of 

 sand grains and attracted by the presence of various kinds 

 of bacteria. They will swallow the bacteria and pass the sand 

 grains by. There is no doubt, however, that the difference 

 between their reaction toward 

 food and their reaction toward 

 inert matter or toward injurious 

 matter is due to a certain relation 

 between the chemical constitution 

 of the protoplasm and the chemi- 

 cal constitution of the outside 

 substances. We should hardly 

 be any more justified in saying 

 that the ameba likes meat juice 

 than we should be in saying that 

 water dislikes oil. In one case, 

 as in the other, the reactions de- 

 pend upon certain relations be- 

 tween the chemical compositions 

 of the two reacting substances. 

 Water does not choose to dissolve 

 sugar and to leave sand undis- 

 solved ; neither can we be sure 

 that a protozoan chooses its food, 

 notwithstanding the fact that it 

 does take some kinds and reject 

 other kinds of objects or materials. 

 It is only when we come to the higher animals that we may 

 begin to speak of choice in this sense ; and even among the 

 highest animals most of the selecting and rejecting depends 

 entirely upon reflexes and instincts rather than upon thought 

 or feeling ; that is, they depend upon the structure of the 

 organism and upon the composition of certain organs rather 

 than upon a conscious purpose or discriminating taste. 



FIG. 



General reaction 



In many one-celled animals every stimu- 

 lation brings about the same response. 

 In the Paramecium the animal, when it 

 runs into an obstacle, whether physical 

 or chemical (0), immediately reverses 

 its movements, backing off a little way, 

 turning to one side, as shown by the 

 arrows, and starting off along a new path 



