252 GENERAL BIOLOGY 



mally the cilia of Paramecium beat in a continuous 

 and specific manner, and the course of the organism 

 through the water is determined by the shape of the 

 cell-body itself. It is as incapable of modifying this 

 movement as a man in a rowboat would be of alter- 

 ing that of the boat's movement. In the latter case 

 the oarsman can only propel his craft forward, 

 backward, or in a curve, depending upon his pulling 

 the oars, pushing them forward, pulling on one oar 

 more than on the other, or finally pulling on one oar 

 and pushing on the other. Indeed these movements 

 reduce to two, pushing and pulling. Now if we 

 postulate that areas of the ciliated surface of 

 Paramecium are capable of local and independent 

 stimulation, then the stronger or weaker beat of the 

 cilia due to such stimulation in certain regions would 

 orient the organism, its own movements determining 

 the direction of its progression. 



The mechanical nature of such tropic responses is 

 evident in cases in which the " tropism " may be 

 arbitrarily reversed by the experimenter. Thus 

 many small animals, like the swarm-spores men- 

 tioned, gather on the side of the dish toward the light. 

 That this is not the expression of a choice or prefer- 

 ence of the organism for one sort of illumination in 

 contrast to another is demonstrated by the fact that 

 changing the temperature, increasing the salinity 

 (of sea- water) , or even agitating the water, may bring 

 about a reversal of the response. In such a way we 

 alter the physiological state of the organism and cause 

 it to react in a different manner. 



