EXPERIMENTAL ZOOLOGY 189 



through the extensive study by zoologists of behavior in the 

 lower organisms. This is important because it is believed 

 among us that only by a much more extended study of the 

 behavior of these forms in which the phenomena are reduced 

 to their lowest terms can we hope to arrive at a correct 

 interpretation of animal behavior at our own end of the scale. 



In the protozoa this study has been made by the examina- 

 tion of the comparatively simple reactions of these forms to 

 stimulation by light, gravity, contact, substances in solution and 

 the like. The reactions to the stimuli are termed "tropisms." 

 Thus we have phototropism, geotropism, thigmotropism, 

 chemotropism. For example, an organism is found to be 

 positively phototropic, that is, it moves toward the light and 

 therefore collects on the light side of the dish, and we will 

 here designate as the "tropism theory" one of the current 

 explanations of how this going toward the light is brought 

 about. While there is some dispute over just what is meant 

 by "the tropism theory" in this connection, I think the follow- 

 ing is a fair representation of what it claims. The animal, 

 let us suppose in this instance a ciliated protozoon, is stimulated 

 by light rays coming from' a certain direction. If at first it 

 is in such a position that one side is illuminated and the other 

 not, it is supposed that the direct stimulation, or lack of 

 stimulation, so affects the sides of the body that the beat 

 of the cilia or locomotor organs is directly stimulated and 

 the position of the animal changed by the stronger beat upon 

 the dark side, until the position is such that the two sides 

 are equally illuminated ; and then, since both are stimulated 

 alike, their beat becomies equal and progression continues in 

 the direction of the source of stimulation, until prevented by 

 some new factor, like the side of the glass dish in which 

 the organism is confined or the surface of the water in a 

 pond. 



The tropism theory has been most actively championed 

 by Professor Loeb and his followers in our own country. 



