250 GENERAL BIOLOGY 



organism at once. It is evident, however, that 

 many classes of stimuli for example, rays of light, 

 and currents of electricity may affect only one part 

 of the organism at a time, producing necessarily 

 an unsymmetrical stimulation. If the more strongly 

 stimulated side were concerned in movement, a change 

 in direction would result. If, for instance, the 

 stimulation should produce an increase of muscular 

 or protoplasmic contraction on that side, the effect 

 would be to turn the organism around until it should 

 reach a position in which both sides were stimulated 

 equally ; in other words, to orient the organism. If, 

 now, the progressive movements still kept up, the 

 direction of the progression of the organism would 

 be toward the source of stimulation. If the stimulus 

 were a depressant instead of an excitant, the orienta- 

 tion would be reversed ; that is, the reaction would be 

 negative. 



Recent experimental work has revealed an ex- 

 traordinary range of responses of this sort among all 

 kinds of animals and plants. The most familiar 

 perhaps is the tendency of green plants to face the 

 source of light. In some cases, as in the sunflower, 

 the plant-head follows the sun as a compass-needle 

 the magnet. The behavior of the unicellular 

 " swarm spores " or of the motile gametes of the 

 lower algse is quite similar. They gather in a mass at 

 the side of the dish exposed to the window. In the 

 insect world the fascination of the moth for the flame 

 has become proverbial. At first glance one might 

 think that a wide gap exists between the mechan- 



