CANKEEWORMS SNODGEASS 331 



Another experiment with our moth will show that she possesses an 

 impulse to go upward. Place an active individual on the side of a 

 vertical block of wood and she immediately starts to ascend it. 

 Turn the block end for end, she reverses and starts upward once, 

 more, and again the experimenter may amuse or instruct himself 

 indefinitely or until he wearies of reversing the block. Any creature 

 possessed of this innate impulse to go upward is said by the savants 

 of tropisms to be endowed with negative geotropism^ signifying that 

 something impels it to go away from the earth. 



Besides phototropism and geotropism there are many other tro- 

 pisms ; that is, if tropism is anything more than a theory, or a name 

 for something we do not understand. Keaction to heat is thermo- 

 tropism\ reaction to water is hydrotropism'^ reaction to chemical 

 substances is chemotropism, the last ordinarily called smell or taste 

 in animals with conscious perceptions. But all tropisms in animals 

 are supposed to act in a purely mechanical way by affecting the 

 muscles of locomotion or those by wdiich the movements of the crea- 

 ture are directed. Thus, in the case of phototropism, if the light is 

 coming at an angle, it strikes on one eye stronger than on the other, 

 and if the animal is endowed at the time with positive phototropism, 

 the reflex nerve current generated by the light on the optic nerve 

 stimulates the muscles most strongly on the side of the body with 

 the eye that receives the most light. These muscles contract, the 

 body bends toward the light until both eyes receive the same stimulus. 

 The creature must inevitably go toward the source of the light. If 

 it is a flying moth it beats itself against the globe or burns itself in 

 the flame, unless it possesses a strong negative thermotropism ; then 

 the heat repels it and it flies in circles about the light. 



The mechanical idea of tropism was first invented to explain the 

 movements of certain plants, and was later extended to animals to 

 account for the definite responses to specific stimuli by such animals 

 as could not be supposed to receive conscious sense impressions or to 

 have the power of voluntary action. Now there is a tendency to 

 explain the acts of all animals as tropisms. A study of the behavior 

 of animals under different conditions, however, necessitates a modi- 

 fication of the extreme mechanistic conception of tropisms as applied 

 to animals. Night-flying moths, for example, are attracted by arti- 

 ficial lights but are nocturnal in their habits; they are positively 

 phototropic only up to a certain degree of intensity of light. We 

 have noted, also, that the nocturnal cankerworm moths become 

 active in a cage as evening approaches, regardless of artificial illumi- 

 nation about them. They appear, therefore, to have regularly re- 

 curring periods of activity which are not influenced by local ex- 

 ternal conditions. Periodic activity of this sort is known as physio- 

 logical rhythm. Many insects and other animals are endowed with it. 



