112 STUDIES IN GENERAL PHYSIOLOGY 



alone; more frequently several causes co-operate, and the 

 movements which are produced in this way, the cause of which 

 is again sought by many in the "will" of the animal, are 

 only the resultant of various causes operating at the same 

 time. In very intense light the full-grown larvae of Musca 

 vomitoria move away from the light in the direction of the 

 light rays: they pass by a piece of meat lying in their way. 

 If the light is sufficiently weak, however, the chemical infhi- 

 ence of the volatile substances arising from the meat ex- 

 ceeds the orienting influences of the light and the larvae crawl 

 to the meat. With other animals which are still more sus- 

 ceptible to chemical stimuli as, for example, the male 

 Lepidoptera, which, as is well known, are attracted to the 

 female from great distances entirely through the effect of 

 chemical stimuli heliotropism may be entirely masked by 

 these chemical stimuli. It is not always an easy matter to 

 say, from the movements which an animal always executes, 

 what are the conditions determining these movements. 



4. Another complicating circumstance is still to be added. 

 Life-phenomena are phenomena of irritability; i. e., they are 

 not dependent solely upon the external causes acting upon 

 the organism at a given moment, but upon these and the 

 conditions present within the organism taken together; and 

 the latter conditions are in themselves variable. The study 

 of animal heliotropism revealed the fact that one and the 

 same animal may react differently toward the light during 

 different periods of its existence. The caterpillars of Por- 

 thesia chrysorrhcea after having fasted through the winter 

 are energetically positively heliotropic. After the animals 

 have eaten, heliotropism may still exist, but intense light, 

 which formerly determined their movements with definiteness, 

 has no more effect upon them than did quite weak light 

 previously. Plant lice become sensitive to light that is to 

 say, positively heliotropic only after they have fed; the 



