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 larvee 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 influ- 
ence of the volatile substances arising from the meat ex- 
ceeds the orienting influences of the light and the larve 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; 7. 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 
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