CHAPTER ITI 
THE TROPISMS 
“The understanding of complicated phenomena depends upon an 
analysis by which they are resolved into their simple elementary 
compounds.’’—Logs, Physiology of the Brain. 
“On s’est fait beaucoup d’idées fausses sur les tropismes; les 
littérateurs, les philosophes; les savants méme, ont disserté sur eux 
d’une facon tout 4 fait fantaisiste, les uns raillant, les autres louant 
sans réserve, ce qu’ils n’avaient pas compris.”—GroRGES Bon. 
THE TROPISMS IN GENERAL 
Certain stimuli exercise a directive effect upon the move- 
ments of animals causing them to go toward or away from 
the source of stimulation. Such movements are commonly 
called tropisms. The flight of a moth toward the candle, 
the gathering of male moths around a box containing a 
female, the movements of protozoans away from regions of 
unusual heat or cold may be taken as illustrations of such 
directed movements. 
No attempt will be made to formulate an accurate defini- 
tion of the word tropism because usage has not established 
the meaning of the term with sufficient precision to render 
this possible. There is considerable difference of opinion 
in regard to the explanation of tropisms, and the kinds of 
behavior to which the term should be applied. In what 
follows phenomena will be described which have been com- 
monly classed as tropisms however diverse they may be in 
character and causation. 
The study of tropisms, which has attracted a large share 
of attention in recent years, was given a great stimulus by 
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