THE TROPISMS IN GENERAL 21 



metrical points receive equal amounts of stimulation. 

 The orientation of animals to heat, chemical substances, 

 gravity, the electric current and currents of air and water 

 is naturally explained in much the same way. Animal 

 instincts may be analyzed into simple tropisms or may 

 be conceived to be developed from them as a basis. 



The tropism theory of Loeb and his followers has met with 

 a certain amount of opposition, especially by Jennings, 

 who showed that in many of the so-called tropisms of the 

 lower organisms there is no definite orientation produced. 

 In the gathering of Paramoecia in weak acid, for instance, 

 the organisms are not forced into line with the diffusion 

 currents and compelled to swim toward the chemical, but 

 individuals which swim into the acid by chance remain 

 there; whenever they attempt to pass from the dilute acid 

 to water they reverse their course, and thus are kept con- 

 fined to one region. While the chemotactic grouping of 

 Paramoecia depends upon a definite reflex, it is produced hi a 

 manner quite different from Loeb's scheme of orientation. 

 Many of the so-called tropisms of the infusoria and other 

 asymmetrical forms were found to take place in accordance 

 with the method followed by Paramoecium. 



In some cases where orientation is effected it may take 

 place more or less indirectly by the selection of random 

 movements. In the earthworm and in the larvae of blow 

 flies which are negatively phototactic it has been shown by 

 the writer that movements which bring the animal toward 

 the light are checked or reversed and only those which hap- 

 pen to direct the animal away from the light are followed up. 

 Whatever immediate orienting tendency the light may have 

 in these cases is relatively unimportant as compared with 

 the element of selection of favorable chance movements in 

 directing the animal away from the light. The tropism 



