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 Parameecia 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 
Paramcecia depends upon a definite reflex, it is produced ina 
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 Paramecium. 
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 larve 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 
