264 THE AMERICAN NATURALIST. [Vor. XXXIV. 
force. If a large number of such organisms are in the field of 
the force, they will soon all show a common orientation, and 
when they move they will all move in the same direction. Ad 
that is necessary for orientation to take place in this way ts that 
the organism shall be differently affected when tt is in one post- 
tion from the way it ts affected in another position. It is diffi- 
cult to imagine any agency that can cause orientation in which 
this condition is not fulfilled. 
When the organism is once oriented with reference to a con- 
stant stimulus, of course no further reaction is necessary; it 
simply keeps up the normal forward motion until this motion 
brings it into new relations with the stimulus, when the motor 
reaction is again caused, resulting in a readjustment of orien- 
tation. In the hypothetical case above supposed, if the organ- 
ism swims in a curve instead of a straight line, it will soon 
come into a position such that the lines of incoming force 
impinge on the sensitive anterior end ; the motor reaction is 
thereby induced until the animal has come again into the 
position where the anterior end is not thus affected. 
Garrey says: “ All these motor reactions have nothing to do 
with ¢vopisms, for these motor reactions are only the expression 
of a very sudden change in the stimuli, while the characteristic 
of the tropism is the stationary condition of the stimuli.” He 
fails to consider the fact that, stationary as the stimuli may be, 
the organisms with which he is dealing are zo stationary ; by 
their own. movements the organisms come into new relations 
with the stimuli, and these new relations may result in the 
production of the motor reaction, causing a readjustment of 
orientation. 
To summarize, the chief points which I would make in criti- 
cism of Garrey's conclusions are as follows : 
1. In all work upon the reactions of organisms to physical 
and chemical agents, it is necessary to consider the structure 
and normal activities of the organisms, as well as the nature of 
the agents acting as stimuli. 
2. Orientation such that * symmetrical points on the surface 
of the body are cut by diffusion lines at the same angle" (or 
by any other lines at the same angle) is impossible in unsym- 
