No. 389.] STIMULI IN UNICELLULAR ORGANISMS. 389 
they move by means of definite organs of locomotion, — flagella, 
or cilia. To make this abstract scheme fit the concrete motions 
of the organs of movement would be much more difficult than 
to invent the scheme. 
If chemotaxis acts in the direct manner supposed in such a 
theory as the above, the organism will of course move directly 
toward a source of attractive stimulus, directly away from a 
source of repellent stimulus, and that moreover without regard 
to the relation of the direction of its axes to the direction of 
motion. As I have shown in detail for Paramecium (0c. cit.), and 
briefly above for Spirostomum and Stentor, this is by no means 
true for the organisms studied. On the contrary, the direction 
of motion has no relation to the position of the source of stim- 
ulus, so that we cannot correctly speak of attraction or repul- 
sion at all. The organism reacts as an individual, not as a 
substance, and the nature of the reaction is conditioned by the 
internal mechanism and the structural differentiations of the 
body of the organism. The essential distinction insisted upon 
by Le Dantec (doc. cit.) between the reactions of a unicellular 
organism and those of a metazoan must therefore, for these 
organisms at least, fall to the ground. It will not do to think 
of the reactions of these organisms as in any way akin to those 
of chemical substances. 
On the other hand, the reactions are equally distant from 
the complexity assumed by those who attribute to unicellular 
organisms most of the psychological powers of higher animals. 
The reactions of these organisms may best be compared with 
the working of a machine in which the wheels are geared to 
turn in but one direction, whatever be the nature of the force 
that sets them in motion. 
DARTMOUTH COLLEGE, HANOVER, N.H., 
February 28, 1899. 
