70 THE BEHAVIOR OF PROTOZOA 
to form a stream which issues in another direction. Stimu- 
lation of the posterior end causes the animal to quicken its 
pace. Stimulating a pseudopod causes it to be withdrawn; 
and by repeated stimulation the animal may be driven 
about at will. 
To the weaker stimuli which are received by coming into 
gentle contact with solid objects, Amceba often ‘reacts 
positively. If, when floating about in the water, one pseu- 
dopod comes in contact with a solid it adheres to it; there 
is a flow of granules toward the point of contact, from the 
rest of the body; this pseudopod enlarges, the others contract 
until most of the body has flowed into the attached pseudo- 
pod, when the Ameba crawls along the surface of the object. 
The utility of the negative response to strong mechanical 
stimuli is obvious since it enables the organism to avoid 
injurious agencies. The positive reaction tends to keep 
the Amceba in contact with solid objects where most of its 
food is secured and where it receives protection. Probably 
also it plays a part in determining the behavior toward food. 
To injurious chemicals Amceba reacts much as to strong 
mechanical stimuli; it creeps away from regions of higher 
temperature, it reacts negatively to light. All these responses 
are purposive in that they are adapted to the preservation 
of the organism. Simple as Amceba apparently is it manages 
to cope very effectively with the conditions of its existence. 
One might conceivably construct a machine which would 
run of itself, gather the food needed to supply the energy 
used in its workings, avoid automatically contact with 
obstacles which would impair its running, move away from 
regions too hot or too cold for its efficient operation, protect 
itself by producing coverings in unfavorable situations, and 
guide itself into the most favorable regions for its mainte- 
nance; but what a wonderfully complicated mechanism it 
