ILLUSTRATED AMONG PROTOZOA. 109 



due weight in estimating the effects of environment 

 upon our hypothetical organism. 



The forces of the environment have produced a 

 deeper effect than we could detect from a physical 

 point of view by a purely physical analysis. We 

 know as a general fact that each reaction of a ner- 

 vously constituted body produces an enduring effect 

 within the body. Each separate action of force, or 

 each stimulus, causes its peculiar mechanical reac- 

 tion. Each sequence of stimulus and reaction will 

 have its effect upon the nervous molecular condi- 

 tion of the organism ; and the more often the 

 sequence is repeated, the more marked will be the 

 effect. With frequent repetition, the response comes 

 more easily and more quickly. Should any of the 

 stimuli form a regular series frequently recurring, 

 then the series of reactions, often repeated, become 

 associated together by co-ordination of the nervous 

 elements. Because of this association, all of the re- 

 actions in the recurrent series tend to occur together. 

 Each reaction of itself tends to produce the succeed- 

 ing reaction. Thus several reactions in a series 

 might ensue upon the action of a single stimulus. 

 For example, let us suppose a nutritious particle to 

 be borne by the current toward our first organism. 

 The molecules which separate from the nutritious 

 particle as gas will be the first to strike the organ- 



