Editorial. 1 4 1 



the infusoria, some change in the relation of the organism to 

 the environment. It is usually, or perhaps universally, a change 

 in the intensity of action of a stimulating agent on the body as 

 a whole or on its most sensitive portion. The organism, hav- 

 ing been in one condition, passes to another, and it is the trans- 

 ition that acts as the effective stimulation. In one of the two 

 classes of reactions to be distinguished, the effective stimulation 

 is due to the fact that as the organism progresses from one re- 

 gion to another it meets different conditions, and to the changes 

 thus caused it reacts. This is the case under natural conditions 

 with the reactions to mechanical stimuli, to heat and cold, to 

 chemicals, to osmotic pressure, and to light when passing to a 

 region of greater or less intensity of illumination. The cause 

 of reaction is analogous to that in our own case in passing from 

 a region of moderate temperature to a hot or cold region. The 

 differences determining reaction are here arranged along the 

 axis of the course, so that it is the backward or forward move- 

 ment that brings them into action. The reaction is a change 

 of movement such as to carry the organism successively in many 

 directions — a series of trial movements. As soon as one of 

 these movements carries the organism away from the stimula- 

 ting agent — that is, in such a direction that the changes to which 

 it subjects the organism lead toward the optimum instead of 

 away from it — the reaction ceases, since the cause for it- has 

 ceased. The organism therefore continues in that direction. 

 The position of the body has little or no effect on the produc- 

 tion of the change that causes stimulation, or on the release from 

 stimulation. The organism might retreat from the stimulating 

 agent backward or forward or sideways ; in the one case as in 

 the other it would be relieved of the stimulating changes. The 

 different individuals may swim away directly or obliquely, their 



alleled under any other conditions. As a result of this interference the reaction 

 of the ciliates to the electric current takes in many respects a different character 

 from the rest of their behavior. I wish therefore to emphasize the fact that the 

 general relations set forth in the text do not apply to the reactions of the ciliates 

 to this agent. Save for the forced cathodic reversal of cilia the response to the 

 electric current would fall in our second class of reactions, as it actually does in 

 the flagellates and rotifers. 



