REGULATION OF REACTIONS 299 



the contraction of the sea anemone Edwardsia when light 

 is flashed on it. It is the change of intensity that causes the 

 response, but there is no evidence of a reaction to a sign 

 here. These creatures are directly interested in the effect 

 of the light which produces the response. Continued illu- 

 mination probably affects the activity of all organisms that 

 respond to light, and change in the sense of reaction 

 when due to the action of light is in all probability due to 

 the effect of continued intensity rather than to change of 

 intensity. 



(4) The actions of organisms may change without any 

 change whatever in external conditions. They may, for 

 example, be positive to light of a given intensity under 

 given conditions at one time and negative to the same 

 intensity under precisely the same external conditions at 

 another. This change in reaction must be due to internal 

 factors. It may take place in course of a few moments, as 

 in Volvox, or it may require weeks, as in the case of the de- 

 velopment of the fly, the larva of which is negative while 

 the imago is positive. Then, again, it may be periodic, 

 as in the sleep movements of many of the plants. 



(5) High light intensity ordinarily causes organisms in 

 the positive state to become negative. There are, how- 

 ever, organisms which do not become negative no matter 

 how high the intensity is, e.g., many plant structures and 

 numerous aquatic larvae. Under natural conditions they 

 do not experience illumination so strong that it is injuri- 

 ous; there is, therefore, no need for a negative response. 

 The positive reactions of insects and other forms which 

 frequently prove fatal under artificial conditions are adap- 

 tive under natural conditions. 



Reversal in the sense of reaction is not a response to a 

 change of intensity. It does not take place until some 

 time after the change is made. There is a time element 

 involved here. It is due to changes occurring within the 

 organisms, caused by continued light conditions, not by 

 changes in such conditions. 



