270 LIGHT AND THE BEHAVIOR OF ORGANISMS 



the processes of changing the intensity; for if it had been 

 the colony on path A would have turned from the source 

 of light at c in place of toward it, and then from it after 

 having been exposed to the high intensity for some little 

 time. The fact that the colony moved toward the arc 

 light some little distance after turning at c, and that it was 

 negative in a much lower light intensity a little later, shows 

 clearly that there is some time required to bring about the 

 changes in the organism w^hich determine whether it shall 

 be negative or positive. Reversal in the sense of reaction 

 is not merely dependent upon the intensity but also upon 

 the time of exposure. It is probably a function of the 

 product of intensity and time. It is therefore evident that 

 the change in sense of orientation in these lower forms is due 

 to continued illumination, while orientation is due to change 

 in the intensity of illumination. If all reactions are regulated 

 by chemical changes, there must be at least two different 

 sets of chemicals involved, one which is influenced by 

 changes of intensity, another by constant intensity. I have 

 discussed the possible nature of the chemical changes asso- 

 ciated with reversal in reactions in a former paper (1907, 

 pp. 1 57-161), and shall refer to it in this volume under 

 theoretic considerations. Chapter XX. 



Reversal in the sense of reaction is of the greatest impor- 

 tance to the well-being of organisms, for, as showm in the 

 preceding chapters, it tends to keep them in the optimum 

 illumination. This is true whether the change of intensity 

 causes reversal in orientation or merely a change in the 

 avoiding reactions, shock-movements, for both of these 

 methods tend to produce aggregations at the optimum. A 

 change in light intensity does not however induce reversal 

 in all organisms which respond to light. I was unable to 

 obtain positive reactions in Stentor coeruleus, Amoeba, 

 and fly larvae; and the same is true for many of the pla- 

 narians, including the land planarian Bipalium kewense, 

 and some other worms, especially in certain stages of their 

 development. 



