370 LIGHT AND THE BEHAVIOR OF ORGANISMS 



There are however photochemical reactions which are 

 suggestively similar to some of the photic reactions in 

 organisms. We have shown in Part IV that in some com- 

 pounds the reactions proceed in one direction in one light 

 condition and in the opposite in another; that the action 

 of the different rays of light is specific, and that in some 

 of these chemical reactions heat and light tend to produce 

 opposite effects. The first two of these reactions are 

 somewhat similar to the change in the sense of reactions 

 produced by changes in light intensity in many organisms 

 and to the specific reactions to different rays. The last 

 is similar to the effect of heat and light on the sense of 

 reaction in Chlamydomonas and various other organisms 

 referred to under (7) above. If then these organisms 

 contain chemical compounds which are affected by light 

 like those referred to above, we can account for the reac- 

 tions mentioned by assuming that they are due to the 

 effect of the light on the chemical changes in the organism. 

 In case of Volvox I was also able to account for a number 

 of peculiarities in the process of reversal in the sense of 

 response by the assumption of reversible photochemical 

 reactions within the organism (Mast, 1907, pp. 157-161). 

 And we might account for the fact that an increase in 

 illumination produces the same effect as a sudden decrease, 

 as in the case of the zooids of Volvox (see (6) above), 

 by assuming two photochemical reactions, one dependent 

 upon the time rate of change of intensity, the other upon 

 the absolute intensity and the time of exposure. In this 

 same way the inhibition of one sort of movement and the 

 augmentation of another in the same organism (Hydra) 

 might be accounted for. So we might continue and account 

 for modifiability, variability, adaptation, etc., by various 

 other assumptions. But all of these assumptions regard- 

 ing chemical changes are so extremely hypothetical that 

 speculation based on them has at present but little 

 value. And it is important to realize that the common 

 belief that light in some way influences the activity 



