CoNGDON, Reactions to Light. 325 



we would soon get a decrease of X and an increase of T. The equation would read 

 X — = 7" + . T being increased from the amount giving neutrality the animal 

 is made negative. A reversal of this process would occur in sufficiently weak 

 light. Acclimatization of the animal consists in changing the proportions of X and 

 T which give the neutral reaction. 



The correlation by Loeb and Mast of reversal of light response and reversal of 

 chemical reaction is suggestive and tempting. Unfortunately it is difficult to test its 

 validity by experiment. Also, while it is beyond question that light may cause 

 chemical change it is doubtful whether it can produce a reversal of reaction. 



A considerable number of agencies have been referred to by which reversal may 

 be brought about. Among them may be named heat, mechanical and chemical 

 stimuli, the various tropisms, condition of development of the individual, tempor- 

 ary physiological states, such as hunger and sexual activity, previous stimuli, and 

 certain stereotyped procedures. Inasmuch as two fairly definite types of light 

 response have by this time become distinguished, there is encouragement to study 

 the effect upon them of the various agencies which have just been named. LoEB 

 and some others have already given a certain amount of attention in this direction. 

 It is one of the tasks which lie next at hand in the comparative psychology of lower 

 animals. 



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