LIGHT REACTIONS OF TERRESTRIAL AMPHIPODS 351 



reduced as the case may be, and so cause a modification of the 

 response. 



The bearing that the above considerations might have on the 

 evolution of a species can only be set forth as a suggestion. It 

 is difficult, however, to ignore the significance of the power of 

 adjustment that these forms show in their choice of methods 

 of reacting to a stimulus, and in their power to control their 

 reactions over against the mechanical effects of a stimulus. 

 This adjustment is shown in their behavior under a glass plate, 

 in the method of jumping toward the light, when it is held 

 above the creatures, and perhaps most important of all, in the 

 controlling of the circus movements. This last mode of beha- 

 vior exhibits a transition from the stage at which the creature 

 is at the mercy of its environment, to a stage at which it is 

 beginning to hold its own against the forces which have shaped 

 it. The reactions of plants and many lower organisms show 

 little or no power of choice or control over stimuli. As we 

 proceed to the higher forms, we find this power of adjustment 

 and control increasing, until we find the animals that have 

 survived mainly through their mastery of the forces of the 

 environment in which they were thrown. 



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