262 LIGHT AND THE BEHAVIOR OF ORGANISMS 



Most of the organisms in Group c have image-forming 

 eyes. The simplest of these appear to serve merely to distin- 

 guish between difference in size and location of illuminated 

 areas, while the more complex serve to distinguish form, 

 distance, and color as well. In many of these organisms 

 the tissue which is sensitive to light is not confined to the 

 eyes. Many of the fishes and amphibia respond to light 

 alter the eyes, including the retina, have been removed, and 

 there are also a number of blind species which respond to 

 light. 



Nearly all of the reactions classified above are probably 

 responses to changes of intensity. They are no doubt asso- 

 ciated with chemical changes caused by changes of light 

 intensity, and affected by other chemical changes dependent 

 upon the effect of continued illumination, the absolute 

 intensity, and the time of exposure. We shall refer to this 

 matter again in the final chapter. 



7. Evolution of Reactions to Light 



It is not my purpose to discuss the problem of the evolu- 

 tion of the reactions to light. Nothing of importance could 

 at present be added, in such a discussion, to the ideas of 

 Jennings on this question expressed in his treatment of the 

 development of behavior (1906, pp. 314-327). I shall there- 

 fore merely suggest, without argument, the order in which 

 the reactions to light seem to have appeared. 



The most primitive responses to light are probably due 

 to the effect of continued illumination on synthetic and 

 growth processes in green plants. Responses of this nature 

 we may assume to have been the basis for the origin of all 

 others, which probably appeared somewhat in the order 

 following: 



(i) Change in the rate of locomotion dependent upon 

 the absolute amount of light energy received. No orienta- 

 tion, but probably aggregation at the optimum intensity — 

 Bacteria. 



