ADAPTATION AND AGGREGATION 239 



are at all. It may be, for all that is known to the contrary, 

 that Loeb is correct in his assumption that the reactions of 

 organisms have their origin in fortuitous chemical combina- 

 tions (see Chapter XX), and that those organisms which 

 live in high light intensity do so because they are positive in 

 their reactions to light, while those which live in low inten- 

 sity do so because they are negative. It must however be 

 conceded that we have as yet but very few, if any,- acts 

 which bear directly on this question. 



2. Different Reactions Observed in the Process of Collecting 

 in Regions having a given Condition of Illumination 



Let us now proceed to ascertain precisely how the dif- 

 ferent organisms respond so as to get into and remain in 

 favorable light conditions. This has been thoroughly inves- 

 tigated for a number of different forms. It is by no means 

 the same in all. We shall consequently consider the dif- 

 ferent methods under several different headings: — a. Ran- 

 dom movements and avoiding reactions; h. Orientation, 

 change in sense of orientation, and avoiding reactions; c. 

 Orientation and extent of movement limited by environ- 

 mental conditions; d. Orientation and movement directly 

 toward the place where the organism comes to rest; e. Ran- 

 dom movements and coming to rest in a given place It 

 should be emphasized here that our aim in this section is 

 not merely to discuss methods of aggregation but also to 

 set forth all different methods of reactions with the view of 

 classifying them. 



a. Random movements and avoiding reactions. — Quite 

 a number of the unicellular organisms get into the region of 

 optimum intensity by random movements. They swim 

 about aimlessly hither and thither, testing conditions in 

 many different places. Thus they sooner or later get into 

 an optimum intensity, where they remain, not by coming 

 to rest, but because whenever they reach the boundary of 

 the optimum region they are stimulated and consequently 



