ADAPTATION AND AGGREGATION 241 



reaction of serpullds to sudden changes of light intensity. 

 Loeb designated the power in serpuUds to respond thus 

 " Unterschiedsempfindhchkeit." 



Besides those already mentioned various other organisms 

 have been found to aggregate by this method, notably Sten- 

 tor coeruleus, Trachelomonas, Chlamydomonas, Chlorogo- 

 nium, Phacus and some swarm-spores, but so far as known 

 Bacterium photometricum is the only form which is entirely 

 dependent upon random movements to get into the region 

 of optimum illumination. All the others are only partly 

 dependent upon random movements in this. Under certain 

 conditions they orient and proceed directly toward the 

 region of most favorable illumination. Many forms how- 

 ever which make use of orientation at times in reaching the 

 optimum light are entirely dependent upon random move- 

 ments and avoiding reactions in case of other sources of 

 stimulation, notably chemicals. 



Reactions of the sort described above have usually been 

 referred to as photopathic. They are supposed by some 

 to be fundamentally different from orienting reactions, 

 which are often called tropic. The former are supposed 

 to be due to the action of light owing to difference or change 

 of intensity, the latter to the action of light owing to con- 

 stant intensity or direction of rays. This distinction how- 

 ever will not hold, for we have clearly demonstrated that 

 orientation in many organisms is due to changes of light 

 intensity on the sensitive tissue. 



b. Orientation, change in sense of orientation, and avoid- 

 ing reactions. — Many organisms, as stated above, have 

 the power of orienting and moving directly toward the 

 region of favorable illumination. In relatively low light 

 intensities they are positive; in relatively high they are 

 negative; in favorable intensity they do not react to light; 

 consequently they tend to remain, but they do not come 

 to rest.^ The rate of movement is apparently not decreased. 



^ Loeb has recently impugned these statements. We shall refer to this 

 matter in detail later (see footnote, p. 266). 



