HISTORICAL REVIEW 45 



merous papers, were brought together and systematized in 

 the well known book on the " Behavior of Lower Organ- 

 isms " (1906). I shall refer to this book almost exclusively 

 in trying to present his views on the factors involved in the 

 phenomena in which we are especially interested — aggre- 

 gation in regions of given light intensity, orientation and 

 change in sense of reaction. 



Aggregation in a region having a given light intensity 

 may be formed, according to Jennings, in either of two ways. 

 (i) The organisms get into the region just as they would 

 into any other region, merely by swimming about in an 

 aimless manner, without orientation and without direct 

 movement toward the region. When they get to the limit of 

 the region and are about to pass out into light of a different 

 intensity the sudden change to which they are subjected 

 produces a stimulation which causes a definite reaction. 

 This reaction consists chiefly In a sudden turn toward 

 a given side, frequently after backing some distance, and 

 procedure on a new course. They respond with this 

 reaction every time they come to the edge of the region and 

 therefore remain In this region. Other individuals behave 

 in the same way and this results In an aggregation. " Motor 

 reflex " was the first term applied to this method of reaction 

 with Its various modifications; later it was designated 

 " motor reaction," and finally " avoiding reaction." The 

 essential feature In the avoiding reaction is the fact that 

 the organism always turns toward the same side regardless 

 of the place of application of the stimulus. The side toward 

 which it turns is determined by Internal factors. Thus It 

 is that the direction of turning bears no definite relation to 

 the position of the source of stimulation. The organism 

 may turn directly toward it or away from It or at any 

 angle to It. The method of aggregation thus described by 

 Jennings for Paramecium is in all essentials like that de- 

 scribed by Engelmann in 1882 and 1883 for Paramecium 

 bursaria, Euglena, Bacterium photometrlcum and other 

 organisms. 



