THE BEHAVIOR OF BACTERIA 39 



falling on a group of individuals of Chromatium was suddenly decreased, 

 a few react to even very slight changes, a larger number to more consid- 

 erable changes, while some hardly react at all. "Nervous" and "apa- 

 thetic" individuals, Engelmann says, can be distinguished in any group. 

 Even in the same individual the reaction may vary. Engelmann found 

 that if the light was suddenly decreased, then restored, and at once de- 

 creased again, the bacteria usually do not react to the second decrease, 

 though they did to the first. 



Among different kinds of bacteria there are, as we have seen, certain 

 constant differences in the reactions. A relation of great significance 

 becomes evident on examining the facts; behavior under stimulation 

 depends on the nature 0} the normal life processes, — especially the meta- 

 bolic processes. Bacteria that require oxygen in their metabolism col- 

 lect in water containing oxygen ; bacteria to which oxygen is useless or 

 harmful avoid oxygen. Bacteria that use hydrogen sulphide in their 

 metabolism gather in that substance. Bacteria that require light for 

 the proper performance of their metabolic processes gather in light, 

 while others do not. When one color is more favorable than others to 

 the metabolic processes the bacteria gather in that color, even though 

 they may under natural conditions have no experience with separated 

 spectral colors. Keeping in mind that all these collections are formed 

 through the fact that the organisms reverse their movement at passing 

 out of the favorable conditions, these relations can be summed up as 

 follows : Behavior that results in interference with the normal metabolic 

 processes is changed, the movement being reversed, while behavior that 

 does not result in interference or that favors the metabolic processes is 

 continued. 



This statement doubtless does not express the behavior completely, 

 yet the general fact which it sets forth is on the whole clearly evident. 

 The result of this method of action is to make the behavior regulatory, 

 or adaptive. Through it, the bacteria, like higher organisms, avoid 

 injurious conditions and collect in beneficial ones. There are some 

 exceptions to this ; the adaptiveness is not perfect, as nothing is perfect 

 under all conditions. The exceptions are perhaps not more numerous 

 in these lowest organisms than in the highest ones. 



Putting all together, the behavior of the bacteria may be summed up 

 as follows : They swim about in a direction determined by the posi- 

 tion of the body axis, until the movement subjects them to an unfavora- 

 ble change ; thereupon they reverse and swim in some other direction. 

 With rapid movements and much sensitiveness to unfavorable influ- 

 ences, this soon results in their finding and remaining in the favorable 

 regions. In the presence of a localized region of favorable conditions 



