38 BEHAVIOR OF THE LOWER ORGANISMS 



direction shall result in such a change as will produce the reversal of 

 movement. Not every change in the environment produces a reaction. 

 A change leading toward a certain optimum condition produces no reac- 

 tion, while a change of opposite character causes the reversal of move- 

 ment. A negative change in the environment — the decrease or cessa- 

 tion of action of a certain agent — may be as effective a stimulus as is 

 a positive change due to the entrance of a new agent into action. This 

 is well illustrated in the reactions to light and oxygen. All these rela- 

 tions we shall meet again, more fully illustrated, in the behavior of 

 infusoria. 



The strength of the change necessary to cause a reaction has been 

 found by Pfeffer to vary in accordance with Weber's law. This as usually 

 formulated expresses certain relations between sensation and stimulus 

 in man. According to this law, it is the relative change in the environ- 

 ment, not the absolute change, that causes a perceptible difference in 

 sensation. Thus if a certain perceptible weight x is pressing on the skin 

 of certain parts of the body, it requires an additional weight of about \ x 

 to produce a noticeable difference in the sensation; if the original weight 

 is 2 X, then an additional weight of f x is required. In general the addi- 

 tional weight must be about one-third the original one before a notice- 

 able difference in sensation is produced. In the bacteria we know noth- 

 ing about sensations, but if we substitute reaction for sensation, similar 

 relations are found to hold good. Pfeffer found that if Bacterium termo 

 is cultivated in o.oi per cent meat extract, they collect noticeably in capil- 

 lary tubes containing 0.05 per cent meat extract, but not in a weaker 

 solution. For producing reaction the inner fluid must therefore be iive 

 times as strong as the outer. If now the culture fluid is raised to a 

 strength of o.i per cent meat extract, then five times this strength — 

 namely, 0.5 per cent — is required to induce the bacteria to collect. If 

 the culture fluid is i per cent, the fluid in the capillary tube must be 

 5 per cent in order to produce the usual reaction. The fluid in which the 

 bacteria collect must be always five times as strong as that in which 

 they live. It is the relative change, not the absolute change, that in- 

 duces reaction. This agreement between the relation of sensation to 

 stimulus in man and that of reaction to stimulus in these low organisms 

 is of great interest. 



There is a considerable amount of variation in the reactions among 

 different individuals of the same species. Thus, Rothert found that 

 specimens of Amylobacter from a certain culture were markedly negative 

 to oxygen and positive to ether, while in specimens from another culture 

 these reactions were hardly observable. Even among individuals of the 

 same culture there is variation. Engelmann found that when the light 



