THEORETIC CONSIDERATIONS 373 



sity in a given way now, and to the same intensity in another 

 way later, it is because the physiological state of the organ- 

 ism has changed. When external changes persistently 

 follow each other, as, for example, shadow and contact in 

 case of the attack of an enemy on Hydroides, the shadow 

 produces a certain physiological state. This state is 

 resolved into another by contact, and this results in a 

 reaction. Repetition tends to cause the resolution of the 

 first physiological state into the second, without contact, 

 and consequently a reaction to the shadow which was 

 formerly given only to a contact stimulus. Thus we have 

 the origin of a reaction to a sign, response to a representative 

 stimulus, as Jennings terms it. The shadow in the case 

 mentioned above represents the contact; it is a sign of the 

 approach of danger. All of this the author has elaborated 

 in a most masterful way in his book " Behavior of the 

 Lower Organisms " (1906). Every step in the develop- 

 ment of the theory is supported by numerous experimental 

 facts and all seems to fit what is known concerning the 

 reactions of organisms. Reactions, according to this 

 theory, are, as stated above, primarily due to physiologi- 

 cal states. External agents ordinarily produce reactions 

 through the effect they have on these states. By the 

 application of this idea all the different phenomena con- 

 nected with reactions to light as summarized at the begin- 

 ning of this chapter can be accounted for. 



But what are these physiological states and of what do 

 they consist? That there are such states in organisms 

 cannot reasonably be doubted, and that the reactions are 

 dependent upon them much as Jennings assumes, seems to 

 me to have been well established in his work. But what 

 regulates the physiological states is a question concern- 

 ing which we have as yet but little knowledge. Jennings 

 assumes that they are regulated entirely objectively, i.e., 

 by the interaction of external and internal physico- 

 chemical processes. This is of course a legitimate assump- 

 tion, an assumption which indeed has some experimental 



