476 H. S. 'Jennings. 



why pain occurs under certain circumstances, pleasure under 

 others. Surely this is not a haphazard matter! There must be 

 some difference in the conditions to induce these differences in 

 conscious states (if they exist) and at the same time to determine 

 the differences in behavior. We are therefore thrown back upon 

 the objective processes occurring. Why are certain conditions 

 accepted, others rejected.^ This is essentially what has often 

 been called the pleasure-pain problem. 



Such facts as are set forth in the preceding paper give us a basis 

 for an objective answer to this question. Organisms are not static 

 structures; processes of complicated character are in continual 

 progress within them. Among these the processes of metabolism 

 are most prominent. Ostwald ('02) has emphasized the point that 

 one of the chief characteristics of living matter is the fact that 

 processes are occurring with much energy within it. The organ- 

 ism is a complex of processes. In the preceding paper we have 

 seen that the reactions of organisms to external agents depend 

 largely on the relation of the action of these agents to the internal 

 processes. 



Let us examine certain cases of this dependence in the simplest 

 organisms — bacteria and protozoa. The green Paramoecium bur- 

 saria requires oxygen in its metabolic processes. While sw*im- 

 ming about it comes to a region where oxygen is lacking. It 

 reacts by turning away and going in some other direction. The 

 white Paramoecium caudatum does the same, and so also do many 

 bacteria. All require oxygen in their metabolic processes; lack 

 of oxygen interferes with these processes, and they react to such a 

 lack by changing their movement and going elsewhere. But 

 there are some bacteria that do not require oxygen in their meta- 

 bolic processes. When these come to a region lacking oxygen 

 they do not react, but keep on and enter this region. In many 

 of these anaerobic bacteria oxygen is known actually to interfere 

 with the physiological processes. When these bacteria come to a 

 region containing oxygen, they change their movement and go 

 elsewhere. In Paramoecia and the bacteria that require oxygen, 

 this does not occur (unless the amount of oxygen rises above the 

 optimum). In all these cases, whenever there is interference with 

 the metabolic processes, the organism reacts by turning away, 

 otherwise it does not. In the reactions of these creatures with 

 reference to light and darkness we see the same thing. In the 



