2 78 POPULAR SCIENCE MONTHLY 



The selecting agent in the race is the environment, and the agent of 

 selection is always life and death. If the organism varies in a way 

 that is suitable, it lives and its progeny multiply. If, on the contrary, 

 some variation be unfavorable, death is its punishment, the animal is 

 eliminated, and nothing further is heard of it in the struggle for 

 existence. 



In the earliest forms, in the paramecium, upon which Professor 

 Jennings has worked, or in the blow-fly larva, that Professor Holmes 

 investigated, we can hardly imagine that there is much more than 

 vague chemical activity or quiescence. When the light is favorable, 

 motion directly ahead is preferred, or no motion at all. The animal 

 moves away rather than towards the light when negatively phototactic 

 merely because there is no physico-chemical tendency to draw back 

 the head when it is turned awav from the light and there is a stimulus 

 which leads to general locomotion. We have to do with the rudiments 

 of pleasure and pain, perhaps, but we can be sure of nothing more 

 than increased tendency toward motion in one position and decreased 

 tendency to movement in the other. It is approximately a mechanical 

 equivalent of pleasure and pain. 



At the next level of complexity in animal learning, the case is not 

 so different. The simplest answer to the question is that the creature 

 is controlled by pleasure and pain. It is not as clear as might be 

 imagined at first sight what this means in last analysis, for, at the very 

 lowest, pain and pleasure must go for their ultimate explanation to 

 the evolution of the species. Other factors are perhaps to be found 

 in the earlier experience of the animal and in even more remote 

 circumstances. While we can not unravel the tangle of factors in- 

 volved in what we call pleasure and pain, yet it may be interesting to 

 indicate that, regarded as a selecting agent, neither is a simple thing 

 but the result of many factors. It is at least worth while to indicate 

 that the deciding factors here are conscious, as opposed to the chemical 

 or physical processes in the organism or to the natural forces in the 

 environment. There may be nothing really new or peculiar in the 

 circumstances or conditions, but it does mean that we classify the 

 manifestations under a new head. This alone makes it worth while 

 to set the selecting agent off as belonging to a special class or group. 



If we bring the different groups under a single general statement, 

 we should have racial progress, due to the chance variations in the 

 animal structures, and have as the selecting agent the environment, 

 which enforces its decrees through the life or death of the organism, 

 or, at least through its nourishing or its failure to flourish. The adapta- 

 tion of the individual would take place in the lower forms through 

 chance responses to ■stimulation, Avhich were in the main not deter- 

 mined by the nature of the stimulus, but which attained their end by 



