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 away 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 involved 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 adaptation of the individual would take place in the lower forms through chance responses to stimulation, which were in the main not determined by the nature of the stimulus, but which attained their end by
