hitherto been regarded as an almost machine-like response to external stimulation is not made with the definiteness that has been supposed. On the contrary, what seems to take place is a feeling about for a favorable condition. When this is found there is either continued motion in this most favored direction or quiescence in the most favorable position. Here again we should have to do with trial and error, but with less complicated possibility of movement and, in all probability, with a different basis for the selection of the favorable condition.
Still a fourth expansion of the category of trial and error is possible in a metaphorical sense. This is to explain the general course of evolution. If we accept in all strictness the conclusions of Weismann, there is no possibility of foreseeing with very great accuracy any change in a race. Changes of one kind appear here, changes of another kind appear there. Aside from the conditions of mating, however, there is no way of tracing to any known causes the changes observed. We are left then with what, at the present stage of knowledge, seem entirely unforeseen and undetermined chance changes in the animal structure, with an accompanying set of instincts and general activities. The real determining factor is, of course, natural selection. In brief the environment determines which of the many forms and functions that originate by chance shall survive. By personification, and even more literally, we can think of the chance variations as the analogue of the trials of the individual animal, and survival as corresponding to the movement that is successful and so retained.
If we should then be permitted to generalize, we should have chance at the basis of all learning, all advancement, all adaptation. The primary facts would be the variation in structure, which would form the basis for all other adaptation. Within the organism at the lowest stage would be found adaptation from moment to moment on the basis of successful chance adjustment. At this stage, however, there is no learning. One adjustment is of no value for later activities, as the animal is at the same level, in the same condition, after the adaptive movement has been made as it was before. At the next stage, in addition to the increased complexity of the organism which makes possible more numerous movements, there is retention of the successful trials. A movement once made is accompanied by a change in the organism which makes that movement more likely to occur in the future. From this point upward there is variation in degree of complexity of possible movements, in the readiness with which movements once attained are retained and repeated, but there is no great change in the mechanics of the problem.
What does change throughout, and what is, after all, on this theory the essential factor in all development, is the selecting agent and the rewards which serve to make one thing permanent rather than another.
