No terminal organ will attain its full development except under the stimulus of constant use. In a blacksmith's arm the stimulus causing its growth is in his mind and not in his arm. He wants certain goods and the way to get them is to use his arm as a smith. The positive side of his development is psychic through which a greater appreciation of goods arises. This leads to work; work involves exercise and exercise leads to growth of the arm. In the case of a musician the stimulus that develops the hand is in his appreciation of music. So long as the love of music persists the stimulus is present which arouses the peculiar growth in the arm. Instead therefore of a new modification in the germ cell being needed to perpetuate the modification of the musician's arm the germ-cell modification has already been made. It exists in the psychic variation creating the love of music. While this persists, the stimulus is present that forces the right growth in the arm. The arm of the musician as compared with that of a laborer is regressive but the stimulus that comes from a love of music keeps active a group of muscles and leads to their growth. There is a general terminal regression counteracted in specific cases by the stimulus due to the activity of central organs. This, I believe, will be found true of all cases purely regressive. They are the effects of progressive changes in the central organs. They should not be confused with degeneration and retardation which indicate a real backward movement.
The other ways in which parent and child differ are through injury and recovery. Any child may be injured in ways that will affect subsequent growth. So in turn he may be free from injuries of parents or, what is more important, his environment may be so modified that injuries to which they were subjected become less frequent or disappear. Ancestors may suffer injury from a disease like malaria or from a parasite like the hook worm for so long a time that the injury seems inherited. A change of environment, however, may prevent the injury and bring back the children to normal standards. Recoveries of this sort are as frequent as the injuries that depress; they need recognition in any scheme to present the differences between parent and child.
It may seem from the last paragraphs that I am getting over into biology where other persons are a better judge of the facts than I am. The change of attitude is, however, more apparent than real. The traits of men and the facts about degeneration were observed long before biology became a science. What biology has done is not to discover new traits, but to enable us to classify them and to show their causes. Only the most obvious facts of biology are needed for this purpose, and they are of importance not to help observation, but to point out remedies. That riches caused men to degenerate and that poverty retards the development of the poor are well-established facts.