4oo THE POPULAR SCIENCE MONTHLY 



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 cer- 

 tain 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 exer- 

 cise 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 modifica- 

 tion 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 ac- 

 tive 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 in- 

 jury 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 disap- 

 pear. 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 in- 

 jury and bring back the children to normal standards. Recoveries of 

 this sort are as frequent as the injuries that depress; they need recog- 

 nition 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 be- 

 fore biology became a science. What biology has done is not to dis- 

 cover 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. 



