Probably the most important years in anyone's life are those eight or ten preceding the twenty-first birthday. During these years Heredity, one of the two great developmental factors, bears its crop, and the seeds sown before birth and during childhood come to maturity. During these years also the other great developmental force known as Environment has full play, the still plastic nature is moulded by circumstances, and the influence of these two forces is seen in the manner of individual that results.