Each organ, each physiological system or process, each mental process, is controlled by its own definite nerve cells in the spinal cord and brain. By exercise of the specific organ or system, the corresponding nerve center is developed; and the development of the nerve center makes possible a more adequate and perfect use of the specific organ or system.
During the growth stage of any part, exercise of the part is not normal but injurious. When sufficient growth has been attained for development to begin, there is an instinctive desire or hunger for exercise of the part. This desire is manifested by the natural, spontaneous activity or interest of the child. For example, during some ten or twelve months the muscles of the legs and back, and corresponding nerve centers in the spinal cord and the brain, are growing. When their growth is attained, these muscles and nerve centers begin to function in the process of standing and walking, and the child makes every effort to walk. To put him on his feet and attempt to teach him before this stage, is to strain unprepared organs, bones, muscles, and nerves. To keep him lying in a vehicle so he cannot exercise when he spontaneously attempts to walk, is to retard or prevent this natural development.
The process of growth and development is not uniform during childhood; neither do all the parts grow and develop at the same time. Growth is periodic and by parts; it is variable for each part or system. There are periods of slow or rapid growth and development at different ages.
Development begins first for the oldest (racially) muscles and parts, and for those that are being used reflexly, that is, arms, legs, trunk, hands, which are known as the fundamental muscles. The finer, accessory muscles and their brain centers do not develop completely until several years after birth.
There are no average children. Every child is somewhat different. In rate of growth, children may normally vary one to two years from the average. In individual children, some factors at any stage will normally be more marked than others. Distinction must therefore be made between (1) chronological age, (2) physiological age, and (3) psychological age. The standards for (2) and (3) are at present the subject of special researches. Physiological age refers to such factors as dentition, development of bones, height, weight, sex maturity. Psychological age refers to mental ability and maturity.
In some children the hereditary force of a specific characteristic is stronger than in other children. Or the environment of one child gives greater stimulus to an instinct at its nascent (beginning) period, and greater opportunity for its use.
The individual who lives most completely in each stage the life normal to that stage, is best prepared for the succeeding stages of life.
To attempt to hurry a child through this process or to permit an arrest of development in any stage or at any point, is to seriously handicap the child’s normal and complete development. Infant prodigies and infantile youths are both abnormal.
In each stage there are some instincts to be especially fostered, some that need encouragement or stimulation, some that require careful direction into useful channels, some to be ignored as only transitory, and a few that may need inhibiting.
The following group of stages has been prepared as possibly most helpful for guidance of parents and teachers in the home. The transition from one stage to another is gradual.