The process of growth itself, whether normal or defective, is beyond the eye of immediate perception. The symptoms of growth, however, can be studied with direct objectivity. Psychologically, growth expresses itself in ascending orders of behavior. The trend and the tempo of development in a human being can be deduced from the manner in which capacities for behavior mature. The possibility of diagnosing developmental types of mental defect in early infancy rests on certain lawful relations between age and growth.
It takes time to grow. Development is profoundly conditioned by duration. Although one cannot say that development is caused by the clock or the almanac, one may insist that development is a function of age, and that a recognition of the age values of behavior and of the behavior values of age constitutes the first essential for developmental diagnosis. The frequent failure to discern certain kinds and grades of mental
GESELL A. THE EARLY DIAGNOSIS OF MENTAL DEFECT. Arch NeurPsych. 1929;22(3):522-529. doi:10.1001/archneurpsyc.1929.02220030099009