=D. Biographical Studies of Individual Children.=--Many books have been written describing the development of individual children. These descriptions doubtless contain much that is typical of all children, but one must be careful not to argue too much from an individual case. Such records are valuable as confirmatory evidence of what has already been observed in connection with other children, or as suggestive of what may be looked for in them. PERIODS OF DEVELOPMENT The period covered by child study may be roughly divided into three parts, namely, (1) infancy, extending from birth to three years of age, (2) childhood, from three to twelve, and (3) adolescence, from twelve to eighteen. While children during each of these periods exhibit striking dissimilarities one from another, there are nevertheless many characteristics that are fairly universal during each period. 1. INFANCY =A. Physical Characteristics.=--One of the striking features of infancy is the rapidity with which command of the bodily organs is secured. Starting with a few inherited reflexes, the child at three years of age has attained fairly complete control of his sense organs and bodily movements, though he lacks that co-ordination of muscles by which certain delicate effects of hand and voice are produced. The relative growth is greater at this than at any subsequent period. Another prominent characteristic is the tendency to incessant movement. The constant handling, exploring, and analysing of objects enhances the child's natural thirst for knowledge, and he probably obtains a larger stock of ideas during the first three years of his life than during any equal period subsequently. =B. Mental Characteristics.=--A conspicuous feature of infancy is the imitative tendency, which early manifests itself. Through this means the child acquires many of his movements, his language power, and the simple games he plays. Sense impressions begin to lose their fleeting character and to become more permanent. As evidence of this, few children remember events farther back than their third year, while many can distinctly recall events of the third and fourth years even after the lapse of a long period of time. The child at this period begins to compare, classify, and generalize in an elementary way, though his ideas are still largely of the concrete variety. His attention is almost entirely non-voluntary; he is interested in objects and activities for themselves alone, and not for the sake of an end. He is, as yet, unable to conceive remote ends, the prime condition of voluntary attention. His ideas of right and wrong conduct are associated with the approval and disapproval of those about him. 2. CHILDHOOD