The term puberty will so often be used in the following chapters that a brief account of the phenomena of puberty may appropriately be given at the outset of this work. Puberty is a name given to the age at which a boy becomes capable of being a father. In temperate climates this age is reached at about fifteen years, though some boys attain it at twelve and some not until seventeen. The one obvious and invariable sign of puberty is a change of pitch in the voice, which assumes its bass character after an embarrassing period of squeaky alternations between the high and low tones. The age is a critical one, as several important changes take place in body and in mind. The reproductive organs undergo considerable development and become sensitive to any stimulus, physical or mental. The seminal fluid, which in normal cases has hitherto been secreted little, if at all, is now elaborated by the testicles, and contains spermatazoa--minute organisms which are essential to reproduction. Under the stimulus of sexual thoughts this fluid is secreted in such quantity as to give rise to involuntary discharge during sleep. These nocturnal emissions are so often found among boys and young men that some physiologists consider them to be quite normal. My experience leads me to doubt this conclusion. Another physical change associated with puberty is the growth of hair on the pubes and on the face: in this latter situation the growth is slow. With the capacity for fatherhood comes a very strong awakening of the sexual instinct, which manifests itself in passion and in lust--the unconscious and the conscious sex hunger. The passion shows itself in a ludicrously indiscriminate and exaggerated susceptibility to female attractions--a susceptibility the sexual character of which is usually quite unrecognised. Among boys who have sex knowledge there is also a tendency to dwell on sexual thoughts when the mind is not otherwise occupied. Passion and lust do not at once develop their full strength; but, coming at a time when self-control is very weak, and coming with all the attraction of novelty, they often dominate the mind even in normal cases, and may become tyrannous when the reproductive system has been prematurely stimulated.