From many points of view adolescence is perhaps the most important epoch in the life of every individual. It is a time of great and significant organic growth, with the development of new organs and functions, and a corresponding transformation of both the emotional and intellectual output. So far as the brain, the most important organ of all, is concerned, one may safely say that before puberty its main function has been acquisition. After puberty vast tracts of brain tissue become active, and an era of rapid development sets in. There is a rapid growth of new nerve connections which occasions both physiological and psychological unrest.[151] An important point to bear in mind, also, is that all periods of rapid development involve conditions of relative instability--one is, in fact, only the obverse side of the other. Dr. Mercier says that with girls "more or less decided manifestations of hysteria are the rule," and with both sexes this instability involves a peculiar susceptibility to suggestions and impressions. Accompanying the purely physical changes the mental and emotional nature undergoes what is little less than a transformation. There is less direct concern with self, and a more conscious concern with others. There is a craving for sympathy, for fellowship, a tendency to look at oneself from the outside, so to speak, a susceptibility to sights and sounds and impressions that formerly had little influence. Each one is conscious of new desires, new attractions, expressed often only in a vague feeling of unrest, with a desire, half shy because half conscious, for the company of the opposite sex. The childish desire for protection weakens; the more mature desire to protect others begins to express itself.