Reconciling discrete psychological typology with a psychobiological continuum

Structure entails arrangements and interrelations of parts that organize th e whole (i.e., personality). It involves stability of traits over time. Ext raversion varies along a continuum towards introversion. Multiple behaviora l and biological variables in several systems vary and are regulated homeos tatically within the normal range. If there is a fixed point for an individ ual, what inhibits variation in the biological parameter?