  For a complex simple Lie algebra of type A_l,B_l,C_l or D_l, given a family
of elements f_\lambda\ in commutative ring Z[\Lambda], we show that f_\lambda\
is just the formal character of the Weyl module V(\lambda) if f_\lambda\
satisfy several natural conditions. Hence we give a necessary and sufficient
condition for constructing a family of g_l-modules from a family of
g_{l-1}-modules.
