Throughout this paper, X will denote a Banach space, S=S(X) and B=B(X) will be the unit sphere and the closed unit ball of X, respectively, and [Gscr ]=[Gscr ](X) will stand for the group of all surjective linear isometries on X. Unless explicitly stated otherwise, all Banach spaces will be assumed to be real. Nevertheless, by passing to real structures, the results remain true for complex spaces.