  We study how much gluon shadowing can be perturbatively generated through the
modified QCD evolution in heavy nuclei. The evolution of small-$x$ gluons is
investigated within the semiclassical approximation. The method of
characteristics is used to evaluate the shadowed distributions in low-$Q$ and
small-$x$ region. In solving the modified evolution equation, we model in
simultaneously fusions from independent constituents and from the same
constituent, both in a proton and in a large loosely bound nucleus of $A\sim
200$. In addition to the actual distributions at small $x$, we study the ratios
of the distributions at an initial scale $Q_0 = 2$ GeV, and show that a strong
nuclear shadowing can follow from the modified QCD evolution.
