  Based on an extended compound nucleus model, isospin effects in statistical
fragment emission from excited nuclear systems are investigated. An
experimentally observed scaling behavior of the ratio of isotope yields
$Y_i(N,Z)$ from two similar emitting sources with different neutron-to-proton
ratios is predicted theoretically, i.e., the relationship of $Y_2/Y_1 \propto
exp(\alpha N + \beta Z)$ is demonstrated. The symmetry energy coefficient
$C_{sym}$ extracted from the simulation results is $\sim$ 27 MeV which is
consistent with realistic theoretical estimates and recent experimental data.
The influence of the surface entropy on the isoscaling behavior is discussed in
detail. It is found that although the surface entropy increases the numercial
values of isoscaling parameters $\alpha$ and $\beta$, it does not affect the
isoscaling behavior qualitatively and has only a minor effect on the extracted
symmetry energy coefficient.
