\begin{figure}[hpb]
\centering
\mbox{
\subfigure[]{\includegraphics[width=3in]{test_fucn_1A.pdf}}
\subfigure[]{\includegraphics[width=3in]{test_fucn_1B.pdf}}
}
\begin{minipage}{4in}
\centering
\caption{Plot of best performing objective function vs. generation for test function $I$ using (a) Parent retention, "Case A", (b) No parent retention, "Case B".}
\end{minipage}
\label{fig:test_func_1}
\end{figure}