We considered the geometry of a double row of vortex pairs in an ideal fluid, and studied the evolution of salt fingers from the geometry systematically. This study illustrates the mechanism of the generation of large-scale convective cells at the final stage of the fingering instability. We discuss why in the growth of double-diffusive salt fingers the emergence of such large convection cells has been implicated phenomenologically as an onset to turbulence, terminating the finger growth. [S1063-651X(99)12010-5].