From the New York Times, May 12, 1989:
Hassler Whitney [was] a mathematician who made important contributions in geometry and . . . was deeply involved in improving the teaching of mathematics. . . . Whitney's specialty was geometry, particularly topology, including the study of multidimensional surfaces like spheres. He demonstrated, for example, that it is possible to plot a tour of the earth that visits each country once and only once - the Hamiltonian cycle.