Geographical separation is arguably fundamental to speciation. John Thomas Gulick (13 March 1832 – 14 April 1923), a missionary from the Hawaiian Islands and one of the earliest evolutionary biologists, was among the first to recognize the critical role for geographical separation in the diversification of ecologically similar Hawaiian land snails. Although Gulick's work is not well-known today, his ideas were discussed by Darwin and Wallace as well as leaders in the Modern Evolutionary Synthesis (e.g., Wright and Mayr) who saw an important role for geographical isolation in speciation. It was perhaps no accident that organisms with low vagility, such as land snails of the Hawaiian Islands (i.e., achatinelline tree snails and ground-dwelling amastrid snails) exemplified the importance of geographical separation in speciation. Here I provide context for Gulick's snail research, showing that the natural setting of the Hawaiian Islands, combined with Gulick's development as a naturalist and evolutionary thinker lead to important insights on speciation, resulting from observations of substantial species richness in achatinelline and amastrid land snails, among the ridges and valleys of the Hawaiian Islands. Gulick's research on lesser-known organisms, island land snails, illustrates key areas for future inquiry, particularly in understanding “nonadaptive” contributions to evolutionary radiations.