ISTHMIAN LINKS: AN IDEA THAT DRIFTED IN A DIFFERENT DIRECTION, 1932-1967

The concept of an isthmian link developed gradually in the work of European explorers, catographers, and scientists across several centuries. The land bridge at Panama provided a way to explain distribution of plants and animals in North and South America, and Beringia offered a connection between North America and Asia. In the early 1930s, two American geologists adapted the idea for a new purpose, to explain pre-Tertiary biographical distributions of terrestrial and marine organisms as an alternative to continental drift. Charles Schuchert and Bailey Willis published articles in 1932 in GSA Bulletin vol. 43 that detailed land bridges across the Pacific and Atlantic oceans made of granite and basalt that served as conduits/transport lanes for the exchange of terrestrial organisms. The notion had several advoctes among the anti-drift camp, notably George Gaylord Simpson and Daniel Axelrod, who chose to ignore contrary evidences and concerns, even from their American colleagues, among them Douglas Houghton Campbell and, to a lesser extent, George S. Myers. Isthmian links proved long-lived in U.S. geological thinking thanks to their transmission through textbooks, notably Charles Schuchert’s many editions of his and Carl Dunbar's textbook on historical geology, although Longwell, Flint & Knopf, in their companion volume on physical geology, took a more thoughtful approach. The acceptance of George Simpson’s explanation for the distribution of mammals as an exemplar of the way organisms could take advantage of land bridges rather than requiring broad continental connections to account for paleobiogeographic distribution patterns, lent credibility to the fixists notions of continental relations. Plate tectonics theory finally subducted the ad hoc land bridges in the late 1960s as continental masses crushed into one another with an abandonment. However, isthmian links such as Mesoamerica and Beringia endure as a defensible part of the Tertiary to explain Simpson’s mammal distributions.