The influence of locality is plainly seen. Convincing evidence of river erosion was obtained in central France, the Pacific Islands, and the Colorado Plateau—regions in which other causes were easily eliminated. Sculpture by glaciers passed beyond the theoretical stage when the simple forms of the Sierras and New Zealand Alps were described. The origin of loess was first discerned in a region where glacial phenomena did not obscure the vision. The complexity of the Glacial period asserted by geologists of the Middle West was denied by eastern students. The work of waves on the English coast impressed British geologists to such an extent that plains of denudation and inland valleys were ascribed to ocean work. In the establishment of principles, the friendly interchange of ideas has yielded large returns. Many of the fundamental conceptions of earth sculpture have come from groups of men so situated as to facilitate criticism. It is impossible, even if desirable, to award individual credit to Venetz, Charpentier, and Agassiz in the formulation of the glacial theory; and the close association of Agassiz and Dana in New England and of Chamberlin and Irving in Wisconsin was undoubtedly helpful in establishing the theory of continental glaciation. From the intimate companionship in field and laboratory of Hutton, Playfair and Hope, arose the profound influence of the Edinburgh school, and the sympathetic cooperation of Powell, Gilbert, and Dutton has given to the world its classics in the genetic study of land forms. The influence of ideas has been closely associated with clarity, conciseness, and attractiveness of presentation. Hutton is known through Playfair, Agassiz’s contributions to glacial geology are known to every student, while Venetz, Charpentier, and Hugi are only names. Cuvier’s discourses on dynamical geology were reprinted and translated into English and German, but Lamarck’s “Hydrogéologie” is known only to book collectors. The verbose works of Guettard, although carrying the same message as Playfair’s “Illustrations” and Desmarest’s “Memoirs,” are practically unknown, as is also Horace H. Hayden’s treatise (1821) on the drift of eastern North America. It has been well said that the world-wide influence of American physiographic teaching is due in no small part to the masterly presentations of Gilbert and Davis. It is surprising to note the delays, the backward steps, and the duplication of effort resulting from lack of familiarity with the work of the pioneers. Sabine says in 1864:[77] “It often happens, not unnaturally, that those who are most occupied with the questions of the day in an advancing science retain but an imperfect recollection of the obligations due to those who laid the first foundations of our subsequent knowledge.”