The dissertation argues that the transformations in Ernest Hemingway's writing style and his philosophy of the natural world between 1932 and 1952 can be attributed to his intense immersion in the environment of the Gulf Stream. This dissertation draws primarily on Hemingway's handwritten fishing logs from 1932, 1933, and 1934 in the Hemingway Collection at the John F. Kennedy Library, which have not been published or thoroughly studied. In 1929, Hemingway portrayed the Gulf Stream as a frontier, and claimed that he wanted to "write like Cezanne painted." Critics interpreted his work as a form of literary naturalism. In 1952, Hemingway portrayed the Gulf Stream world as a harmonious, organic whole, and he claimed that he would like to have his work illustrated by Winslow Homer. The distinct differences in the portrayal of themes, setting, and character between To Have and Have Not (1937) and The Old Man and the Sea (1952) are explored to illustrate the dimensions of the transformations within Hemingway's work. Numerous specific passages in the fishing logs served as seeds for scenes in these works, as Hemingway gathered raw material for his fiction. Through his scientific study of the climate, marine life, and birds of the Gulf Stream from 1932 to 1939, Hemingway's understanding of the integration of the natural world broadened. The new knowledge of "what to leave out" of his fiction refined his method of writing from the "iceberg principle," in which seven-eighths of the story is omitted. The precise observations of the logs, inscribed through hundreds of pages, generated the stylistic and philosophic transformation that occurred between 1932 and 1952.