It is well known that American botanist Asa Gray's 1859 paper on the floristic similarities between Japan and the United States was among the earliest applications of Charles Darwin's evolutionary theory in plant geography. Commonly known as Gray's "disjunction thesis," Gray's diagnosis of that previously inexplicable pattern not only provoked his famous debate with Louis Agassiz but also secured his role as the foremost advocate of Darwin and Darwinism in the United States. Making use of previously unknown archival materials, this dissertation examines the making of Gray's disjunction thesis and its relation to his collecting networks. I first point out that, as far back as the 1840s, Gray had identified remarkable "analogies" between the flora of East Asia and that of North America. By analyzing Gray and his contemporaries' "free and liberal exchange of specimens," I argue that Gray at the time was convinced that "a particular plan" existed in nature, and he considered that the floristic similarities between Japan and eastern North America manifested this plan. In the 1850s, when Gray applied himself to enumerating collections brought back by professional collectors supported by the subscription system and appointed in governmental surveying expeditions, his view of nature was then replaced by one that regarded the flora as merely "a catalogue of species." I argue that it was by undertaking the manual labor of cataloging species and by charging subscription fees for catalogued species that Gray established his status as a metropolitan botanist and as the "mint" that produced species as a currency for transactions in botanical communities. Finally, I examine the Gray-Darwin correspondence in the 1850s and the expedition that brought Gray's collector to Japan. I argue that Gray's thesis cannot be considered Darwinian as historians of science have long understood the term, and that its conception was part of the United States' scientific imperialism in East Asia. In light of recent studies focusing on the history of field sciences, this dissertation urges that a close examination of a biogeographical discovery like Gray's thesis is impossible without considering the institutional, cultural, and material aspects that tie the closets of naturalists to the field destinations of collectors. / History of Science