The article deals with correspondence in natural history in the eighteenth century between England and North America. The corpus discussed consists of correspondence between John Bartram and Peter Collinson, and between Alexander Garden and John Ellis. The approach used in the study is qualitative and rhetorical; the main point considered is how the letters construct scientific centre and periphery in the eighteenth-century Atlantic world. A central concept is the “colonial exchange”, whereby “raw materials” from the colonies — in this case plant and animal specimens, along with proposed identifications and names — are exchanged for “finished products”, in this case codified scientific knowledge contained in publications.