In its broadest sense, a scientific network is a set of connections between people, places, and things that channel the communication of knowledge, and that substantially determine both its intellectual form and content, and its material and social effects. In practice, such networks can be very personal and intimate, as between close friends and colleagues, or more formal and bureaucratic, joining state institutions and universities, or national and colonial authorities. In the nineteenth-century, letter writing was one of the most important activities for building and maintaining such connections. Darwin’s networks extended from his family circle and close friends to readers, observers, and experimenters across the globe, most of whom he never met. His contacts were socially diverse, including women and men from different classes, nationalities, and professions. He extended the social and geographic range of his contacts in large part by tapping into the networks of others, such as Joseph Dalton Hooker and Asa Gray, who were at leading scientific institutions and who carried out extensive administrative correspondence. Other contacts such William Bernard Tegetmeier and George Frederick Cupples, introduced him to communities of pigeon fanciers and dog breeders.