Letter writing is often seen as a part of scientific communication, rather than as integral to knowledge making. This section shows how correspondence could help to shape the practice of science, from the process of specialization, to the design of experiments, the use of instruments, the work of collecting, and the construction of theory. Darwin was not simply a gentleman naturalist and broad theorist. He actively participated in a number of scientific specialties, including geology, descriptive and taxonomic zoology, microscopy, and physiological botany, as well as pioneering other nascent fields such as animal ethology and child psychology. Working from home and sometimes on phenomena that had been little investigated, he had to design new experimental procedures and techniques, often using simple household materials. But he also corresponded with leading institutional practitioners, and availed himself of the most advanced laboratory methods and equipment. Darwin used letters as a speculative space, trying out theories and refining them in dialogue with constructive critics.