This paper explores Darwin's 14,000 plus letters and suggests that in spite of the enormous amount of published material on Darwin and his work, there remains much untapped information in his correspondence. A quantitative analysis of his correspondence, reveals that many of Darwin's most important sources and projects have not been researched. I provide examples in two of his correspondents, William B. Tegetmeier and John Scott, who were extremely important to Darwin's work in domestic animal breeding and plant hybrid studies, respectively. In addition, Darwin's work on seed viability and distribution are discussed to illustrate both the extent of his correspondence network and the complexity of his many sub-projects. The appendices suggest avenues for the further research of Darwin's correspondence by correlating the amount of correspondence with the amount of published material on the correspondents.