This second of three volumes of Mueller’s selected correspondence covers the central period of his life, years during which his letter-writing expanded and diversified to an astonishing extent. Throughout the period, Mueller continued as Government Botanist of Victoria, for much of the time he was also director of the Melbourne Botanic Garden. The volume includes both official and private correspondence documenting his work in these two capacities, the political difficulties associated with the latter position that eventually led to his dismissal from the Garden, and his wider role as one of the leading figures in Australian scientific life. His international standing is shown by letters exchanged with many of the world’s other leading naturalists, as well as by the honours showered upon him. His collaboration with George Bentham in the preparation of the latter’s Flora australiensis is thoroughly documented, as are his exchanges with Bentham and others on the basis of biological classification and on Darwin’s controversial ideas about the origin of species, his active participation in an international network of exchanges of plants and animals for acclimatization or museum purposes, his leading role in furthering the exploration of inland Australia, and various aspects of his personality and private life. There is a substantial historical introduction, and the biographical register begun in Volume 1 is extended to cover names newly appearing in this volume.