The Genera of Birds, listing over 2,400 genera is the most important work by the ornithologist George Robert Gray. Due to the huge effort taken to compile the work accurately, it became the standard ornithological work of the nineteenth century. Published between 1844-9, the books included physical characteristics, habits and over 46,000 references. Gray intended the work to be illustrated by the best natural history artists and hired David William Mitchell, who illustrated the first two volumes. Due to other preoccupations, Mitchell could not illustrate the third volume, leaving Gray to hire Joseph Wolf, who had recently arrived from Germany. Wolf would go on to become one of the best regarded illustrators of natural history in the second half of the nineteenth century.