William Yarrell was a London businessman, a stationer and bookseller, who became an expert on British birds and fish, writing standard reference works on both.  He was a member of several science and natural history societies, including the Linnean Society, and was a founder member of both the Zoological Society of London and the Entomological Society of London. 

As a student, Darwin gave his cousin, William Darwin Fox, a present of a stuffed swan, identified byYarrell as a new species. And when he was preparing for the Beagle voyage Yarrell helped him get the best deals on equipment and also gave him advice on how best to prepare specimens.  After his return it was Yarrell who took him as a visitor to the Linnean Society.

Yarrell continued to play an important role in Darwin's career: they exchanged publications, Darwin sending Journal of researches and Yarrell sending the first volume of his History of British Birds, and Darwin sought the older man's advice on suitable engravers for the Zoology of the Beagle.  When Darwin wanted to study the effects of artificial breeding, it was Yarrell who pursuaded him to keep pigeons himself at Down House, something Darwin came to enjoy greatly, and also introduced him to William Bernhard Tegetmeier, who became one of Darwin's most important collaborators, providing specimens, engravings, and information for decades.  It was to Tegetmeier that Darwin wrote on hearing of Yarrell's death, lamenting 'our old & excellent friend'.