from Today in Science History:
In 1831, Charles Darwin returned home from a geology field trip in North Wales to find letters from Revd. John Henslow and George Peacock informing him that he will soon be invited on a scientific voyage of HMS Beagle. He was 22 years old, and had just graduated from Cambridge University. The offer was to be a naturalist on H.M.S. Beagle for a two year survey of South America, leaving on 25 Sep. Although he immediately accepted the offer, his father and sisters were opposed. They regarded it as an idle pursuit that would delay his expected career in the clergy. His father, however, was prepared to change his mind, but only if Darwin could find a qualified man who viewed the exploit as worthwhile. Darwin spent the next two days doing just that.
Naturalist, or gentlemenly companion to Fitzroy? The two letters referred to are here and here.
The Dispersal of Darwin 