Like many a young man who has enjoyed a comfortable childhood, Francis Darwin simply drifted into a career, in his case medicine. There is no record that the choice caused him any great anguish or soul searching. There were, after all, plenty of precedents in his family for entering medicine, albeit often unhappy ones, and medicine seemed a sensible choice given his general interest in biology, encouraged in childhood first by his father and later by his school. He might not have had a great passion for the subject itself, or have been driven by any religious vocation to heal the sick, but there is no evidence that he was strongly pulled in any other direction. Family and friends alike commented that, above all, Francis was a steady man, not given to whims or transient enthusiasms. He was, however, burdened with a heavy sense of duty.

On graduating in 1870, aged twenty-two, from Trinity College, Cambridge with a BA degree (first class), he moved on to St George's Hospital, where he registered to take an MB degree from the University of London. Required to carry out small pieces of research and to write a thesis towards his final degree, Francis soon found an interest in, and flair for, physiology. Perhaps he was enthused with the spirit of the age, for these were indeed exciting times for the subject in Britain. New ideas and approaches, already well established in France by men like Claude Bernard, and in Germany by Carl Ludwig and others, were at last infiltrating Britain's staid institutions. Leading the establishment of physiology as a respected discipline in British university laboratories were Michael Foster and John Burdon-Sanderson.

It was Foster who in the late 1860s had introduced at University College, London (UCL) the very first course in experimental physiology in Britain. When in 1870 Foster moved on to a chair in Cambridge, he was replaced by Sanderson. Quickly realising that he could not on his own make the laboratory a success, Sanderson strove to build around himself a group of the most able young physiologists.