Charles Darwin spent 20 years perfecting his ideas on evolution and how it operates. What motivated him to channelise his energy and focus for so long on his work?

Thomas J Barloon and Russell Noyes Jr of the University of Iowa College of Medicine, US, have suggested that he suffered from panic disorder and that his illness turned him into a scholarly recluse. Victims of this psychotic illness suffer unexpected attacks of intense terror and anxiety which last for several minutes and is accompanied by palpitations, breathlessness, trembling, sweating, nausea and dizziness. They often change their behaviour drastically to avoid situations similar to those that triggered the initial reaction.

In Darwin's case, as the researchers state, the condition was probably aggravated by agoraphobia, the fear of being in open or public places. That he suffered from a mysterious condition which prevented him from leaving home, is well recorded. Darwin himself described this as a "sensation of fear' and often turned down invitations to travel. Psychiatrists like James C Ballenger of the Medical University of South Carolina, US, find the theory to be a credible explanation of Darwin's illness.