This month marks the 150th anniversary of the most important event in the evolution of the study of science, not only in Oxford but everywhere else too: the great British Association debate of June 30, 1860, at the University Museum, Parks Road, between Thomas Henry Huxley (1825-1895), known as ‘Darwin’s bulldog’, in the science corner, and Samuel Wilberforce, Bishop of Oxford (1805-1873), in the Church corner.

Charles Darwin (1809-1882) had published his earth-shatteringly significant book, The Origin of Species seven months before. But where was he on that fateful day? Answer: he was at Moor Park, near Farnham in Surrey, receiving hydropathic treatment for a debilitating illness at a sort of spa set up there by Dr Edward Wickstead Lane.

I gather this piece of knowledge from a new booklet, just 12 pages long, published by Huxley Scientific Press — a very small Oxford enterprise run by Sophie Huxley (a distant relative of Thomas Huxley) and her husband. It is called Darwin's Mysterious Illness and is written by Robert Youngson, a retired doctor and the author of popular scientific books, as well as many important medical reference works. In it, Dr Youngson speculates on what Darwin’s malady might have been, describing it as a puzzle unsolved for 150 years.

After Darwin’s return from his five-year journey of discovery aboard HMS Beagle in 1836, he wrote to a friend, Bernhard Studer, in 1847: “Shortly after my return from my long voyage, I had a tedious and severe illness, and I have never since recovered my strength and suppose I never shall . . . I appear quite well, but from being a strong man, I am incapable of any continued muscular exertion; or indeed of much exertion of mind, for even conversation, if it excites me, tires me in a very short time, so that I am compelled to live a most retired life.”

Dr Lane wrote: “When the worst attacks were on, he seemed almost crushed with agony, the nervous system being severely shaken, and the temporary depression resulting distressingly great.” And Darwin’s son Francis, wrote in his Reminiscences of My Father’s Everyday Life (c1884): “For nearly 40 years he never knew one day of the health of ordinary men, and thus his life was one long struggle against the weariness and strain of sickness.”

Dr Youngson examines — but seems to doubt — the possibility of the fatigue being due to Chagas’ Disease, described for the first time by Brazilian physician Carlos Chagas (1879-1934). This is caused by the South American blood-sucking cone-nosed assassin insect Triatoma infestans, or benchuca, sometimes called the ‘kissing bug’ because of its painless bite, which all too often kills its victim through heart failure.

Certainly Darwin was exposed to the attentions of this horrid product of evolution. He recorded an incident that occurred during the Beagle voyage in the village of Luxan in the Argentinian province of Mendoza in March 1835: “At night I experienced an attack (for it deserves no less a name) of the Benchura, a species of Reduvius, the great black bug of the Pampas. It is most disgusting to feel soft wingless insects, about an inch long crawling over one’s body.”

But Darwin did not die of heart failure; instead he died many years later, aged 73, of coronary thrombosis. His doctors and friends suspected old-fashioned hypochondria. He could work for, at the most, two hours at a stretch; then he needed to lie down on a sofa and have a novel read to him. Prof John Tyndall at the Royal Institution remarked that illness allowed Darwin plenty of time “to ponder a great deal”.

Huxley even joked: “If only I could break my leg, what a lot of scientific work I could do.”

As for that debate at the University Museum, controversy still rages as to the exact exchange of words between Wilberforce and Huxley. It went something along the lines of Wilberforce asking whether Huxley was descended from an ape on his mother’s or his father’s side — and Huxley replying that he would rather be descended from an ape than a bigoted prelate.