Darwin was very ill at one point, next to dying (his words) In spite of his scepticism towards homeopaty, he actually used it. Maybe in desperation; later he experiented with homeopatic dilutions on plants.

"“I grieve to say that Dr Gully gives me homeopathic medicines three times a day, which I take obediently without an atom of faith.”
Despite Darwin's skepticism about homeopathy, he experienced the power of these medicines.
After just eight days of his arrival, Darwin experienced a skin eruption all over his legs. It is interesting and important to note that patients who receive homeopathic treatment do not always get better immediately. In fact, in around 20-30% of patients with chronic symptoms, patients experience a “healing crisis,” usually an externalization of symptoms to the skin or an excerbation of old symptoms that were previously suppressed with conventional drugging. The fact that Darwin was skeptical of Gully's treatment and that Darwin experienced this initial worsening of symptoms suggests that Darwin's reaction was clearly not a placebo effect.Some skeptics assert that Gully's “water-cure” treatment may have provided the therapeutic result, not homeopathy, though Darwin acknowledged on March 24, 1849, that Dr. Gully had not even begun the sweating process of his treatment, one of the important parts of water-cure treatment. Actually, it was just 2 weeks after arriving that Darwin wrote "I much like and think highly of Dr. Gully" and more.
On March 28, 1849, he had not have any vomiting for 10 days (a rare experience for him):
By April 19, 1849, Darwin wrote,
“I now increase in weight, have escaped sickness for 30 days, which is thrice as long an interval, as I have had for last year; & yesterday in 4 walks I managed seven miles! I am turning into a mere walking and eating machine.”

"In Darwin's book on his experiments with Drosera, he expressed complete amazement at the hypersensitivity of a plant to extremely small doses of certain chemicals: “Moreover, this extreme sensitiveness, exceeding that of the most delicate part of the human body, as well as the power of transmitting various impulses from one part of the leaf to another, have been acquired without the intervention of any nervous system” (Darwin, 1875, 272).
Yet, Darwin also discovered that Drosera is not sensitive to every substance. He tested various alkaloids and other substances that act powerfully on humans and animals who have a nervous system but produced no effect on Drosera. He concluded that the “power of transmitting an influence to other parts of the leaf, causing movement, or modified secretion, or aggregation, does not depend on the presence of a diffused element, allied to nerve-tissue” (Darwin, 1875, 274).
Darwin confirmed an important homeopathic observation that living systems are hypersensitive to only certain substances. Sadly and strangely, conventional scientists have attacked homeopaths for using extremely small doses of substances without any appreciation for the homeopaths' credo that living systems—whether human, animal, or plant—will be hypersensitive to a limited number of substances (and the homeopathic method of individualizing treatment is a refined method to find this substance or substances)."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2816387/
http://www.homeopathic.com/Articles/History/Despite_Darwins_Initial_Skeptism_a_Homeopath.html