Dear Sir,

Will you pardon me troubling you with a brief inquiry?. I have been appointed to read the address in Medicine at the approaching meeting of the British Medical Association—to be held in Manchester in the beginning of August.¹

My subject is the parasitic nature of infective & contagious diseases. I propose to develop the idea that pathogenic organisms are variations of some common bacterium of similar morphology—that the Bacillus anthracis for example (the organism of Splenic fever) is a parasitic variation of the Bacillus Subtilis.² And the question I wish to ask you is this:— do you know of any animal or plant which has varied in this way—i.e. has acquired the parasitic habit.—or can you refer me to any source of information on the subject. I have been looking over your volumes on “the variation of plants & animals under domestication” without finding what I want.³ My excuse for troubling you is the great pressure on my time—and the short period between this and the 7^th of August.

I hope you will pardon my intrusion on your valuable leisure.

Believe me | yours faithfully | W^m. Roberts

C. Darwin Esq F.R.S. etc