In the opening of his _Inquiry_, Pearson was good enough to say of Jenner, “I would not pluck a sprig of laurel from the wreath that decorates his brow”; but, disputing the origin of genuine Cowpox in Horsegrease, he might have asked himself, what sprig of laurel he had left. That Cowpox originated in Horsegrease was not Jenner’s discovery. As Pearson ascertained in the London milk farms, “There was such a notion entertained in several parts of the country, whatever might be its foundation.” (P. 86.) But the definition of Horsegrease Cowpox as the form of Cowpox that justified the faith of the country-folk in the power of the disease to avert Smallpox, was Jenner’s solitary distinction--the principle and motive of his _Inquiry_, which, to prove fallacious, was to extinguish his title to regard. Cowpox apart from Horsegrease was clearly taught by Jenner to have no influence on the constitution, and to be attended with no erysipelas. “Let me call your attention,” he wrote to Pearson, 27th September, 1798, “to a similarity between the Smallpox and the Cowpox when inoculated. The symptoms of absorption first disturb the system, and, secondly, the system feels the consequences of the local sores. Exactly so with the Cowpox; and _as the Cowpox inflammation is always of the erysipelatous kind_, when it spreads over the skin to any great extent, it produces symptoms not unlike the confluent Smallpox.” (P. 100.) Pearson foresaw that if the principle of inoculation with Cowpox were established it would lead to other applications-- The Cow Poison appears to alter the human constitution, so as to render it insusceptible of a different morbific poison, namely, the variolous in producing the Smallpox. This fact is, I believe, quite a novelty in physiology and pathology: it indicates a new principle in the mode of prophylactic practice. And we now see a principle upon which diseases from various other morbific poisons may possibly be prevented from taking place, such as the Measles, Ulcerous Sore Throat, Hooping Cough, Syphilis, etc., namely, in consequence of destroying the excitability of the constitution to such poisons by the agency of different, and perhaps less hurtful ones. Whether the Cowpox preserves the constitution from other morbific poisons, besides the variolous, is an undecided question. (P. 79.) Like Jenner, he also recognised in Cowpox a counter-irritant--a safe sort of fever that might be used to drive off other diseases--