MY DEAR SIR,--You may be assured that a person may be repeatedly affected, both locally and generally, by the Cowpox, two instances of which I have adduced, and have many more in my recollection; but, nevertheless, I have some reason to suspect that my discriminations have not been, till lately, sufficiently nice.... Certain it is, that the skin is always subject to the ulcerative effects of the virus; but whether the constitution can repeatedly feel the primary effects of it, I have experiments in view to determine. (P. 99.) This passage is commended to those who hold with Mr. John Simon that Jenner delivered to the world “a Master-piece of Medical Induction,” the fruit of thirty years of incessant thought, watching and experiment. It is plain that in 1798 the very elements of the problem were by him undetermined, and the most obvious objections unforeseen and unconsidered. Pearson’s strongest opposition was reserved for the asserted origin of Cowpox in Horsegrease. He said-- It has no better support than the coincidence in some instances of the two diseases in the same farm in which the same servants are employed among the Horses and Cows. I have found that in many farms the Cowpox breaks out although no new-comer has been introduced to the herd; although the milkers do not come in contact with the Horses; although there are no greased Horses; and even although there are no Horses kept on the farm. It appears that the Cowpox does not break out under the most favourable circumstances, if it be occasioned by _the Grease_. “I have had,” writes Sir Isaac Pennington, Cambridge, 14th September, 1798, “Dr. Jenner’s book some weeks, and the particulars stated in it are really astonishing. I have made inquiries upon the subject at Cottenham and Willingham, in which two parishes 3000 milch Cows are kept; also a great many Horses of the rough-legged cart kind (much liable to the scratches or grease) half the parishes being under the plough, and the men much employed in milking. But I cannot find that any pustulous eruptions on the teats of the Cows, or on the hands of the milkers, have ever been heard of.” (P. 82.)