Under these circumstances we can well understand the favourable reception given to an operation which produced a slight, _non-infectious_ disease, which yet was alleged to protect against small-pox as completely as did the inoculated disease itself. This was Vaccination, which arose from the belief of farmers in Gloucestershire and elsewhere that those who had caught cow-pox from cows were free from small-pox for the rest of their lives. Jenner, in 1798, published his _Inquiry_, giving an account of the facts which, in his opinion, proved this to be the case. But in the light of our present knowledge we see that they are wholly inconclusive. Six of his patients had had cow-pox when young, and were inoculated with small-pox in the usual way from twenty-one to fifty-three years afterwards, and because they did not take the disease, he concluded that the cow-pox had preserved them. But we know that a considerable proportion of persons in middle age are insusceptible to small-pox infection; besides which even those who most strongly uphold vaccination now admit that its effects die out entirely in a few years—some say four or five, some ten—so that these people who had had cow-pox so long before were certainly _not_ protected by it from taking small-pox. Several other patients were farriers or stable men who were infected by horse-grease, not by cow-pox, and were also said to be insusceptible to small-pox inoculation, though not so completely as those who had had cow-pox. The remainder of Jenner’s cases were six children, from five to eight years old, who were vaccinated, and then inoculated a few weeks or months afterwards. These cases are fallacious from two causes. In the first place, any remnant of the effects of the vaccination (which were sometimes severe), or the existence of scurvy, then very prevalent, or of any other skin-disease, might prevent the test-inoculation from producing any effect.[1] The other cause of uncertainty arises from the fact that this “variolous test” consisted in inoculating with small-pox virus obtained from the _last_ of a series of successive patients in whom the effect produced was a minimum, consisting of very few pustules, sometimes only one, and a very slight amount of fever. The results of this test, whether on a person who had had cow-pox or who had not had it, was usually so slight that it could easily be described by a believer in the influence of the one disease on the other as having “no effect”; and Dr. Creighton declares, after a study of the whole literature of the subject, that the description of the results of the test is almost always loose and general, and that in the few cases where more detail is given the symptoms described are almost the same in the vaccinated as in the unvaccinated. Again, no careful tests were ever made by inoculating at the same time, and in exactly the same way, two groups of persons of similar age, constitution, and health, the one group having been