Vaccination was the successor of Inoculation (or, more precisely, Variolation), entering into a possession already acquired in the human mind. It had been observed from of old that some forms of disease rarely recur in the same person in a lifetime; and thus when scarlet fever, or measles, or smallpox broke out in a family, it was considered prudent to let the disease have its course, and thereby obtain immunity from fear of future infection. It was this confidence, that smallpox once undergone was finally disposed of, that was the justification of the practice of inoculating the disease when introduced from the East in the first quarter of last century. Inasmuch, it was argued, as none can have smallpox more than once, why not induce it artificially, and pass through the illness at a convenient season? But Nature, though compliant, does not always accept the course we ingeniously prescribe for her. Smallpox as naturally developed (so to speak) is a crisis of impurity in the blood, and if the requisite conditions are absent, it cannot be adequately excited. Hence variolation was an uncertain and hazardous operation. It took with some and was indistinguishable from an attack of ordinary smallpox; it took partially, or not at all with others; and the operation was frequently followed by malaise, disorders of the skin, and grave constitutional derangements. Nor were the variolated secure from smallpox. They occasionally had smallpox with their neighbours, and then it was said, “There must have been some mistake about the inoculation; for it is impossible that anyone can be successfully inoculated and have smallpox.” Further, the variolated, while labouring under the induced malady, conveyed the disease to their attendants and visitors; and thus smallpox was propagated by the means intended to avert it. THE PRECURSOR OF VACCINATION.