It has been said that beliefs and observances in themselves most irrational wear a different aspect when viewed in the light of their origin and history. It is so with vaccination. Had it come upon the world as we know it, with failure and disaster, equivocation and apology, rejection would have been inevitable; but when we turn to the past we discover that our damnosa hæreditas has a tradition that goes far to account for, if not to excuse, the folly which remains. 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. At the close of last century, variolation had become the custom of the upper and middle classes of England. The trouble and the peril were disliked, but were accepted in the name of duty. The variolation of their children was an anxiety that weighed like lead on the hearts of affectionate parents; and glad and grateful they were when the operation was accomplished without serious mishap. Patients designed for variolation were dieted, purged, and bled; and smallpox from sufferers of sound constitution was diligently inquired for. Mild smallpox was in great demand and was propagated from arm to arm. When Dr. Dimsdale operated on the Empress Catharine he did not venture to convey smallpox direct to the imperial person. He looked out a case of “benign smallpox” with which he inoculated a strong young man, and from the young man the Empress. Unless we realise the inconveniences, the uncertainties, the disasters and the horrors of the practice of variolation, albeit minimised, excused and denied by its professors, we can never understand the enthusiasm with which vaccination was received as its substitute. The promise conveyed in vaccination was a relief inexpressible, bearing with it a show of reason that was well nigh irresistible. The argument ran thus: No one can have smallpox twice, and the mildest attack is as protective from subsequent attack as the severest. Therefore it is that in inoculation with smallpox we find security.