Jenner died in 1823, and at that date three kinds of virus were in use; first, cowpox from horsegrease or horsepox; second, cowpox; third, horsepox. These of course were subject to inscrutable modification in transmission from arm to arm: it is the distinct sources we have to recognise. A patient intent on vaccination might have said to himself, Which shall it be? Shall I be cowpoxed? or, shall I be horsepoxed? or, shall I be horsepoxed cowpoxed? How such an inquirer would have been answered had he set his perplexity before his medical adviser, I can only conjecture. Probably he would have been rebuked for his intrusion into matters outside his province. The little girl who quenched the scepticism of her comrade with the dictum, “It is so, for ma says so; and if it isn’t so, it is so, if ma says so,” illustrates the manner of rebuff administered to those who pry into professional mysteries. It is for you to pay and for us to think is a formula by no means limited to ecclesiastics. SMALLPOX COWPOX. Jenner was pleased to describe cowpox as a mild form of smallpox; but for what reason, outside his pleasure, he did not explain. Nevertheless the suggestion has borne fruit. When virus has fallen short, it has been asked, Why, if cowpox be mild smallpox, should not cows be inoculated with smallpox, and a crop of virus be raised? Various such attempts have been made, in which Mr. Badcock of Brighton has been especially distinguished. Mr. John Simon, writing in 1857, said, “Mr. Badcock, from 1840 to the present time, has again and again derived fresh stocks of vaccine lymph from cows artificially infected by him; having vaccinated with such lymph more than 14,000 persons, and having forwarded supplies of it to more than 400 medical practitioners.” When it is remembered that virus for half a dozen or more vaccinations is taken from a single arm, and that this process of reproduction is repeated every week, some idea may be formed of the extent to which this smallpox cowpox has been diffused over the country. The original assertion that vaccination conferred life-long immunity from smallpox was unwillingly abandoned under stress of experience, until no respectable practitioner pretended that the rite afforded more than a partial or temporary security. In promotion of smallpox cowpox, however, Jenner’s most extravagant claims were revived. In Mr. Simon’s words, for the recipient of smallpox cowpox, “Neither renewed vaccination, nor inoculation with smallpox, nor the closest contact and co-habitation with smallpox patients, will occasion him to betray any remnant of susceptibility to infection.” Untrue even of variolation, it is unnecessary to controvert such a figment: it suffices to place it on record.