It is now twelve years since it first became possible to study plague by modern methods, and yet no conclusion on its aetiology has been reached which has met with general acceptance. Two causes, as it seems to me, have chiefly contributed to this disappoinment. One consists in the appeal which has been persistently made to bacteriology for the lights which in fact are not in its power to shed; the other is neglect of the epidemiological method, or at least failure to apply it effectively. In consequence, a field has been occupied by the bacteriologist which could not be profitably laboured by him. The epidemiologist has not seriously disputed this usurpation of his domain, and for his apparent apathy he has not been without excuses of some weight.