There is doubtless some particular application of the population principle in the earlier instance as in the later, but not the same application in both. The conditions at the beginning of the three hundred years’ reign of plague in Britain were different from those at the end of it. The increased prevalence of smallpox in the generation before the last great outburst of plague, and the infantile or puerile character of the epidemic fever of 1661, as described by Willis, show that the incidence of infectious mortality had already begun to shift towards the age of childhood. It looks as if the conditions of population, intricate and obscure as they must be confessed to be, were somehow determining what the reigning infectious maladies, with their special age-incidence, should be. Such a gradual change is the more probable for the reason that infectious mortality came in due time to be mostly an affair of childhood. The plague, which was the great infection of the later medieval and earlier modern period, was peculiarly fatal to adult lives; on the other hand, the mortality from infectious diseases in our own time falls in much the larger ratio upon infants and children. It looks as if this change, now so obvious, had begun before the end of plague in Britain, having become more marked in the generation following its extinction. The direct successor of plague, so far as concerns age-incidence and nosological affinity, was the pestilential or malignant typhus, which came into great prominence in 1685-86, in circumstances that seemed to contemporaries to forebode a return of the plague. But before we come to that, there remains a little to be said of some other fevers, especially of the comatose fever of 1673-76, which was largely an affair of childhood.