The comparative rarity of enteric fever in the chief towns of Scotland and Ireland continued for a good many years longer, indeed until after the differences between typhus and typhoid were perceived and admitted by all. Even at the London Fever Hospital, during twenty-four years (1848-71) after Sir William Jenner’s diagnostic points were strictly looked to in its wards, much the greater part of the admissions were of typhus; in only two periods, 1850-55 and 1858-61, during both of which there was comparatively little fever of any kind in London, did the admissions for enteric fever slightly exceed those for typhus; on an annual average of the twenty-four years ending 1871, the cases of the former were only about a fifth part of the whole. The cases of enteric fever increased decidedly after 1865. Murchison thought that the increase might be accounted for in part by the enlargement of the Fever Hospital, and by the unusually high temperature of certain years, the summers and autumns of 1865, 1866, 1868 and 1870 having been remarkable for their great heat and prolonged drought; but, he adds, “it is not a little remarkable that this increased prevalence of enteric fever in the metropolis has been contemporaneous with the completion of the main drainage scheme[385].” Still more recently, the relative proportions of typhus and enteric fever have been reversed, so that there have been years with little or no typhus but with a good deal of enteric fever. There are some persons, unacquainted with the history, who cannot imagine that it was ever otherwise than now, who think of the former times of medicine, not as differing in social, economic, and various other respects from their own, but only as being less clever at diagnosis. There are others who realize clearly enough the historical matter of fact, but find it necessary to explain the almost contemporaneous decline of typhus and rise of typhoid by some hypothesis of the latter being “evolved” out of the former. This evolutional doctrine makes the mistake of ascribing to the species of disease the same comparative fixity of characters that belongs to the species of animals and plants. Beside the latter, the species of disease are the creatures of a day. In the nosological field, the origin of species is not analogous to the evolution of a new species of animal or plant out of an old, as in the hypothesis of Darwin, for the reason that every species of disease is evolved directly and, as it were, _pro re nata_, out of a few simple conditions of human life, variously mixed but always there to give occasion to one infective malady or another, which may have a shorter existence, like sweating sickness, or a longer, like plague. Edinburgh experiences offer a ready criticism of the evolutional doctrine. Typhus declined, and typhoid rose; but it was in the old tenement houses of the Canongate, Cowgate, Grassmarket, and High Street that typhus declined, and it was mostly in the new streets across the