t the deeper
investigators went the more difficult the problem became, while M. Elie
Metschnikoff of the Pasteur Institute made a similar admission. The
difficulty lies chiefly in the variable characters assumed by the
organism and the variable effects produced by it. The type reached by
cultivation through a few generations may differ so widely from the
original in appearance and behaviour as to be hardly recognizable,
while, on the other hand, of two organisms apparently indistinguishable
one may be innocuous and the other give rise to the most violent
cholera. This variability offers a possible explanation of the frequent
failure to trace the origin of epidemic outbreaks in isolated places. It
is commonly assumed that the micro-organism is of a specific character,
and always introduced from without, when cholera appears in countries or
places where it is not endemic. In some cases such introduction can be
proved, and in others it can be inferred with a high degree of
probability, but sometimes it is impossible to trace the origin to any
possible channel of communication. A remarkable case of this kind
occurred at the Nietleben lunatic asylum near Halle, in 1893, in the
shape of a sudden, explosive and isolated outbreak of true Asiatic
cholera. It was entirely confined to the institution, and the peculiar
circumstances enabled a very exact investigation to be made. The facts
led Professor Arndt, of Greifswald, to propound a novel and interesting
theory. No cholera existed in the surrounding district and no
introduction could be traced, but for several months in the previous
autumn diarrhoea had prevailed in the asylum. The sewage from the
establishment was disposed of on a farm, and the effluent passed into
the river Saale above the intake of the water-supply for the asylum.
Thus a circulation of morbid material through the persons of the inmates
was established. Dr Arndt's theory was that by virtue of this
circulation cholera was gradually developed from previously existing
intestinal disease of an allied but milder type. The outbreak occurred
in winter, and coincided with the freezing of the filter-beds at the
waterworks. The theory is worth notice, because a similar relation
between the drainage and the water-supply frequently exists in places
severely attacked by cholera, and it has repeatedly been observed that
the latter is preceded by the prevalence of a milder form of intestinal
disease. The inference is not th