at cholera can be developed _de novo_,
but that the type is unstable, and that a virulent form may be evolved
under favourable conditions from another so mild as to be unrecognized,
and consequently undetected in its origin or introduction. This is quite
in keeping with the observed variability of the micro-organism, and with
the trend of modern research with regard to the relations between other
pathogenic germs and the multifarious gradations of type assumed by
other zymotic diseases. The same thing has been suggested of diphtheria.

Epidemicity.

Cholera is endemic in the East over a wide area, ranging from Bombay to
southern China, but its chief home is British India. It principally
affects the alluvial soil near the mouths of the great rivers, and more
particularly the delta of the Ganges. Lower Bengal is pre-eminently the
standing focus and centre of diffusion. In some years it is quiescent,
though never absent; in others it becomes diffused, for reasons of which
nothing is known, and its diffusive activity varies greatly from equally
inscrutable causes. At irregular intervals this property becomes so
heightened that the disease passes its natural boundaries and is carried
east, north and west, it may be to Europe or beyond to the American
continent. We must assume that the micro-organism, like those of other
epidemic diseases, acquires greater vitality and toxic energy, or
greater power of reproduction at some times than at others, but the
conditions that govern this behaviour are quite unknown, though no
problem has a more important bearing on public health. Bacteriology, as
already intimated, has thrown no light upon it, nor has meteorology.
Some results of modern research, indeed, tend to assign increasing
importance to the relations between surface soil and certain
micro-organisms, and suggest that changes in the level of the subsoil
water, to which Professor Max von Pettenkoffer long ago drew attention,
may be a dominant factor in determining the latency or activity of
pathogenic germs. But this is largely a matter of conjecture, and, so
far as cholera is concerned, the conditions which turn an endemic into
an epidemic disease must be admitted to be still unknown.

On the other hand, the mode of dissemination is now well understood.
Diffusion takes place along the lines of human intercourse. The poison
is carried chiefly by infected persons moving from place to place; but
soiled clothes, rags and other