their propagation upon extremely small
living solid particles, to which the title of _microzymes_ is applied. An
animal suffering under either of these terrible diseases is a source of
infection and contagion to others, for precisely the same reason as a tub
of fermenting beer is capable of propagating its fermentation by
"infection," or "contagion," to fresh wort. In both cases it is the solid
living particles which are efficient; the liquid in which they float, and
at the expense of which they live, being altogether passive.

Now arises the question, are these microzymes the results of
_Homogenesis_, or of _Xenogenesis?_ are they capable, like the
_Toruloe_ of yeast, of arising only by the development of pre-existing
germs? or may they be, like the constituents of a nut-gall, the results
of a modification and individualisation of the tissues of the body in
which they are found, resulting from the operation of certain conditions?
Are they parasites in the zoological sense, or are they merely what
Virchow has called "heterologous growths"? It is obvious that this
question has the most profound importance, whether we look at it from a
practical or from a theoretical point of view. A parasite may be stamped
out by destroying its germs, but a pathological product can only be
annihilated by removing the conditions which give rise to it.

It appears to me that this great problem will have to be solved for each
zymotic disease separately, for analogy cuts two ways. I have dwelt upon
the analogy of pathological modification, which is in favour of the
xenogenetic origin of microzymes; but I must now speak of the equally
strong analogies in favour of the origin of such pestiferous particles by
the ordinary process of the generation of like from like.

It is, at present, a well-established fact that certain diseases, both of
plants and of animals, which have all the characters of contagious and
infectious epidemics, are caused by minute organisms. The smut of wheat
is a well-known instance of such a disease, and it cannot be doubted that
the grape-disease and the potato-disease fall under the same category.
Among animals, insects are wonderfully liable to the ravages of
contagious and infectious diseases caused by microscopic _Fungi_.

In autumn, it is not uncommon to see flies motionless upon a window-pane,
with a sort of magic circle, in white, drawn round them. On microscopic
examination, the magic circle is found to consis