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 consist of innumerable spores, which have been thrown off in all directions by a minute fungus called Empusa muscoe, the spore-forming filaments of which stand out like a pile of velvet from the body of the fly.  These spore-forming filaments are connected with others which fill the interior of the fly’s body like so much fine wool, having eaten away and destroyed the creature’s viscera.  This is the full-grown condition of the Empusa.  If traced back to its earliest stages, in flies which are still active, and to all appearance healthy, it is found to exist in the form of minute corpuscles which float in the blood of the fly.  These multiply and lengthen into filaments, at the expense of the fly’s substance; and when they have at last killed the patient, they grow out of its body and give off spores.  Healthy flies shut up with diseased ones catch this mortal disease, and perish like the others.  A most competent observer, M. Cohn, who studied the development of the Empusa very carefully, was