disturbances; and we might certainly expect secondary disturbances in the harmonic processes of the whole metabolism of the host, without the micro-organisms as such exerting any direct action.
Finally, we have to consider yet another possibility, that certain micro-organisms produce poisonous substances within themselves, and give them off externally. It is, at present, very problematical, what view we are to take of these substances. Are they substances which play a part in the metabolism of the micro-organisms themselves, or are we dealing with agents which, when passed to the exterior, affect the nutritive medium in some way or another, that is, by either decomposing or reconstituting it? It is quite conceivable that certain micro-organisms have agencies at their disposal, which are able to modify particular nutritive cultures in a particular way. Many pathological observations have proved, that certain micro-organisms require a so-called mixed infection, that is, that certain bacteria alter the cell substance of the host in such a way, that some other kind of bacterium finds conditions favourable for its existence. It seems that this is also the case with some kinds of tumours, such as sarcoma and carcinoma, where a preparation of the medium by certain substances is of great importance. In the future we shall be compelled to appreciate all these possibilities. If we could succeed in limiting more precisely the
