Page:The Harveian oration 1888.djvu/41

33 for a period somewhat less than six weeks that mice seem to be impervious to its attacks. But how it is that this attenuation takes place—whether it is due to the effect of heat alone or to the effect of oxygen—bacteriologists are by no means agreed. Further it has been shown that when the attenuation of the bacillus has been brought about in a certain way (by the addition for instance of carbolic acid or potassium bichromate to the cultivation) its virulence is not regained when cultivated in fresh material at the ordinary temperature. But, attenuated in a different fashion, it does regain its virulence under those conditions. Again the virulence of a parasitic organism is materially altered according to the animal through which it has been transmitted. "In the course of Pasteur's interesting researches on swine plague he found that pigeons inoculated with the virus of swine plague died in six or eight days after suffering in the first instance from symptoms like those of fowl cholera. If the disease is transmitted from pigeon to pigeon, the organism after a time acts more violently, and the animal dies sooner. If, now, pigs are inoculated from these pigeons, death occurs more quickly than when inoculated from a pig, the organism having become more virulent. With the rabbit the converse is the case. The virus kills rabbits, but if it is passed through a series of rabbits it is no longer able to kill pigs" (Watson Cheyne).

Now, if this is the case, if the virulence of parasitic or pathogenic organisms can be thus modified, it seems incredible that organisms of similar structure, though apparently harmless saprophytes, cannot be cultivated in some way or other so as to become pathogenic. And when the bacteriologists tell us that it is impossible, that only means, I suppose, that they have not yet discovered the way. Specific diseases and pathogenic bacteria