This very extraordinary fact is, then, established that the virulence may be entirely gone while yet the microbe lives. The cultures offer the spectacle of a microbe indefinitely cultivable, yet, on the other hand, incapable of living in the bodies of fowls, and in consequence deprived of virulence. May not this domesticated microbe, as M. Bouley calls it, be compared to those inoffensive microbes of which there are so many in nature? May not our common microbes be those organisms which have lost their former virulence? But may not these harmless microbes, become infectious in some particular circumstances? And if there are fewer virulent maladies now than there were in times past, might not the number of these maladies again increase? Questions multiply as the facts relating to the attenuation of a virus suggest inductions, awaken ideas, and throw new lights upon a problem which, until within these last few years, has remained so obscure. Formerly it was believed that these viruses were morbid entities. A virus was a unity. This opinion has still its declared upholders. According to Pasteur a virus has different degrees of virulence; it can pass from the weakest virulence to the maximum. Modifying, at will, the virus of fowl cholera, Pasteur inoculates some hens, for instance, with a virus too attenuated to protect from death, but which nevertheless is effectual in securing them against a virus stronger than itself. The second virus will preserve them from the attacks of a third virus, and thus passing from virus to virus they end by being guaranteed against the most deadly virulences. The whole question of vaccination resolves itself into knowing at what moment a certain degree of virus attenuation is a guarantee of protection against the mortal virus. It seems that between small-pox and cow-pox facts of a similar kind take place. It is probable that vaccination rarely gives perfect security against the infection of a very malignant small-pox; moreover, during epidemics of small-pox many persons who have been previously vaccinated are attacked, and some even die of the disease. As regards the practice of vaccinating fowls against the cholera peculiar to them--which, though it certainly is not of the same importance as human vaccination, is a scientifically _capital_ fact--we may hope that whatever the differences of receptivity in different races, or in different individuals of the same race, there will be found vaccines to suit them all, special care being taken to resort to the employment of two successive vaccines of unequal power, employed after an interval of ten or fifteen days. The first vaccine may always be chosen of a degree of weakness which will not in any case cause death, and yet of sufficient strength to prevent dangerous consequences from the second vaccine, which would in some cases be fatal if employed at once, and to enable it to act as a vaccine against the most virulent virus.