Pasteur had proved that viruses are not morbid entities, that they can assume numerous forms, and especially physiological properties, dependent on the medium in which they live and multiply. The virulence belongs to living microscopic species, but is at the same time essentially modifiable. It may be weakened or intensified, and each of these states is capable of being made permanent by culture. A microbe is virulent in an animal, when it has the power of swarming in the body of that animal, after the manner of a parasite, and of producing, by the renewal of its own life, disturbances which cause disease and death. If this microbe has lived in any species of animal--that is to say, if several times over it has passed from the body of one individual into that of another of the same kind, without having been subjected to any sensible exterior influence during its passage--we may consider that the virulence of this parasite has reached a fixed and maximum state for the individuals of that race. The splenic fever parasite pertaining to sheep, for instance, varies little from one subject to another or from one year to another in the same country; this must be attributed, doubtless, to the fact that, in its successive passages through the sheep, the habit of the parasite to live in sheep has, so to speak, attained a definite state. It is thus with the virus of the Jennerian vaccination. But the virulence of a virus which is not at its maximum may be essentially modified by its passage into a succession of individuals of the same race. It will be remembered how, when Pasteur and his assistants wished to increase progressively the virulence of the virus of chicken cholera and splenic fever, so as to bring them at last to their maximum intensity, these viruses were first inoculated into young subjects, and from them successively into older ones. 'The Academy remembers, without doubt,' said Pasteur in a recent communication, 'that, some time ago, we discovered a microbe virus in the saliva of hydrophobia. This microbe, though very virulent for rabbits, is shown to be harmless for adult guinea-pigs, but it kills rapidly guinea-pigs only some hours or days old. In following out this inoculation from young guinea-pigs, we have seen the virulence increase, and easily arrive at the point of causing death to older guinea-pigs. There was even at last a marked difference in the lesions. The increase of virulence, by successive passages through individuals of one race, was clearly shown. 'But the new and unexpected result that I wish to point out to the Academy consists in this: that the microbe, after having increased its virulence by successive passages through the bodies of guinea-pigs, shows itself to be less virulent in relation to rabbits than it was before.