For a considerable period, those of a sceptical disposition asserted that the advance of bacteriological knowledge was of service only in surgical cases. But Pasteur showed that this was an erroneous view. Working with Chamberland and Roux, Pasteur demonstrated that many infectious diseases could be prevented by the use of attenuated virus; he succeeded in saving the lives of many animals and of men, bitten by rabid dogs and affected by hydrophobia, a disease formerly almost invariably fatal and among the most horrible to which man is liable. In the latter direction, medical science is developing at an extraordinary rate, and is achieving results of a remarkable nature. Among recent discoveries, I may mention that of the curative properties of the blood serum of animals which have been subjected to the action either of microbes or of the soluble products of microbes. Von Behring, working with the Kitasato, a Japanese investigator, has shown that a serum of this nature, prepared with the poison produced by the microbe of diphtheria (the poison was discovered by Roux in collaboration with Yersin), is capable not only of protecting those in good health from diphtheria, but of curing those who have been attacked by the disease. The serum fails to act only when it is employed in advanced cases of diphtheria. Anti-diphtheritic serum, introduced into medical practice about eight years ago, has been tried in every way and has been proved to possess both preventive and curative properties. If patients still die from diphtheria, it is only because the treatment has been applied too late or insufficiently. The use of the anti-diphtheritic serum has reduced the mortality in cases of diphtheria from 50 or even 60 per cent. to 12 or 14 per cent. The number of infant lives that have been saved by this method must be enormous. The beneficent discovery of the curative value of serums has been applied to other diseases and is giving very encouraging results. I cannot go into details here, but it is enough to say that in the last quarter of a century medicine has entered a new epoch, and has taken its place among other exact sciences based on the experimental method. Although it is not surprising that in so short a space of time science has not yet conquered all the ills affecting humanity, this failure has provoked the most severe criticism.