One of the objections that has been made regarding the acceptance of this organism as the sole cause of cholera has been the difficulty of reproducing in animals, by injections of the spirillum, a disease that generally resembled human cholera. Haffkine adopted the ingenious plan of using diluted serum obtained from rabbits as the medium for the development of the spirillum, and transferring a few drops from this to a less diluted serum, and so on until the micro-organism lived in the undiluted serum. If the spirillum thus acclimatized, so to speak, be injected into the blood-vessels of a healthy rabbit, the animal will die with all the symptoms of cholera; this serum might be called a virulent culture. By passing an ordinary bouillon culture of the spirillum through a series of guinea-pigs he could also obtain a virulent culture that rapidly killed if injected into the abdominal cavity, but that, when injected under the skin, produces phenomena similar to those described by Ferran, and results, as the latter stated, in rendering the animal immune against inoculation with cholera in any strength whatever. Rabbits and pigeons were rendered immune in the same way. These results induced Haffkine to try the inoculations on himself and seven other persons; the phenomena observed in each of these individuals were similar to those reported by Ferran, and a second inoculation made after an interval of seven or eight days produced far less general and local disturbance than the first. While differing slightly in the methods employed, all these later experimenters, it may be seen, have confirmed Ferran's original report, although none of them mentions his name or the priority of his discovery.
What is the value of these inoculations? This may in part be answered by the question, What is the harm? All observers concur in stating that in animals, as well as in man, the inoculations, made in moderate doses, are harmless beyond producing a slight local irritation and temporary malaise. And any one who has been subjected to the inoculations can easily determine at any future period whether he is then protected, by receiving another inoculation; just as a later determines the protection of the individual by an earlier vaccination.
It is necessarily conceded that the factors that enter into the question of natural are not those of artificial infection. But it is necessary to recall the facts stated in the first portion of this paper; and it is seen that there are good grounds for believing that immunity against natural infection is correlated with immunity against artificial infection. There may be no exact and absolute demonstration of this fact, for no vaccinated person willingly associates with a small-pox patient, or has himself inoculated with matter from a small-pox sore, in order to determine his immunity toward that disease. The vital statistics of this cen-
