the case of the cholera microbe owing to its special liability to variation, both in its morphological and in its pathogenic characters. On this subject Haffkine remarks: " When the cholera bacillus was first discovered its properties were described with extreme precision, which helped in concentrating for a long time all studies on well-defined and carefully chosen specimens. Little by little, as the field of observation grew larger, a number of varieties have been found with characteristics differing so largely as to annihilate almost completely the original description. When we open the intestine of deceased cholera patients and investigate the microbes there, the adopted methods will bring to the surface vibrios in which the external forms, instead of the characteristic comma or spirillum, will vary between a coccus and a straight thread; the number and disposition of the cilia, the secretion of acids, the form of growth in broth, will vary; instead of giving in gelatin a discrete and well-defined figure of liquefaction, the variation will extend from the complete loss of this property to a rapid dissolution of the whole medium; there will be varieties which grow luxuriantly in given media, and others which do not grow there at all; some will be phosphorescent in the dark, and others not; some will give the indol reaction, and others will be deprived of this property, —and so on. The first thing to be done is to select carefully among these the most typical specimens, rejecting the others, and then to try their pathogenic power. We shall find such a divergence in strength that the extreme forms will not be believed to be the cholera species. There will be commas deprived of any virulence demonstrable on animals, and others which will kill the most resistant species. Some will be fatal to a guineapig at a dose of 1/100 of a culture tube, and others harmless in doses 500 times stronger. The average comma dies out when introduced under the skin of an adult animal; others will spread in the system and give rise to a fatal septicaemia. The ordinary comma will be without effect on birds; but several specimens have been isolated, and believed to be typical, which easily killed pigeons by hypodermic
