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2, — It is now a couple of centuries since Leeuwenhoek 

 communicated to the Royal Society an account of what was 

 apparently the first discovery of bacteria. In a letter to 

 Nature Professor Cohn, a few months ago, called the 

 attention of the scientific world to the fact that towards the 

 close of 1083, Leeuwenhoek announced the discovery of 

 active microscopic organisms, including bacilli, bacteriums, 

 and vibrions. 'J'he Dutch microscopist discovered them in 

 the white substance adhering to his teeth, and bearing in 

 mind the important part which thermal conditions have 

 plaj^ed, or have been supposed to play in recent experiments 

 on the sterilisation of "culture" infusions and the "attenu- 

 ation" of microbes, it is worth while to note that, failing on 

 a subsequent occasion to perceive the movements of bacteria 

 in the same substance, he assumed that they had been killed 

 by the hot coffee which he had taken at breakfast. The 

 discovery not only gave a new vitality to the discussions on 

 spontaneous generation, both sides finding therein arguments 

 in favour of their special opinions, but also suggested ideas 

 as to the propagation of contagious diseases and the nature 

 of infection. In the papers communicated to the Hoyal 

 Society during the last quarter of the 17th and the 1st 

 quarter of the 18th centuries we find most of the ideas which 

 are still the leading ideas of micro-biologists. With refer- 

 ence to generation in general it is argued that the animalcule 

 is the germ furnished only by the male, and that the 

 female merely supplies the nidus requisite for its develop- 

 ment. Springing from this hypothesis we have the 

 suggestion that the nidus affects or modifies the germ, which 

 I take to imply that, given the germ, the nidus in which it 

 is implanted determines the species evolved therefrom. 

 Experiments were made with rain water, mineral water, 

 infusions of pepper-corns, bay-berries, oats, barley, and 

 wheat, and the scum collected from these infusions was 

 discovered to be masses of organisms. Round and elongated 



