SPONTANEOUS GENERATION OF MICROBES 19 



the more highly organised things ceased to be held among 

 scientists. But this idea as to the primary origin of living 

 things did not disappear. On the contrary, during the eigh- 

 teenth and nineteenth centuries it reached its fullest develop- 

 ment in connection with the simplest living things, the 

 micro-organisms. 



Hypotheses concerning the spontaneous 

 generation of microbes. 



Almost at the same time as Redi was carrying out his 

 celebrated experiments, a new world of living creatures 

 invisible to the naked eye was opened up by the Dutch 

 scientist Anthony van Leeuw^enhoek (1632-1723), with the 

 help of magnifying glasses made Avith his own hands. In 

 letters to the Royal Society in London he described in detail 

 these small ' living animalcules ' discovered by him in rain 

 water which had stood for a long time in the air, in various 

 infusions, in excrement, in the tartar of teeth, etc. With his 

 glass van Leeuwenhoek saw representatives of almost all the 

 classes of micro-organism known to us at the present day. 

 He gave descriptions, ^\"hich were surprisingly accurate for 

 those times, of infusoria, yeasts, bacteria, etc.^° 



The curious discoveries of the Dutch scientist attracted the 

 most general attention and provoked many similar studies. 

 Micro-organisms -^vere discovered wherever decay or fermenta- 

 tion of organic substances was going on. They were foimd 

 in different sorts of plant infusions and decoctions, in decay- 

 ing meat, in stale broth, in sour milk, in fermenting wort 

 etc. Substances which quickly become tainted or which 

 decay easily had only to be kept in a warm place for some 

 time when microscopic living things, which had not been 

 there before, at once began to develop in them. As the belief 

 in the spontaneous generation of living things was current 

 at the time, it was unhesitatingly assumed that it extended 

 to cover the spontaneous generation of living microbes from 

 inanimate matter in these decoctions and infusions. 



Van Leeuwenhoek himself did not propose this idea. He 

 maintained that the micro-organisms fell into his infusions 

 from the air. This opinion was confirmed by the experiments 



