ix PROBLEM LIMITED TO MICROSCOPIC FORMS 97 



If he doubts of this let him go to Egypt, and there he will 

 find the fields swarming with mice, begot of the mud of 

 Nylus, to the great calamity of the inhabitants." 



As accurate inquiries into these matters were made, the 

 number of cases in which equivocal generation was sup- 

 posed to occur was rapidly diminished. It was a simple 

 matter when once thought of to prove, as Redi did in 

 1638, that no maggots were ever "bred " in meat on which 

 flies were prevented by wire screens from laying their eggs. 

 Far more difficult was the task, also begun in the seventeenth 

 century, of proving that parasites, such as tape-worms, arise 

 from eggs taken in with the food ; but gradually this pro- 

 position was firmly established, so that no one of any 

 scientific culture continued to believe in the abiogenetic 

 origin of the more highly organized animals any more than 

 in showers of frogs, or in the origin of geese from 

 barnacles. 



But a new phase of the question was opened with the in- 

 vention of the microscope. In 1683, Anthony van Leeuwen- 

 hoek discovered Bacteria, and it was soon found that however 

 carefully meat might be protected by screens, or infusions by 

 being placed in well-corked or stoppered bottles, putrefaction 

 always set in sooner or later, and was invariably accom- 

 panied by the development of myriads of bacteria, monads, 

 and other low organisms. It was not surprising, considering 

 the rapidity with which these were found to make their 

 appearance, that many men of science imagined them to be 

 produced abiogenetically. 



Let us consider exactly what this implies. Suppose we 

 have a vessel of hay-infusion, and in it a single Bacterium. 

 The microbe will absorb the nutrient fluid and convert it 

 into fresh protoplasm : it will divide repeatedly, and, its 

 progeny repeating the process, the vessel will soon con- 



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