THE ORIGIN OF LIFE 7 



in the food and has sealed the jar so tightly that no more 

 microorganisms can get in. But how would she feel if she 

 believed that these tiny creatures could spring to life spon- 

 taneously in any organic material without having to get in 

 from the outside? Naturally she would think her canning 

 might be unsuccessful no matter how carefully the work 

 was done. 



And yet that is just what was believed in the eighteenth 

 century. It was not supposed that these tiny organisms 

 needed any living parent ; in other words, scientists believed 

 in spontaneous generation. They thought, in fact, they were 

 being very conservative in limiting this possibility to mi- 

 croscopic organisms; for at an earlier day it was generally 

 believed that even moderately large animals could come into 

 existence without living parents. In the ancient days we 

 find even intelligent writers giving directions for producing 

 mice. All you had to do was to put a piece of cheese in a 

 dark cellar, cover it with rags, and in a few days the cheese 

 would have turned into mice! The results of this experiment 

 would often be the same to-day if we selected a cellar not 

 too tightly constructed, although we should have a slightly 

 different explanation as to where the mice came from. As 

 a matter of fact, long before the eighteenth century all 

 serious^minded people had given up the idea that creatures 

 as large as mice, eels, or worms could arise spontaneously; 

 but only very recently it had been believed that meat turned 

 into maggots. It took a very clever scientist to think of 

 keeping meat screened so that flies would not get at it and 

 then to observe that the maggots did not appear ; this same 

 scientist (Francesco Redi) studied the maggots and dis- 

 covered that they were immatiure flies. From that time on 



