26 GENERAL ZOOLOGY 



maggots. Van Helmont, a deservedly eminent scientist of 

 the sixteenth century, was a firm believer in such spon- 

 taneous generation, and soberly stated that mice could be 

 produced by placing some dirty linen in a receptacle, to- 

 gether with a few grains of wheat or a piece of cheese. 

 Such crude ideas show the low level of the scientific thought 

 of the time. The belief that organisms commonly origi- 

 nated from inorganic materials or decaying substances was 

 prevalent. 



The first critical experiments which helped to do away 

 with the belief in spontaneous generation were performed 

 by an Italian poet and physician, Franchesca Redi. He 

 demonstrated clearly that fly maggots were not engendered 

 spontaneously in spoiled meat, but developed from eggs 

 deposited by flies. He set three jars on his window sill- 

 one was uncovered; one covered with gauze so that the air 

 might enter freely but no flies could come in; the other was 

 corked tightly. Though the meat in all the bottles spoiled, 

 maggots appeared only in the uncovered jar. The flies 

 were seen to lay their eggs, and the hatching of maggots 

 was observed. Redi's epoch-making discovery seems very 

 simple to us, but it aroused a storm of discussion and dis- 

 pute in his time. 



It would perhaps be expected that the invention of the 

 microscope would at once have furnished the means to dis- 

 prove spontaneous generation, but it had the opposite 

 effect. This instrument opened up a new world of minute 

 " animalculae," and with these the advocates of spon- 

 taneous generation made their last stand. It was found 

 that if dry hay was put in water, cooked to destroy any 

 living germs which might be present, and allowed to stand 

 for a day or two, that the " infusion" was filled with myriads 

 of " animalcules." This experiment was believed to prove 

 that the organisms were generated from the disintegrating 

 hay. Later, however, it was urged that the animalcules 

 might enter the infusion from the air. Experiments were 

 tried in which infusions were sealed up in tubes, both before 



