166 PRINCIPLES OF ANIMAL BIOLOGY 



were few men fitted by reason of their learning and . temperament to 

 observe carefully and to plan and execute critical experiments by which 

 these ideas might be tested. Authority ruled. Eventually, however, 

 there arose among men a spirit of inquiry which impelled them to question 

 authority and to demand verification by means of observation and experi- 

 ment. One si<eptical inquirer was the Italian Redi (Fig. 123). 



Experiments Discrediting Spontaneous Generation. — Previously 

 to Redi's time people accepted apparently without question the idea 

 that maggots were generated spontaneously in meat. Redi, as a result 

 of his observations on the actions of flies about meat, conceived the idea 

 that there might be some connection between the flies and the maggots. 

 To test the new idea he devised some experiments. He put some meat 

 in a jar and covered it with parchment. The meat putrefied but there 

 were no maggots. When he substituted a fine gauze cover for the parch- 

 ment the flies perceived the odor of meat about the dish, but since they 

 could not reach the meat they laid their eggs on the gauze where the 

 eggs hatched. By a series of experiments of this sort Redi showed how 

 organisms might arise in a variety of situations without appealing to 

 the idea of spontaneous generation. 



By the introduction, early in the 17th centurj'-, of the microscope as 

 a means of study opportunity w.as given to investigate a whole new 

 world of living things whose existence could only have been suspected. 

 Leeuwenhoek (Fig. 6) discovered bacteria and Protozoa about 1683. 

 Many investigators believed that these minute things were the beginnings 

 of more complex organisms which were in process of originating de novo. 

 These ideas were retained with considerable tenacity for a hundred 

 years or more, until experiments were performed which rendered them 

 untenable. 



Spallanzani, 1777, objected to the methods employed in making 

 the cultures of microorganisms used in support of the theory of spontane- 

 ous generation. He filled flasks with culture solutions in which bacteria 

 and other low organisms were supposed to originate, boiled the contents 

 and hermetically sealed the flasks. These flasks were then subjected 

 to conditions thought to l)e favorable to the formation of new living 

 things, but no organisms appeared. When, in response to criticisms of 

 his method, Spallanzani tapped the glass making a slight fracture in 

 order to let in the air, organisms always appeared. No more critical 

 experiments seem to have been performed until about 1836 when Schultze 

 perfected a method for admitting air to the culture medium. He drew 

 the air through a scries of tulx^s containing acids or strong alkalies which 

 he believed would kill any organisms in the air. Schwann in 1837 and 

 others used still other methods of removing possible organisms from the 

 air before allowing it to come in contact with the culture medium. Cot- 

 ton wool was used by some to plug up the containers. Pasteur (Fig. 124) 



