220 ANIMAL BIOLOGY 



The difficulties came from two chief sources. In the first place, 

 Redi himself had been baffled by the presence of parasites within 

 certain internal organs of higher animals, such as the brains of 

 sheep. How did they get there if not by spontaneous generation? 

 The answer to this had to await the working out of the marvel- 

 lously complex life histories of the parasitic worms and allied 

 forms which showed that they all arise from parents like them- 

 selves. In the second place, improvements in microscope lenses 

 contributed to the discovery of smaller and smaller living creatures. 

 Countless numbers and myriads of kinds of "animalcules" ap- 

 pearing almost overnight in decaying organic infusions aroused 

 widespread interest and amazement, and proved to be the chief 

 riddle. The plausible explanation seemed to be spontaneous gen- 

 eration. (Figs. 14, 18, 26, 251, 252.) 



Among others, Needham studied the problem and believed that 

 he had demonstrated the spontaneous origin of minute organisms 

 in infusions that he had boiled and sealed in flasks. His results 

 attracted considerable attention because the famous French nat- 

 uralist, Buffon, found in them support for his theory that the bodies 

 of all organisms are composed of indestructible living units, which 

 upon the death of the individual are scattered in nature and later 

 brought together again to form the units of new generations. 



Needham's and similar results, however, were shown by Spal- 

 lanzani and others to be inconclusive because they were obtained 

 by insufficient sterilization and sealing of the flasks containing the 

 infusions. But at this point objections came from another source: 

 the chemists who had recently discovered the important part 

 played by free oxygen for life processes and for the putrefaction 

 and fermentation of organic substances. They argued that the 

 treatment to which Spallanzani and other experimenters subjected 

 the infusions might well have changed the organic matter, ex- 

 cluded oxygen, etc., so that it was impossible for life to be produced. 



This objection was met by a long series of experiments by various 

 investigators during the first half of the last century, which showed 

 conclusively that thoroughly sterilized infusions never developed 

 living organisms even when air was admitted, provided the latter 

 had been rendered sterile by heat or by having all suspended 

 'dust' particles removed. Thus the chemists were answered — 

 the infusions possessed all the conditions necessary to support life 

 — but life did not arise. The biologist who contributed most to 



