LIFE AND LIVING THINGS 27 



and after boiling. Spallanzani, in the middle of the 

 eighteenth century, wrote: "I used hermetically sealed 

 vessels. I kept them for an hour in boiling water, and 

 after opening and examining their contents after a reason- 

 able interval, I found not the slightest trace of animalcule, 

 though I examined with the microscope the infusions from 

 nineteen different vessels." The question was still dis- 

 cussed, however, and even as late as 1858 a French scientist 

 read a paper before the National Academy in which he 

 announced that animalcule had been raised in boiled in- 

 fusions exposed only to " artificial air," or oxygen. He 

 maintained that the organisms could not have arrived as 

 air-borne particles. 



This paper excited the interest of Pasteur and led to the 

 work which finally laid the idea of spontaneous generation 

 to rest. "He showe'd that sterilized cultures always be- 

 came infected when exposed to air; that properly filtered or 

 sterilized air never caused infection; that Alpine air almost 

 free from germs scarcely ever produced a growth of organ- 

 isms; that city air nearly always produced contamination; 

 and that, in absence of added germs from without, culture 

 media remained sterile for years. The sources of error in 

 the work of his opponents were elucidated, and their con- 

 trary results explained on such grounds." Tyndall alfe'# 

 furnished evidence of a physical nature which corroborated, 

 the discoveries of Pasteur. He was trying to get a beam of 

 light which was perfectly free from dust particles and made 

 an apparatus in which the light was passed through win- 

 dows in an air-tight box. When the box was absolutely 

 dust-free there was no fermentation within it, thus demon- 

 strating that the origin of bacteria and similar organisms 

 was due to air-borne particles. 



At the present time, the only generalization we can make 

 concerning the origin of living things is, "all life comes from 

 life." Every living organism originates from a preexisting 

 individual of its own sort. This conclusion is unsatisfying, 



