GENERAL ZOOLOGY 



the meat and to grow as they consumed the meat. The pupal stage and the 

 emergence of the adult flies were observed. Maggots also hatched from eggs 

 transferred from the gauze to the meat. The meat in the parchment-covered 

 jars decomposed without the appearance of maggots. Redi made other obser- 

 vations on the development of insects and reached the conclusion that all 

 spontaneous generation was presumably due to the introduction of living 

 "germs" from without. 



In 1676, the Hollander, Antony van Leeuwenhoek, discovered with the 

 microscope, which had recently come into use as a scientific instrument, what 

 he described as "little animals observed in rain, well, sea, and snow water 

 as also in water wherein pepper had lain infused." Among other forms of 

 life he observed some of the larger bacteria and many protozoans. During 

 the eighteenth century the observations of Leeuwenhoek were extended by 

 other workers until the important types of microscopic animals became 

 known. Although larger organisms were seen to arise from eggs or seeds, it 

 could still be believed that microorganisms arose spontaneously if conditions 

 were suitable. This belief was not unnatural in view of the sudden appear- 

 ance of these forms in the great numbers often observed in laboratory cultures. 

 Some biologists from Redi onward, reasoning by analogy with higher organ- 

 isms, believed that microorganisms arose from pre-existing forms. Others 

 clung to belief in spontaneous generation. In spite of repeated failures to 

 find evidence of abiogenesis, the question was reopened on theoretical grounds 

 by Pouchet in 1859. 



Final Establishment of Biogenesis. The work of Louis Pasteur (1822 

 1895) and his contemporaries, about 1860 1864, was stimulated by this re- 

 opening of the problem. A series of brilliant researches by this great French- 

 man, by the CJerman Koch (1843-1910), and by others finally showed that 

 even the smallest organisms arise by division from parent forms. Species of 

 protozoans and of bacteria were followed stage by stage until the life cycles 

 of representative types were known in their active and in their resting phases. 

 The English physicist, Tyndall, during investigations upon light about 1876, 

 studied the "floating matter of the air" and showed that it teems with spores 

 and other resistant stages of microorganisms which need only settle upon a 

 proper medium to germinate. The English surgeon, Lister (1827 1912), and 

 others who investigated the germ theory of disease as applied to surgery 

 demonstrated that the germs found in wounds are not generated within the 

 body but are introduced, as the spores or the active stages of such minute 

 organisms may be introduced into a sterile culture medium. The extension 

 of these demonstrations and of the Cell Theory completed the overthrow of 

 abiogenesis and established biogenesis as the true explanation of the origin of 

 new individuals. The saying of an earlier time, omne viimm ex ovo, every 

 living thing from an egg, and a later one, omms cellula e cel/ula, every cell 

 from a cell, express the facts as now established. 



The long controversy over biogenesis was related throughout its history to 

 the observation that infectious diseases spread and multiply like living organ- 



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