204 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



unnatural in view of the sudden appearance of innumerable minute 

 forms of life as often observed in laboratory cultures. Some 

 biologists, from Redi onward, reasoning by analogy with higher 

 organisms, always believed that such micro-organisms arose from 

 preexisting forms, although many clung to the idea of spontaneous 

 origin. The process by which hfe was assumed to arise sponta- 

 neously may be called abiogenesis, in contrast to biogenesis or the 

 genesis of life from hfe. The persistent behef in the possibility of 

 abiogenesis led Spallanzani, in 1775, and Schwann, in 1837, to 

 perform extensive experiments, the results of which were against 

 the theory of spontaneous origin. In spite of these repeated fail- 

 ures to find any positive evidence for abiogenesis, the question was 

 reopened on theoretical grounds by Pouchet in 1859. 



Final Establishment of Biogenesis. — The work of Pasteur and 

 others was stimulated by this final recrudescence of the idea of 

 spontaneous generation, as induced by Pouchet and his followers. 

 Then came the series of brilhant researches by this great French- 

 man, by the German, Koch, and by others, which finally showed 

 how even the smallest organisms arise by cell division from parent 

 forms. Encysted stages of protozoa and spore stages of bacteria 

 were recognized and followed stage by stage until the hfe cycles of 

 representative types were fully estabhshed alike in their active and 

 in their resting stages. The English physicist, Tyndall, in the 

 course of his investigations upon light, studied the " floating matter 

 of the air " and showed that it teemed with spores and other 

 resistant stages which needed only to settle upon a proper medium 

 in order to germinate (Fig. 107). The English surgeon, Lister, 

 and others who investigated the germ theory of disease as applied 

 to surgery, established the fact that the germs found in wounds 

 and in specific diseases were not generated spontaneously when 

 conditions became right for them within the animal body, but were 

 introduced into it as the spores or active stages of such minute 

 organisms might be introduced into a sterile culture medium. The 

 progressive extension of such demonstrations and the further exten- 

 sion of the Cell Doctrine to the origin of higher organisms com- 

 pleted the overthrow of abiogenesis and established biogenesis, 

 or the origin of hving organisms from preexisting organisms, as 

 the true explanation of the source of new individuals, although 

 there will always remain the theoretical possibihty that proto- 

 plasm may be synthesized under conditions of laboratory experi- 



