578 REPRODUCTION. 



criminately from the organic materials of the infusion ; the particular 

 form which they might assume in any case being determined by the 

 physical conditions present. Their inevitable appearance in organic 

 infusions, under ordinary temperatures and exposures, contributed 

 to this belief. The substance of the infusion might be previously 

 baked or boiled ; the water in which it was infused might be purified 

 by distillation from all organic contamination ; and yet infusoria would 

 make their appearance at the usual time and in the usual abundance, 

 provided the infusion were exposed to moderate warmth and to the 

 access of air. But these conditions are essential to all organic life, and 

 are not, therefore, especially requisite for infusoria. 



Consequently, the infusoria must either have been spontaneously 

 generated from the materials of the infusion, or else their germs must 

 have been introduced from without In the latter case these germs 

 must be wafted about by the atmospheric currents, in a comparatively 

 dry and inactive condition, to resume their development when brought 

 in contact with moisture and the requisite organic material. 



The researches relating to this question continued from 1775, when 

 they were carried on by Needham and Spallanzani, throughout the 

 greater part of the present century, in the hands of Cuvier^ Schultze, 

 Helmholtz, Milne-Edwards, Longet, Pouchet, Pasteur, Wyman, Tyn- 

 dall, and Bastian. The main object of investigation was to discover, 

 if all previous living germs were destroyed by heat, and the access 

 of others prevented by hermetically sealing the vessels or thoroughly 

 purifying the air introduced, whether, under such circumstances, infu- 

 sorial life would be developed. 



The general result of these experiments was that such precautions 

 diminished and often prevented the production of infusoria. Spallan- 

 zani* had already shown, in 1776, that organic infusions in hermeti- 

 cally sealed flasks, if boiled for two minutes, failed to produce any of 

 the larger and more highly organized animalcules ; and that boiling for 

 three-quarters of an hour prevented the appearance of the more minute 

 and simpler kinds. 



Schultze ( performed similar experiments, with the addition of admit- 

 ting to the organic infusion purified air. He placed his infusion in a 

 glass flask, the stopper of which was provided with two narrow tubes, 

 bent at right angles. When the infusion had been thoroughly boiled, 

 and all air expelled from the flask by the vapor of ebullition, he fastened 

 to each tube a series of bulbs containing on one side sulphuric acid, and 

 on the other a solution of potassium hydrate ; so that the air which 

 reentered the flask while cooling must pass through these fluids, and 

 thu? be cleansed of all living organic matter. The apparatus was then 

 kept in a warm place for two months, during which time the air was 

 renewed daily by suction through the tubes, without any infusoria being 



* Opuscoli de Fisica animale e vegetabile. Modena, 1776, vol. i., p. 10. 

 f Poggendorfs Annalen, 1836. Band xxxix., p. 487. 



