32 LECTURE iir. 



conceivably minute and light ova will be raised with the dust by the 

 first pufF of wind, diffused through the atmosphere, and may there 

 remain long suspended ; forming, perhaps, their share of the particles 

 which we see flickering in the sunbeam, ready to fall into any 

 collection of water, beaten down by every summer shower into the 

 streams or pools which receive or may be formed by such showers, 

 and, by virtue of their tenacity of life, ready to develope themselves 

 wherever they may find the requisite conditions for their existence. 



The possibility^ or, rather, the high probability, that such is the 

 design of the oviparous generation of the Infusoria, and such the 

 common mode of the diffusion of their ova, renders the hypothesis of 

 equivocal generation, which has been so frequently invoked to 

 explain their origin in new-formed natural or artificial infusions, 

 quite gratuitous. If organs of generation might, at first sight, seem 

 superfluous in creatures propagating their kind by gemmation and 

 spontaneous fission, equivocal generation is surely still less required 

 to explain the origin of beings so richly provided with the ordinary 

 and recognised modes of propagation. Many experiments have, 

 however, been detailed, in which adequate precautions appeared to 

 have been taken to prevent the possibility of the entry of fertile 

 germs into the fluid experimented on, after means had been taken to 

 destroy all that it might contain. From these experiments, the mere 

 access of atmospheric air, light, and heat to the infusions has been 

 deemed to include all the conditions required for the primary form- 

 ation of animal or of vegetable organisms. The results in favour of 

 such a view are, however, explicable by supposing that due pre- 

 cautions had not been adopted at the beginning of the experiment to 

 exclude every animal or germ capable of development in the infusion, 

 or to gain satisfactory assurance that the air subsequently admitted 

 contained nothing of the kind. The only experiment in which these 

 difficulties appear to have been fully overcome, is that in which the 

 requisite apparatus was conceived by Professor Schulze of Berlin. 

 He filled a glass flask half full of distilled water, in which were mixed 

 various animal and vegetable substances: he then closed it with a 

 good cork, through which were passed two glass tubes, bent at right 

 angles, the whole being air-tight : it was next placed in a sand bath, 

 and heated until the water boiled violently. While the watery vapour 

 was escaping by the glass tubes, the Professor fastened at each end 

 an apparatus which chemists employ for collecting carbonic acid : 

 that at the one end was filled with concentrated sulphuric acid, and 

 the other with a solution of potash. By means of the boiling heat, it 

 is to be presumed that every thing living and all germs in the flask 

 or in the tubes were destroyed ; whilst all access was cut off by the 



