298 THE BEGINNINGS OF LIFE. 



filtered infusions exposed to the air beneath a bell-jar 

 had been derived from ova or from dried adult forms, 

 which (after dropping into the infusion) had subse- 

 quently propagated themselves and multiplied therein, 

 then such ova or dried adult forms ought to drop just 

 as freely into receptacles of distilled water presenting 

 an equal area and similarly exposed. These ova or 

 dried animals are sufficiently large to be easily dis- 

 coverable, when present; and by placing the water 

 after a time in a conical vessel, any particles which it 

 contains may be allowed to sink before the supernatant 

 fluid is slowly drawn off, either by a pipette, or, better 

 still, by a siphon of small bore. The microscopical 

 examination of the small quantity of fluid which re- 

 mains will very rarely show a trace of a ciliated In- 

 fusorium, either adult or in the form of egg. And yet, 

 if the weather has been warm, in the course of four 

 or five days the surface of the organic infusion similarly 

 exposed beneath a bell-jar will have become covered 

 with a thick pellicle, and the infusion itself, if not 

 affording an acid reaction, may be found to contain an 

 incalculable multitude of ciliated Infusoria, of one form 

 or another. 



It may well be asked, whence come these swarming 

 myriads of animalcules ? Can they have been derived 

 from certain germs floating in the limited atmosphere 

 to which the infusion has been exposed? If so, one 

 would think that it must be from a very limited 

 number, seeing that none are to be found in the 



