38 MANUAL OF ZOOLOGY. 



bacteria and the vibrios now exhibit a vibratile or serpentine 

 movement through the surrounding fluid. 



3. After a varying period, the bacteria and vibrios become 

 motionless, and disintegrate so as to produce again a finely 

 molecular pellicle. 



4. Little spherical bodies now appear, each of which is pro- 

 vided with a vibratile cilium'with which it moves actively 

 through the infusion. (Monas lens.) 



5. Varied forms of ciliated Infusoria some of which pos- 

 sess a mouth and are otherwise highly organised make their 

 appearance in the fluid. 



The above is the general sequence of the phenomena which 

 have been observed, and the following are the two theories 

 which have been advanced to account for them : 



a. By the advocates of spontaneous generation, or " Hetero- 

 geny," it is affirmed that the Infusoria, which finally appear in 

 the infusion, are produced spontaneously out of the molecular 

 pellicle, the molecules of which are also of spontaneous origin, 

 and are not derived from any pre-existing germs. 



b. By the " panspermists," or the opponents of spontaneous 

 generation, it is alleged, on the other hand, that the production 

 of Bacteria, Vibrios, Monads, and Infusoria, in organic infu- 

 sion, is due simply to the fact that the atmosphere, and pro- 

 bably the fluid itself, is charged with innumerable germs too 

 minute, perhaps, to be always detectable by the microscope 

 which, obtaining access to the fluid, and finding their favour- 

 able conditions, are developed into living beings. 



A large number of elaborate experiments have been carried 

 out to prove that atmospheric air is absolutely necessary for the 

 production of these living beings, and that if the air be pro- 

 perly purified by passage through destructive chemical reagents, 

 no such organisms will be produced, provided that the infusion 

 have been previously boiled. As the results of all these 

 experimental trials have hitherto proved more or less contra- 

 dictory, it is unnecessary to enter into the question further, and 

 it will be sufficient to indicate the following general considera- 

 tions : 



a. The primary molecules which appear in the fluid are 

 extremely minute, and if they are developed from germs, these 

 may be so small as to elude any power of the microscope yet 

 known to us. As they subsequently become converted into 

 bacteria and vibrios, and as there can be little dispute as to 

 these being truly living organisms, we are obliged to believe 

 that they must have had some definite origin. It appears, 

 however, to be hardly philosophical to assume that they form 



