THE BEGINNINGS 01 IIFE. 30 1 



be found in the tall glass, and an abundance of ciliated 

 Infusoria; whilst the shallow vessel presents only an 

 exceedingly thin and scarcely apparent proligerous mem- 

 brane, and not a single ciliated Infusorium. When the 

 conditions are reversed — when the quantity of fluid 

 is much diminished in the tall glass, and very much 

 increased in the shallower one — so as to reverse the 

 relative depths of the solutions in the differently shaped 

 vessels, then the ciliated Infusoria are still found with 

 the deeper solution and the thicker pellicle, and only 

 an abundance of Bacteria where the solution is shal- 

 low and the pellicle scanty. Remembering that the 

 existence of either ciliated Infusoria, or of the ova of 

 these, to any notable extent, in the atmosphere, is a mere 

 matter of hypothesis, which its advocates have failed to 

 justify; and remembering, on the other hand, the now 

 frequently demonstrated mode of evolution of the cilia- 

 ted Infusoria in the pellicle from modified aggregations 

 of motionless Bacteria^ — it must be evident to all that 

 the above-mentioned experiments seem inexplicable if 

 we attempt to explain them by the atmospheric germ- 

 theory, though they are quite consistent with the doc- 

 trines of heterogenists. Both solutions are exposed to 

 the same air, and therefore to the same possible source 

 of ova; yet in the solution of the one vessel, after 

 the lapse of a few days, ciliated Infusoria are found; 

 in the other, there are none. Let the conditions of 

 depth of the solutions in the two vessels be reversed, 

 and then again the ciliated Infusoria are met with in 



