34 MANUAL OF ZOOLOGY. 



germs too minute, perhaps, to be always detectable by the 

 microscope which, obtaining access to the fluid, and finding 

 there favourable 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 

 properly 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 

 contradictory, it is unnecessary to enter into the question 

 further, and it will be sufficient to indicate the following general 

 considerations : 



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 coalesce to form the bac- 

 teria 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, how- 

 ever, to be hardly philosophical to assume that they form 

 themselves out of the inorganic materials of the infusion; 

 since this implies the sudden appearance, or creation, of new 

 force, for which there seems to be no means of accounting. 



1). The nature of the vibrios and bacteria must be looked 

 upon as quite uncertain. To say the least of it, they are quite 

 as likely to be plants as animals ; and the most probable hypo- 

 thesis would place them near the filamentous Confervee. 



c. What has been said above with regard to the origin of 

 the bacteria and vibrios applies equally to the origin of the 

 Monads, which appear in the infusion subsequently to the 

 death of the vibrios. 



d. These Monads, as shown by recent researches, are pro- 

 bably to be looked upon as the embryonic, or larval, forms of 

 the higher Infusoria which succeed them. 



e. Many of the Infusoria, which finally appear, are of a 

 comparatively high grade of organisation, being certainly the 

 highest of the Protozoa, and being placed by some competent 

 observers in the neighbourhood of the Trematode Worms 

 (Annuloida). It is, therefore, very unlikely that these should 

 be generated spontaneously ; since if this ever occurs, it is 

 reasonable to suppose that the creatures thus produced will 

 be of the lowest possible organisation (such as the Gregarinidae 

 for example), and will be far below the Infusoria in point of 

 structure. 



/. The reproductive process in many of these same Infusoria 



