36 MANUAL OF ZOOLOGY. 



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 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 

 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. 



b. 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 the former near the filamentous Confervse. 



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. 



<?. 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. 



f. The reproductive process in many of these same Infusoria 

 is perfectly well known, and it consists either in a true sexual 

 process, for which proper organs are provided (as in Paramoe- 

 cium), or in a process of gemmation or fission. It is there- 

 fore improbable that they should be generated in the manner 

 maintained by the heterogenists, since this mode of reproduc- 

 tion would appear to be superfluous. 



g. In the absence of any direct proof to the contrary, it is 

 safer to adopt an explanation of the observed phenomena 



