SPONTANEOUS GENERATION. 1 29 



that the bacteria and vibriones are to be regarded as be- 

 longing to the vegetable kingdom. 



Lastly, at a still later period, the fluid may be found to 

 ccmtain forms of the so-called " Infusorian Animalcules." 

 These are undoubted animals, and though not standing 

 very high in the zoological scale, they are by no means the 

 humblest or most lowly organised members of the animal 

 kingdom. 



The phenomena just recounted are altogether beyond 

 doubt, and may be observed by any one for himself with a 

 little trouble and a tolerably good microscope. Their ex- 

 planation, however, has been the subject of one of the 

 most vigorous controversies which has ever divided the 

 scientific world into two opposing camps ; and it cannot 

 be regarded as by any means near its final settlement. The 

 point to be settled is this : — How does a fluid which, to 

 begin with, is wholly without living beings, become the 

 home of unquestionable living organisms ? Two answers 

 have been given to this question. The oldest theory, and 

 one which was in vogue long anterior to the discovery of 

 the facts just mentioned, was, that these living beings formed 

 themselves spontaneously and de novo out of the dead 

 materials of the fluid. Very ancient is this belief, that 

 living beings could be produced by the spontaneous action 

 of a genial and prolific nature upon dead matter ; and 

 many animals, both real and imaginary, have been asserted 

 to have been generated in this fashion. Nowadays, how- 

 ever, the theory of spontaneous generation has been wholly 

 given up as regards all the cases in which the ancients placed 

 credence ; and it has become entirely restricted to a group 

 of minute organisms, the very existence of which has only 

 been known since the microscope has reached something 

 like its present perfection. Stated briefly, then, as far as 

 concerns the facts above described, it is held by one school 

 that the microscopic organisms which make their appear- 

 ance in organic infusions, after exposure to the air, have 



