l66 GENERAL PRINCIPLES OF ZOOLOGY. 



the evolution theory renders a consideration of this ques- 

 tion necessary at this point. 



I. GENERATIO SPONTANEA, ARCHEGONY. 



Theory of Spontaneous Generation. The old zoolo- 

 gists, even Aristotle himself, believed that many animals, 

 including even highly-organized forms, like frogs and most 

 insects, arose by spontaneous generation from the mud. 

 Not until the seventeenth and eighteenth centuries did this 

 doctrine find energetic opponents, in Spallanzani, Fran- 

 cesco Redi, Rosel v. Rosenhof, Swammerdam, and others, 

 who sought to adduce the experimental proof that all 

 animals lay eggs which must be fertilized by the sper- 

 matozoon of the male in order to develop further. By 

 their conclusive investigations the doctrine of spontaneous 

 generation was driven into the realm of the natural history 

 of the lower animals. Here it found a new foundation in 

 the occurrence of parasites inside of animals which, at the 

 beginning of their life without doubt, must have been free 

 from these internal inhabitants. Parasitologists main- 

 tained that the parasites arose quite anew from the super- 

 fluous plastic material of their host ; finally, by a series of 

 epoch-making researches, the way was discovered by which 

 the young forms of the parasite, developing from eggs, 

 find their way into the body of their host. It was until 

 recently considered a proof of the doctrine of spontaneous 

 generation that, after a time, animal and plant life, (uni- 

 cellular organisms, infusorian animalcules, etc.), became 

 evident in water supposed to contain no living thing what- 

 ever; further, that organic fluids became foul by the de- 

 velopment of the lowest of the plants, the bacteria. At 

 the present time, we know that in all these cases germs of 

 organisms, which have been carried about by the air, are 

 the causes of the new development of life. If the germs 

 are killed by heating the glass and boiling the fluid, and if 

 by suitable means the entrance of new germs is prevented, 

 then such a "sterilized fluid" remains permanently un- 



