530 



NATURE OF REPRODUCTION. 



Fig. 168. 



Different kinds of INFUSOKI A. 



riods of their growth. Ehrenberg has described more than 300 

 different varieties of them. They are generally provided with cilia 

 attached to the exterior of their bodies, and are, for the most part, 

 in constant and rapid motion in the fluid which they inhabit. 



Owing to their presence in 

 animal and vegetable watery 

 infusions, they have received 

 the name of "infusoria," or 

 " infusorial animalcules." 



Now these infusoria are 

 always produced under the 

 conditions which we have de- 

 scribed above. The animal 

 or vegetable substance used 

 for the infusion may be pre- 

 viously baked or boiled, so 

 as to destroy all living germs 

 which it might accidentally 

 contain; the water in which 

 it is infused may be carefully 

 distilled, and thus freed from all similar contamination; and yet the 

 infusorial animalcules will make their appearance at the usual time 

 and in the usual abundance. It is only requisite that the infusion 

 be exposed to a moderately elevated temperature, and to the access 

 of atmospheric air; conditions which are equally necessary for 

 maintaining the life of all animal and vegetable organisms, what- 

 ever be the source from which they are derived. Under the above 

 circumstances, therefore, either the animalcules must have been 

 produced by spontaneous generation in the watery infusion, or their 

 germs must have been introduced into it through the medium of 

 the atmosphere. No such introduction has ever been directly de- 

 monstrated, nor have even any eggs or germs belonging to the 

 infusoria ever been detected. 



Nevertheless, there is every probability that the infusoria are 

 produced from germs, and not by spontaneous generation. Since 

 the infusoria themselves are microscopic in size, it is not surprising 

 that their eggs, which must be smaller still, should have escaped 

 observation. We know, too, that in many instances the minute 

 germs of animals or plants may be wafted about in a dry state by 

 the atmosphere, until, by accidentally coming in contact with warmth 

 and moisture, they become developed and bring forth living organ- 



