166 THE MECHANICAL FUNCTIONS. 



* ^4. Infusoria. 



Infusory animalcules, or Infusoria, were so named 

 by Muller, a Danish naturalist, from the circum- 

 stance of their swarming in all infusions of vege- 

 table or animal substances which have been kept 

 for a sufficient time. They are, in general, far too 

 minute to be perceptible to the naked eye : it is to 

 the microscope alone, therefore, that we owe our 

 knowledge of their existence, and of the curious 

 phenomena they present : yet even the best instru- 

 ments afford us but little insight into their real 

 organization and physical conditions. On this ac- 

 count it is extremely difficult to assign their true 

 place in the scale of animals. By most systematic 

 writers they have been regarded as occuj^ying the 

 very lowest rank in the series, and as exemplifying 

 the simplest of all possible conditions to which ani- 

 mal life can be reduced. Monads, which are the 

 smallest of visible animalcules, have been spoken 

 of as constituting " the ultimate term of animality ;" 

 and some writers have even expressed doubts whe- 

 ther they really belong to the animal kingdom, and 

 whether they should not rather be considered as 

 the elementary molecules of organic beings, sepa- 

 rated from each other by the effects of chemical 

 decomposition, and retaining the power of sponta- 

 neous, but irregular and indeterminate motion. It 

 was conceived that all material particles belong to 

 the one or the other of two classes ; the first, wholly 

 inert, and insusceptible of being organized ; the 

 second, endowed with a principle of organic apti- 



