184 THE MECHANICAL FUNCTIONS. 



existence, and of the curious phenomena they 

 present : yet even the best instruments afford us 

 but little insight into their real organization and 

 physical conditions. On this account it is ex- 

 tremely difficult to assign their true place in the 

 scale of animals. By most systematic writers 

 they have been regarded as occupying the very 

 lowest rank in the series, and as exemplifying 

 the simplest of all possible conditions to which 

 animal life can be reduced. Monads, which are 

 the smallest of visible animalcules have been 

 spoken of as constituting " the ultimate term of 

 ariimality ;" and some writers have even ex- 

 pressed doubts whether they really belong to the 

 animal kingdom, and whether they should not 

 rather be considered as the elementary molecules 

 of organic beings, separated from each other by 

 the effects of chemical decomposition, and re- 

 taining the power of spontaneous, 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 aptitude, or 

 capability of uniting into living masses, and con- 

 stituting, therefore, the essential elements of all 

 organization. According to this view, all vege- 

 tables or animals in existence would be mere 

 aggregations of infusory animalcules, which gra- 

 dually accumulate by continual additions to 



