INTRODUCTION. 



All that is cognizable to sense is reducible to two 

 classes of existences — the one passive, inert, and 

 governed by the general laws of nature ; the other 

 active, combating and modifying these laws. The 

 first class comprehends inorganic or inanimate ex- 

 istence ; the second, animate or organic existence. 

 Animate beings are composed of organs which, 

 though varying in number, figure, and function, in 

 the infinite varieties of living creatures, may, ne- 

 vertheless, be classed under two heads : 1st. Or- 

 gans destined to preserve the individual. 2d. 

 Organs appropriated to the perpetuation of the 

 species. To the first class belong the organs of 

 nutrition, locomotion, and sensation ; to the second, 

 the organs of generation. A scale or gradation of 

 animated beings may therefore be established, on 

 the ground of their possessing more or fewer of 

 these organs. 



The simplest animal with which we are ac- 

 quainted is to be found in water either in a stagnant 

 state or impregnated with decayed vegetable mat- 

 ter : it is of microscopic minuteness, a single 

 living point, without any organ whatever, and called 

 Monas. A drop of putrid water contains myriads 

 of these in motion. One degree higher in the 



