4 INTRODUCTION. 



great divisions, which have appropriately enough been called 

 kingdoms,, viz. the ANIMAL the VEGETABLE and the MINE- 

 RAL kingdoms. These divisions are not less proper than con- 

 venient ; and although some writers believe it possible to trace 

 a continuous but progressive connection from the most perfect 

 animal in the scale to the inert and lifeless rock, yet there seems 

 no good reason for supposing that such a chain exists, or if ex- 

 isting, that all the links shall ever be discovered. The works of 

 the Author of Nature are indeed all in consistent harmony with 

 one another, and there is a mutual dependence, advantageous 

 to all, among the various classes of organized beings : but be- 

 tween the lowest form of vegetable or animal life, and the most 

 symmetrically disposed crystal in the mineral kingdom be- 

 tween a living body and inert matter there is an immeasu- 

 rable distance ; and between the highest of the lower animals 

 and Man, of all beings alone endowed with the power of reason 

 and the faculty of speech, a distance still more incalculable. 



Animals have been defined to be organized bodies which have 

 life and sensation, and are capable of voluntary motion ; Vege- 

 tables organized bodies, endowed with a vital principle, but want- 

 ing sensation ; and Minerals unorganized bodies, without life, 

 and of course without sensation. 



It has been found impossible to give a satisfactory definition 

 of Life ; and physiological writers have therefore limited their 

 efforts to communicate some idea of the vital principle by re- 

 marking its effects. Life, where its effects are most easily recog- 

 nized, seems to consist in the faculty with which certain corpo- 

 real combinations are endowed, of existing for a certain period 

 under a determinate form, and assimilating to their substance a 

 part of the surrounding bodies ; at the same time restoring to 

 the elements part of their own substance. This vital principle, 

 which, when allied to matter, controls its affinities and directs 

 its forms, is not palpable to the senses in an uncombined shape, 

 and it is only from its effects on material substances that its ex- 

 istence is demonstrated. Baron Cuvier compares the mechani- 

 cal action of life on matter to a vortex more or less rapid, more 

 or less complicated, where the supply and the waste of particles 

 occasion a constant movement. While this movement subsists, 

 the body which exercises it lives ; when the movement is stop- 



