INTRODUCTION 7 



But, before we give ourselves up to the interest 

 of this subject, it will gratify us to express our 

 conviction that the perfection of the plan of ani- 

 mal bodies, the demonstration of contrivance and 

 adaptation, but more than these, the proof of the 

 continual operation of the power which originally 

 created the system, are evinced in the property of 

 life, in the adjustment of the various sensibili- 

 ties, in the fine order of the moving parts of 

 the body, in the circulation of living blood, 

 in the continual death of particles and their re- 

 moval from the frame, in the permanence of 

 the individual whilst every material particle of his 

 frame is a thousand times 1 changed in the pro- 

 gress of his life. But this is altogether a distinct 

 inquiry, and we are deterred from touching upon 

 it, not more from knowing that our readers are 

 not initiated into it, than from the depth and 

 very great difficulty of the subject. 



1 The old philosophers gave out that the human body was seven 

 times changed during the natural life. Modern discoveries have 

 shown that the hardest material of the frame is changing con- 

 tinually ; that is, every instant of time, from birth to death. 



