INTRODUCTION 3 



that belong to living matter. Every organism assimi- 

 lates food, substances circulate through all parts of its 

 body, it breathes, it moves and it is able to produce its 

 kind. These are the functions that we are to study. To 

 explain their simple universal features we must have re- 

 course to the principles of physics and chemistry. No 

 adequate explanation can be made of any process with- 

 out a knowledge of these principles, for in all purely 

 physical operations the animal obeys certain natural 

 laws, usually without knowledge on its part, of what it 

 is doing and certainly without volition. But as the pecu- 

 liar way in which the functions manifest themselves is 

 dependent upon structure, to explain the manner of their 

 occurrence in any particular animal we must have re- 

 course to anatomy. 



