40 DEl'ELOPAIENT AND HEREDITY. 



and its place taken by other similar particles. To 

 account for all these movements of each particle, 

 we are compelled to suppose a certain combination 

 of forces acting in a variety of directions, and 

 impelling the particle through its devious course 

 so long as it remains a part of the living aggre- 

 gate. Every movement of each particle means 

 that a certain amount of energy has been imparted 

 to it, and every time the motion of the particle 

 deviates from the straight line, we know that an 

 additional amount of energy has acted upon it. 

 We know that part of this energy in the organism 

 has its source in the oxidation of the food ; but 

 this alone, coming from a single source, could not 

 maintain a mobile equilibrium of forces. The only 

 other source of energy for the organism lies in 

 the existence and movements of matter outside of 

 its own body. The energy of such movements is 

 transmitted to the organism in the form of pres- 

 sure, impacts, strains, heat, light, electricity, atmos- 

 pheric vibrations, etc. 



If we expect to explain organic phenomena as 

 mechanical processes we must allow due weight 

 to these forces of the environment as causes of 

 organic change. 



