28 DEVELOPMENT AND HEREDITY. 



motions of matter requires a certain definite amount 

 of energy to bring it about ; and we find a corre- 

 sponding source of energy in the chemical changes 

 which convert the potential energy of nutriment into 

 kinetic energy. But it is evident that this simple 

 liberation of energy by chemical action is not of 

 itself sufficient to account for the ensuing wonder- 

 fully complex motions of the parts of the organism. 

 To guide masses of matter through the devious 

 courses necessary to build up the body of a living 

 organism, it is necessary not only to have a sufficient 

 energy of motion, but at certain definite points along 

 the line that motion must meet with a resistance 

 sufficient to deflect and guide the mass to its given 

 destination. The forces of cohesion and gravity 

 undoubtedly have some effect, but the great com- 

 plexity of results cannot be attributed to so small a 

 number of simple constant forces. What, then, is 

 the source of this energy of resistance .'' The energy 

 of the primary motion and the energy of resistance 

 cannot have the same source, for it is a fundamental 

 principle that masses set in motion by energy from 

 a common source tend to pursue a straight line, and 

 so, rnoving radially from the centre, they cannot in- 

 terfere with each other. Therefore, while we ascribe 

 the primary motion of the masses to the energy set 

 free by chemical action, we must ascribe the modi- 



