4 Dynamic Evolution 



Forces act in straight lines, — curved motion 

 being the resultant of two or more forces 

 acting simultaneously on the same bodies. 

 The magnitude of forces, the directions of 

 their actions, and all resultants of two or more 

 forces are subject to mathematical computa- 

 tions. Hence, all causes operate in accord- 

 ance with mathematical laws. The mere fact 

 that some composites of forces, particularly 

 in living organisms, are so complex that they 

 have not yet yielded to mathematical compu- 

 tation, does not negative this proposition. 



A result may be the simple effect of a 

 simple cause, or a composite of effects pro- 

 duced by a composite of causes. In any 

 composite of causes, each individual cause 

 produces its own effect uninfluenced by the 

 presence or absence of other causes, — the 

 result being a composite of the effects. If a 

 composite of causes is variable in its 

 composition, the result is correspondingly 

 variable. 



The variability in animals is due to the 

 variability in the composites of causes which 



