194 The Evolution Hypothesis. 



relation of environment. The cause of variation in 

 the units is thus in part internal to the organism ; but 

 the organic whole, as well as every part of it, is ex- 

 posed continually to the impact of the forces consti- 

 tuting the larger environment in which the organism 

 lives. We have, then, a series of contrasts between 

 the environed and the environment, running from the 

 individual unit to the organism, and outward to the 

 entire universe, which complicates the question and 

 renders Mr. Spencer's explanation hardly explicable. 

 "We may regard it thus: The unit has an internal 

 system of forces equilibrated in harmony with the 

 ancestral form of organism; all other units making 

 up the living thing have similarly their equilibrations ; 

 but the incident forces bear in upon the whole and 

 upon the parts, disturbing the equilibrium. Every 

 such disturbance modifies the internal relations of 

 each several unit, and the inter-relations of the units 

 to one another and to the entire aggregate. In re- 

 gaining equilibrium the organic structure is modified. 

 Now, if the modified form arise in this way, it is not 

 easy to see how the direction of movement is due 

 altogether to the units, the incident forces only com- 

 municating the power. The principle in question 

 seems right in the face of Newton's second law of 

 motion that change of motion is proportioned to 

 the impressed force, and takes place in the direction 

 of the straight line in which the force acts. Mr. 

 Spencer may take refuge in the distinction between 



