Modification of Unit Characters 



107 



the pure stock (the diameter of the circle referred to) is 



indicated by < > . Of course, even "pure 



stock" varies somewhat, since no two individuals are 



If 



The Control 



FIG. 27. Diagram illustrating CASTLE'S selection experiment 

 with hooded rats. 



exactly alike, biology recognizing what is called "indi- 

 viduality." The point, however, is that the compara- 

 tively small variation in a pure stock is not due to 

 germinal differences but to responses called out by 

 varying external conditions, such as nutrition, light, etc. 



