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 



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. 



