go 



Outline of Genetics 



the same. A selection on the basis of fluctuations within a pure 

 line, therefore, should not result in any permanent improvement ; 

 in fact, it has been demonstrated many times that no such improve- 

 ment can be effected in this way. When selection is made, 



- Pure-Bred 

 Generations 



/ Selection 

 Begins 



The Control 



Fig. 15. — Diagram illustrating Castle's selection experiment with 

 hooded rats. 



however, among varying doses of cumulative factors, an entirely 

 different situation is faced, for in such a case we are not dealing 

 with a pure line. 



The significance, therefore, of Castle's results may be real- 

 ized. He bred his original pure line for many generations and 

 found that it varied only within very narrow limits; and these 



