THEORY OF EVOLUTION 179 



that can show an effect only when hoodedness 

 is itself present. That this is not an imaginary 

 objection but a real one is shown by an experi- 

 ment that Castle himself made which furnishes 

 the ground for the second objection. 



Second. If the factor has really changed its 

 potency, then if a very dark individual from 

 one end of the series is crossed to a wild rat and 

 the second generation raised we should expect 

 that the hooded F 2 rats would all be dark like 

 their dark grandparent. When Castle made 

 this test he found that there were many grades 

 of hooded rats in the F 2 progeny. They were 

 darker, it is true, as a group than were the 

 original hooded group at the beginning of the 

 selection experiment, but they gave many in- 

 termediate grades. Castle attempts to explain 

 this by the assumption that the factor made 

 pure by selection became contaminated by its 

 normal allelomorph in the F t parent, but not 

 only does this assumption appear to beg the 

 whole question, but it is in flat contradiction 

 with what we have observed in hundreds of 

 Mendelian cases where no evidence for such 

 a contamination exists. 



