THEORY OF EVOLUTIOX 179 



that can show an effect onlv when lioodedness 

 is itself present. That this is not an imaginary 

 ohjection 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 F2 rats would all be dark like 

 their dark grandparent. AAHien Castle made 

 this test he found that there were many grades 

 of hooded rats in the F2 progeny. They were 

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

 oi'iginal hooded group at the beginning of the 

 selection experiment, but tliey 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 Ft parent, but not 

 only does this assumption appear to beg the 

 whole question, but it is in flat contradiction 

 witli wliat we have observed in hundreds of 

 JNIendelian cases where no evidence for such 

 a contamination exists. 



