180 THEORY OF EVOLUTION 



Later Castle erossed some of the extracted 

 rats of average grade (3.01) from the plus 

 series to the same wild race and got Fo hooded 

 rats from this cross. These F2 hooded rats did 

 not further approach the ordinary range ])ut 

 were nearer tlie extreme selected plus hooded 

 rats (.3.38) than were the F2's extracted from 

 the first cross (2..59). Castle concludes from 

 tliis that multiple factors can not account for 

 the result. As a matter of fact, Castle's evi- 

 dence as publish cd does not establish his con- 

 clusion because the wild rats used in the second 

 experiment may have carried plus modifiers. 

 This could only be determined by suitable tests 

 wliich Castle does not furnish. This is the 

 crucial point, without which the evidence carries 

 no conviction. 



Furthermore, from Castle's point of view, 

 these latest results would seem to increase the 

 difficulty of interpretation of his first Fo ex- 

 tracted cross, and it is now the first result that 

 calls for explanation if one accepts his later 

 conclusion. 



These and other objections that might be 

 taken up show, I think, that Castle's experi- 



