THEORY OF EVOLUTION 177 



broader stripe in the other. As the diagram 

 shows {fig. 88) Castle has sueeeeded in pro- 

 ducing in one direction a race in which the 

 dorsal stripe has disappeared and in the other 

 direction a race in which the black has extended 

 over tlie back and sides, leaving only a white 

 mark on the bellv. Neither of these extremes 

 occurs, he believes, in the ordinary hooded race 

 of domesticated rats. In other words no mat- 

 ter how many of them came under observa- 

 tion the extreme types of his exi^eriment would 

 not be found. 



Castle claims that the factor for hoodedness 

 must be a single Mendelian unit, because if 

 hooded rats are crossed to wild gray rats w^th 

 uniform coat and their offspring are inbred 

 there are produced in F2 three uniform rats to 

 one hooded rat. Castle advances the hypothe- 

 sis that factors — by which he means Mendelian 

 factors — mav themselves vary in much the 

 same way as do the characters that they stand 

 for. He argues, in so many words, that since 

 we judge a factor by the kind of character it 

 produces, when the character varies the factor 

 that stands for it mav have changed. 



