THEORY OF EVOLUTION 177 



broader stripe in the other. As the diagram 

 shows (fig. 88) Castle has succeeded 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 the back and sides, leaving only a white 

 mark on the belly. 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 experiment 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 with 

 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 — may 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 may have changed. 



