THE FACTOR HYPOTHESIS 



157 



BiCP blue or maltese (dilute black). 



bICP = chocolate. 



biCP = silver-fawn (dilute chocolate). 



The results of crossing the hybrids formed from the 

 combinations indicated at the left in the table below are 

 shown at the right where the expectation is given in 

 parentheses after the actual results. 



It will be seen that the actual results, even when 

 such small totals are concerned, approximate very 

 closely the expectation and are entirely consistent. 



D. CASTLE'S BROWN-EYED YELLOW GUINEA-PIGS 



Furthermore Castle has shown that in guinea-pigs 

 there is an independent gene for extension of pigment 

 distinct from the uniformity gene already mentioned. 

 The absence of this extension gene ("restriction") is 

 manifested by a lack of black or brown pigment every- 

 where except in the eyes and to a slight extent in the 

 skin of the extremities, while the distribution of yellow 

 is wholly unaffected by it. 



That such "extension" and "restriction" genes 

 really exist, is proven in the following way: 



When a brown (chocolate) guinea-pig is crossed 

 with an ordinary black-eyed yellow one, the young are 

 all black pigmented, but by cross-breeding these hybrid 



