250 



MULTIPLE FACTORS 



and there was little increased variability, but in 

 F2 the variability was increased greatly: the ex- 

 treme grades of the selected stocks again appeared, 

 together with all intermediate values. The F. curve 

 was not, however, one that could be reconstructed 

 by simply compounding the Pi and Fi curves in 

 1:2:1 ratio as in Morgan and Bridges' cross of 

 thorax pattern, for the proportion of intermediates 

 was much greater than in a distribution so calcu- 



FiG. 6L — Series of arbitrary grades of hooded rats used in classifying 

 results of selection experiment. Above the figures the numbers assigned 

 to the grades are given (see text). (After Castle and Phillips.) 



lated ; this again showed that more than one pair of 

 modifying factors was involved. 



One of the most exhaustively studied cases of the 

 effect of selection on a mixed population is that 

 carried out on hooded rats by Castle and his co- 

 workers, particularly Phillips. The pattern of 

 hooded rats is shown in Fig. 61. The dark pigment 

 covers the head and extends as a stripe down the. 

 back. The extent of the hood and the breadth of 

 the dorsal band are so variable that in one direction, 



