196 



MULTIPLE FACTORS 



a cross between this " reverted" strain and the se- 

 lected 100 per cent, abnormal strain failed to give a 

 definite result. From the offspring of this cross, 

 again crossed to the 100 per cent, strain, Lutz selected 

 another abnormal strain, which produced from 95 to 

 100 per cent, abnormals for eight successive genera- 

 tions. He then selected back again for normals and 

 in six generations he obtained a strain wiiich produced 

 no more abnormals than does a wild strain. Selec- 

 tion for reduced veins was also successful, but was not 

 carried on very extensively. 



FIG. 61. 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.) 



It is not clear what interpretation should be placed 

 upon these experiments of Lutz, but it seems probable 

 that mutations affecting the venation occurred 

 several times, and were selected. 



One of the most exhaustively studied cases of the 

 effect of selection on a mixed population is that car- 

 ried out on hooded rats by Castle and his co-workers, 

 particularly Phillips. The pattern of hooded rats is 



