470 



THE AMEBIC AN NATURALIST 



[Vol. LI 



The third cross was between wild males and females 

 of grade — 2 or 2^ in the tenth generation of the minus 

 selection series. As a control for these animals it is pos- 

 sible to use the progeny of 2^ parents in the tenth gen- 

 eration of the minus selection series (see line 5, Table 

 VI). The 474 young so obtained vary between grades 



— 3i and — 1. 



Among the ninetj'-one young obtained, eighteen, or 

 approximately 20 per cent., fall within the limits of vari- 

 ability of the hooded grandparents. The range of vari- 

 ability in the Fo generation is considerable, being from 



— 2 on one side to + 2f on the other. 



Three crosses between animals of the plus selection 

 series and wild rats are recorded. In the first cross (see 

 lines 7 and 8, Table VI) females of grade plus three 

 of the third generation of the plus selection series 

 were used. The breeding capacity of these animals may 

 be fairly judged by considering the progeny obtained 

 from plus three parents in the third generation of the 

 plus selection experiment. The 143 young so obtained 

 range from grade + f to -f 3f . Only twenty-one Fo young 

 are recorded, varying between grades + If and -f 3^. It 

 is interesting to note that all of them fall ivithin the range 

 of variation of the grandparental hooded race. 



In the second cross females of grade -f 3:^, fifth gen- 

 eration and females of grade -f 34, sixth generation, were 

 crossed with wild males. The progeny obtained from 

 animals of similar grade and generation within the selec- 

 tion experiment can serve as controls. The 320 young so 

 obtained vary between grades -|- H and + 4|. The thirty- 

 eight F2 young obtained vary between grades + 1^ and 

 + 35, again, in every case, reduplicating the grand- 

 parental forms. 



The third cross, which includes only small numbers and 

 is therefore of less relative value than the two previous 

 crosses, was between a female of +41, tenth generation 

 and a wild male. The control animals gave twenty-five 

 young ranging from grades + 3i to -f 4f . The Fg gen- 



