No. 608] MULTIPLE FACTORS IN MICE AND RATS 471 



eration, which consists of only sixteen animals, ranges 

 between the grades +2 and +3|. Six of the sixteen 

 young fall within grades characteristic of the grand- 

 parental animals (lines 11 and 12, Table VI). 



Crosses between hooded and Irish rats have given com- 

 parable results (Table VII). Rats of grades —1^ gen- 

 eration 3^ minus selection, were crossed with Irish rats. 

 As a control for the hooded animals the progeny of rats 

 of grade — 1| in the third and fourth selection generation 

 may be used. The total of the progeny so obtained is 112. 

 It will be noticed from Table VII, line 1, that they vary 

 between grades — 2^ and + 1. Only eight or 8.8 per cent, 

 of the generation vary outside of the grandparental 

 grades. The range of variability in this generation is be- 

 tween — 2 and -f 1^. The second cross is between females 

 of grade — IJ, from the fifth selection generation of the 

 minus series and Irish males (lines 3 and 4, Table VII). 

 As a control for these the progeny of — 1^ parents of a 

 similar generation may be used. The 143 progeny so ob- 

 tained fall within the grades — 2^ and — f inclusive. The 

 fifty-three Fo young range between grades — If and + 1. 

 Almost exactly 50 per cent, of them reproduced the grades 

 of the hooded grandparental types. 



The third cross is between females of minus two grade 

 in generation 7} of the minus selection experiment and 

 Irish males. As a control, the progeny obtained from the 

 females of — 2 grade in the seventh and eighth selection 

 generations may be used. Such animals produced 2,013 

 young, ranging from — 2f to — 4. Sixty-six young 

 ranged from grades — 2 to + 2. Of these, 75.8 per cent, 

 reproduced the grades characteristic of the hooded grand- 

 parental race. 



Two experiments are recorded showing crosses of plus 

 selection animals with Irish males. In the first of these, 

 the females used were of grade -f 2| in the sorond gen- 

 eration of the plus scl(M-tioii cxporiincnr. Aiiimnls of 

 similar grade and goiKM-atioii may a<-t a> (•(inti-«i!>. a ItluMiifh 

 the number of young o])tained from thorn i> very small. 



