No. 608] MULTIPLE FACTOBS IX MICK AM) HATS 469 



does nevertheless show that 5.4 per cent, of the seventy- 

 three young reproduce the degree of spotting charac- 

 terizing one of the parental races. 



The total number of animals observed, however, is con- 

 siderably smaller than in the previous experiment and 

 the results are therefore subject to greater error. 



We may now consider several crosses made between 

 hooded and wild rats. The generation in such crosses 

 consists almost entirely of self pigmented animals. It is, 

 therefore, in a comparison of the range of variability 

 observed in the F2 generation with that observed in the 

 pure hooded race from which the hooded grandparent was 

 taken, that we may expect to find evidence for or against 

 segregation. The first cross to be recorded is between a 

 female of grade — If in the 21 generation of the minus 

 selection series, and a wild male. As a control for this 

 female, the progeny of — If parents in the second and 

 third selection generations may be taken. It will be seen 

 that their progeny varies between grades — 2 and — i 

 (see line 1, Table VI), in a total of 90 animals. The F2 

 generation shows a range of variability between grades 

 — 1| and grade -f 3^. It is interesting to note, however, 

 that twenty-three or 37 per cent, of the sixty-two animals 

 observed fall within the range of variation of the hooded 

 grandparent. A comparison of grades — 14 and — 2 in 

 Fig. 2 will show how closely Fo animals approach the 

 extreme condition of the hooded grandparental race. 



In the second cross between hooded and wild rats, 

 females of minus two grade of the sixth generation of the 

 minus selection series were used. As controls for these 

 females the progeny of — 2 parents in the sixth generation 

 of the minus selection experiment may be used (see line 

 3, Table VI). The 969 young so obtained fall between 

 the grades — 2| and — |. The forty-eight young show a 

 large range of variability from - 1} to + 2f . Of the 

 forty-ci-ht (Obtained, twelve, or 2r. per rent., fall within 



parental generation. 



