CROSSES WITH WILD RATS. 17 



The F2 (or second generation) offspring, however, include about 1 

 hooded individual in 4. In a total of 962 F2 young, 230 were hooded, 

 or 24 per cent. This summary includes only those litters in which 

 dominants as well as recessives were recorded. In many litters only 

 the hooded young were recorded, as the special object of the investiga- 

 tion was to ascertain whether the extracted recessives were like the pure 

 hooded race in grade or not. In the above summary also the hooded 

 grandparent was in every case a female. The reciprocal cross is more 

 difficult to obtain, but one wild female rat, caught in 1911, has bred 

 quite regularly in captivity, though each time she has murdered her 

 hooded mate prior to the birth of the young. Her Fo grandchildren 

 derived from matings with males of the minus series include 32 hooded 

 and 96 non-hooded individuals, exactly 25 per cent hooded. 



A second cross of selected animals of the minus series was made 

 between a wild male and four females of grade — 2 derived respectively 

 from generations 5j, 5^, 6^, and 7. As a group these mothers are more 

 nearly comparable with generation 6, Table 21, than with any other 

 uncrossed group. As the Fi progeny of these four mothers by a wild 

 male were mated i?iter se, it is possible to deal with their hooded grand- 

 children only as a group. The character of these is indicated in the 

 second row of Table 42. They number 48 individuals and have a 

 mean grade of +0.25, showing a modification in a plus direction similar 

 to that observed in the previous case. 



A third cross in which the minus series is concerned was made between 

 females of grade —2 and —2.25, generation 10, and wild males. The 

 F2 offspring include 91 hooded individuals classified as to grade in the 

 third row of Table 42. Their mean grade is +0.24, confirming fully 

 the results obtained in the two previous experiments. 



With these three cases we may compare three cases in which animals 

 of the plus selection series were crossed with a wild male. (See the last 

 three rows of Table 42.) Females of grade +3.00, generation 3, were 

 crossed with a wild male. From this mating resulted 21 hooded grand- 

 children, ranging in grade from +1.75 to +3.50, mean +2.56. These 

 grandchildren, it will be observed, in no case are of minus grade, as are 

 about half the grandchildren when the grandparent is of minus grade. 

 There is also no clear evidence of modification of the hooded character 

 by the cross in this case. The grade of the extracted hooded individual 

 is just about what uncrossed individuals of grade +3.00 produced in the 

 corresponding generation of the plus series. 



In the next case two females of the plus series, belonging to genera- 

 tions 5 and 6, respectively, were crossed with a wild male and their 

 children were bred inter se. There resulted 38 hooded grandchildren, 

 as shown in the next to the last row of Table 42. The range of the 

 grades of these hooded grandchildren was similar to that of the grand- 

 children in the foregoing case, but their mean was somewhat higher, as 



