18 PIEBALD RATS AND SELECTION. 



we should expect, since they are descended from more highly selected 

 individuals; for the hooded grandparents in this case were of grade 

 +3.25 (generation 5) and +3.50 (generation 6), whereas the grand- 

 parent in the foregoing case was of grade +3.00 (generation 3). 



It is a noteworthy fact that in both these cases the wild cross does not 

 seem to have increased the pigmentation in extracted hooded indi- 

 viduals, as it did when the minus series was crossed, but rather to have 

 diminished it; yet the difference between observed and expected is not 

 great. We might disregard it altogether, if a similar but more striking 

 result were not observable in the third case as well as in another series of 

 crosses presently to be described. 



The third case (last row of Table 42) involves a cross between a 

 female of grade +4.25, generation 10, plus series, and a wild male. 

 The F2 offspring include 16 hooded individuals of mean grade +3.15. 

 Animals of this grade in the uncrossed race in this generation produced 

 young of mean grade +3.84. 



Before leaving this subject it is important to observe the considerable 

 difference between the extracted hooded grandchildren of the minus- 

 series rats, as a group, and those of the plus series. The latter is unmis- 

 takably a plus-series group; the former is on the border line between 

 the two series. (Cf. Plates 2 and 3.) 



CROSSES WITH BLACK "IRISH" RATS. 



As a control on the results given by the wild crosses, we may examine 

 the results obtained by crossing the plus and minus selected races with a 

 black Irish race. The Irish race used for this purpose consisted of ani- 

 mals black everywhere except on the bell}^ On the system of grading 

 used in this paper they would range from +4f to +5f , +6 being an all- 

 black rat. 



Crosses of minus-series hooded rats with Irish produced Irish Fi off- 

 spring with rather more white on the belly than the Irish parents 

 possessed. In the F2 generation hooded individuals reappeared in 

 approximately the expected 25 per cent. In a total of 764 second gen- 

 eration young, 171, or 22.4 per cent, were hooded. The grade of pig- 

 mentation of these extracted recessives as compared with that of their 

 hooded grandparents we may now consider, as was done in the case of 

 the wild crosses. (See Table 43.) 



Six individuals of the minus selection series, of generation 3h, and of 

 mean grade -1.50, were crossed with Irish rats producing Irish off- 

 spring which were mated inter se. Among the grandchildren appeared 

 the usual proportion of recessives (hooded), 90 in number. The dis- 

 tribution of these as regards grade of pigmentation is shown in the first 

 row of Table 43. Their mean grade is - 0.62, that of uncrossed hooded 

 rats of the same grade as the hooded grandparents being - 1.31 in gener- 



