CROSSES WITH BLACK '' IRISH " RATS. 19 



ations 3 and 4. In other words, the cross has considerably increased the 

 pigmentation in the hooded grandchildren over what was to be expected 

 had the cross not taken place. Nevertheless the increase in this case is 

 less than in the similar cross with wild rats. (Compare Table 42.) 



In the second row of Table 43 is shown the grade distribution of 

 extracted hooded grandchildren of two mothers of grade —1.87 and 

 generation 4. The mean of the 53 hooded grandchildren is in this case 

 — 0.73, that of uncrossed hooded parents of the same grade and genera- 

 tion being 1.18. This average is probably too low. An examination 

 of the means of adjacent classes (Table 19) indicates that it should be 

 about 1.35. 



In the third row of Table 43 is shown the grade distribution of the 

 extracted recessive grandchildren of a — 2.00 male, minus-series rat, of 

 generation 7^. The 66 grandchildren are of mean grade —0.94, ex- 

 pected — 1.75. 



Comparing the three experiments (first three rows of Table 43) , we 

 see that the more advanced grandparents, in grade and generations of 

 selection, have the more adva,nced grandchildren; but in every case 

 these are less advanced than grandparents of the same sort would have 

 given had they not been crossed. Hence crossing with Irish has clearly 

 had the effect of increasing the pigmentation in the minus series in the 

 same way (but in lesser degree) as did crossing with wild animals. 



The results of crossing hooded rats of the plus series with Irish ones 

 are shown in the last two rows of Table 43. Several rats of mean grade 

 -F2.25 and of generation 2 were crossed with Irish, and their Irish 

 young were then bred inter se, producing 239 hooded grandchildren. 

 These ranged in grade from —1.00 to -|-3.25, their mean being +1.27. 

 The grade of uncrossed rats of like grade and generation to the hooded 

 grandparents is +1.80. Hence here, as in the cross with wild rats, the 

 pigmentation has not been increased, but decreased by the cross, con- 

 trary to what we should expect. Further, the departure from expecta- 

 tion is greater in this cross than in the wild cross. These conclusions 

 are supported by the results shown in the last row of Table 43. In the 

 experiment here recorded a+3.00 rat of generation 3 was mated with 

 an Irish rat. The hooded grandchildren derived from this cross were, 

 as shown in the table, of mean grade +0.95, expected about +2.50. 

 Since the number of animals recorded in this experiment is compara- 

 tively small, the quantitative result is less important than that of the 

 foregoing experiment, but qualitatively the two are in entire agreement. 



The various crosses of the selected minus and plus series with wild 

 rats and with Irish rats respectively are consistent with each other. 

 In every case the cross ina'eases the pigmentation of the minus series and 

 decreases that of the plus series; in other words, it undoes the work of 

 selection to some extent. Does this mean that the condition created by 

 selection was in reality an unstable one, so that an outcross tends to do 



