166 INHERITANCE IN RATS. 



Obviously these facts do not harmonize with the assumption that 

 the regression observed in the first F 2 was due to loss of modifying fac- 

 tors accumulated during the ten preceding generations of selection; 

 for no further loss occurs in the second F 2 . On the other hand, a 

 partial recovery is made of what was lost in the first F 2 . This suggests 

 the idea that that loss may have been due to physiological causes non- 

 genetic in character, such as produce increased size in racial crosses; for 

 among guinea-pigs (as among certain plants) it has been found that Fj. 

 has an increased size due to vigor produced by crossing and not due to 

 heredity at all. This increased size persists partially in F 2 , but for the 

 most part is not in evidence beyond FV I would not suggest that the 

 present case is parallel with this, but it seems quite possible that similar 

 non-genetic agencies are concerned in the striking regression of the first 

 F 2 and the subsequent reversed regression in the second F 2 . 



Whatever its correct explanation may be, the fact of the reversed 

 regression in a second F 2 is very clear, as other cases than those already 

 discussed will show. 



A hooded rat of grade +4 and generation 10, c?6348, had by a wild 

 female several young of the character already described for the young 

 of 9 5513. These, mated brother with sister, produced a first F 2 (table 

 141) of 90 rats, 22 of which were hooded, 68 being non-hooded, again 

 a good 1 : 3 ratio. The hooded young ranged from +2 to +4 in grade, 

 their mean being 3.28. Of the 22 hooded individuals, 1 male and 7 

 females were mated with wild rats to obtain a second F 1; and the 

 second FI animals were then mated brother with sister to obtain the 

 desired second F 2 . The character of this is shown family by family in 

 table 143. It contained 497 individuals, of which 121 were hooded 

 and 376 non-hooded, a ratio of 1 : 3.1. The weighted mean of the 8 

 selected grandparents is 2.93, which is 0.35 below the mean of the 22 

 first F 2 hooded animals which they represent. The mean of the second 

 F 2 hooded young is 3.22, which indicates a reversed regression of 0.29 

 on the grade of the grandparents, but shows no significant difference 

 from the mean of the grandparental group (3.28). 



All except one of the 8 families classified in table 143 show unmis- 

 takably the reversed regression. This exceptional family consists of 

 the grandchildren of 9 9747. They have a mean grade of 2.90, sub- 

 stantially the same as that of the entire group of grandparents but con- 

 siderably lower than that of their own hooded grandmother. Appa- 

 rently she did not come up genetically to her phenotypic grade. This 

 the other grandparents of the group did. For those of lowest grade 

 (2, 2f ) produced lower-grade hooded grandchildren than did the grand- 

 parents of highest grade (3|, 4), as was found to be the case also, in 

 table 142. 



We may next trace the inheritance of the hooded character through 

 a third but smaller family produced by two successive crosses with wild 



