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 F x 

 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 F\. 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 F! 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 



