164 



INHERITANCE IN RATS. 



brother with sister, thus precluding the possibility of introducing 

 modifying factors from other sources. In making the second set of 

 crosses, the extracted individual has, wherever possible, been crossed 

 with its own wild grandparent. In the few cases in which this was 

 impossible, wild animals of the same stock have been used. This stock 

 consisted of a colony of wild rats which invaded the basement of the 

 Bussey Institution apparently from a nearby stable. Owing to faulty 

 construction of the building they were able to breed in spots inaccessible 

 to us, and it took many months of continuous and persistent trapping 

 to secure their extermination. During this period we trapped a hun- 

 dred or more of them, all typical Norway rats, colored all over, without 

 even the white spot occasionally seen on the chest of wild rats. Two 

 generations of rats from this wild stock have been reared in the labora- 

 tory, and all have this same self-colored condition. 



The hooded animals used in the experiments to be reported on in 

 this connection consisted of 4 individuals of the plus-selected series, 

 a male and 3 females, as follows : 



TABLE 140. 



'See figure 35, plate 7, for significance of the grades. 



Each of these animals was mated with a single wild mate, and their 

 children were weaned directly into breeding cages containing a male 

 and two or three females (brother and sisters). In the case of two 

 matings, F! males of the same parentage were at the time lacking and 

 males from a different cross were used. The results of such matings are 

 tabulated by themselves and serve a useful purpose as controls. The F! 

 animals all closely resembled their wild parents, but many of them had 

 a white spot on the chest. They ranged from grade +5| to +6 (self). 



The F 2 animals are classified in table 141, where it appears that 73 

 of them were hooded and 219 non-hooded (i. e., like FJ, an exact 1 : 3 

 ratio. More than half of this F 2 generation consists of the grand- 

 children of 9 5513, produced by breeding her children brother with 

 sister, those children all having been sired by the same wild rat. Her 

 grandchildren include 41 hooded and 107 non-hooded young. The 

 hooded young range in grade from + lf to +4, their mean grade being 

 +3.05, a considerable regression from the grade of the grandmother, 

 which was 4.25. 



Hooded rats of the same grade and generation as the grandmother, 

 when bred with each other, produced young of mean grade +3.84. 



