22 HEREDITY IN RABBITS, RATS, AND MICE. 



Male 6071 (table 37), although of higher grade, gave no better 

 results. His own brother, 0^6072 (table 38), born in the same litter 

 and graded the same, did much better. He was bred very extensively 

 and gave a record very similar to that of his father, cf 6370, who was 

 of the same grade but had half a generation less of selected ancestry. 

 Male 6072 had 75 homozygous English young, of mean grade 1.97 

 (father's record 1.79); he also had 159 heterozygous English young 

 of mean grade 4.63 (father's record, 4.66). Increase in the grade of 

 the young with increase in the mother's grade is very clearly shown 

 among the young of this sire. (See table 38.) 



The next male tested was 6964 (table 39) a son of cf 6071. He was 

 discarded after a set of matings which showed him probably not 

 better than his uncle, 6072, who was still in service. He had 21 homo- 

 zygous young of mean grade 1.49 (his uncle's record was 1.97) and 29 

 heterozygous English young of mean grade 4.68 (his uncle's record 

 being 4.63). Next was tested cr7699 (table 40), son of 6^6072, who 

 shared with his half brother, cf 6370, the position of best sire so far. 

 All were of the same grade, 5. This male was mated with all available 

 does and produced 354 recorded young. He has a better record than 

 any sire so far tested. By heterozygous does he has sired 75 homozy- 

 gous young of mean grade 2.31 and 149 heterozygous young of mean 

 grade 4.80. 



Another male of the same grade and generation as the foregoing, 

 indeed his half-brother, being also a son of 6072, was tested, but 

 appeared not to be superior to 7699 and so was soon discarded. This 

 animal, 9532 (table 41), sired 16 homozygous English young of mean 

 grade 2.53 and also 27 heterozygous English young of mean grade 4.73. 



Three sons of the superior male, 7699, have since been tested, viz, 

 9806, 1212, and 534 (tables 42-44). The first one shows no probable 

 superiority over his father, but the last two are more promising, each 

 having produced a total of over 60 heterozygous English young with a 

 mean close to grade 5. In the case of their father the corresponding 

 group of young were of grade 4.80. The homozj-gous young produced 

 by their father were of mean grade 2.31; those produced by the sons 

 were of mean grade 2.87 and 2.95 respectively. Accordingly, as regards 

 both heterozygous and homozygous j'oung, the sons have a distinctly 

 better record. This, no doubt, was due in part to the fact that their 

 mates were of higher grade or from more highly selected stock, but it 

 was not wholly due to this cause, for their half-brother (9806, table 42) 

 did not show the superiority which they showed, even when mated 

 with females of high grade and advanced generations. Hence we must 

 conclude that these two males, 1212 (table 43) and 534 (table 44), were 

 genetically superior to their father. 



Table 45 shows the grade distribution of the young produced by a 

 homozygous Enghsh male, 1173 (plate 3, fig. 6), when mated with does 



