20 



HEREDITY IN RABBITS, RATS, AND MICE. 



from U to 4, the mean being 2.80. Evidently the heterozygous 

 English" produced by these matings with unrelated does were much 

 more variable than those produced by matings of the original English 

 individuals with each other. 



It now occurred to us to see to what extent this variabiUty could 

 be carried farther in a plus direction by selection. Accordingly we 

 chose the grade 4 individual produced by 9 1492 as the starting-point 

 of the experiment. This individual had been recorded as d'2711. 

 He constitutes generation 1 of the selection experiment now to be 

 described, all animals produced in those experiments deriving their 

 English from him. He was mated with a black-and-tan doe, with 

 three black does, and six Himalayan (albino) does, all free from 

 racial white spotting (EngHsh or Dutch). These matings produced 

 56 young (table 31, b) equally divided between heterozygous English 

 and self. The English young were of higher grade than the English 

 young produced by 0^2545. (Compare tables 30 and 31.) They 

 ranged from grade 3 to grade 5, mean 3.89, as compared with a mean 

 of 2.80 for the young of cf 2545, produced in similar matings. 



Male 2711 was later mated to 5 of his heterozygous English daugh- 

 ters produced in the matings already described, and also to one of the 

 resulting grand-daughters. The character of their young is shown 

 in table 31, a. The mothers form generations 1^ and 2^ of the selected 

 English race. They vary in grade from 3|- to 5. They produced three 

 classes of young: low-grade English (homozygotes), high-grade 

 English (heterozygotes), and selfs. Their respective numbers were 

 12, 11, and 8. The mean of the low-grade English group was 1.52, 

 that of the high-grade group was 3.93, which agrees closely with the 

 grade of the same group produced by matings with non-English (self) 

 does (table 31, b). 



Grouping the mothers by grade, the relation shown 

 herewith is observed between grade of mother and 

 grade of young. This indicates that selection of 

 higher-grade parents would probably result in produc- 

 ing higher grade young. 



A son of 0^2711, viz, cr5086, generation 1^, grade 4|-, was chosen 

 to succeed him in the selection experiment. This buck was mated 

 with 10 different heterozygous English does, 5 of which had also been 

 mated with his father. (See table 32.) There resulted 20 homozygous 

 English young, 44 heterozygotes, and 35 selfs. The mean grade of the 

 homozygotes was 1.38, that of the heterozygotes was 3.96, averages 

 not very different from those which characterized the young of d^2711 

 (table 31). Consequently no advance can be claimed as a result of 

 the selection of cf5086. He was mated also with 3 black-and-tan 

 does (table 32, b), producing thus 10 English j^oung of mean grade 

 3.15, a lower average than that given by cf 2711 in matings with self 



