42 
GENETICS: CASTLE AND HADLEY 
After this series of matings had been completed, a second series was 
begun in which the same 13 females were mated with one of the darkest 
bucks produced in the Series I matings (a son of 9 16 E). The selected 
buck was d'40A (fig. 6) grade 3.75, considerably darker than his father 
(fig. 5). This series of matings produced 189 English young, together 
with a like number of self (non-English) young. The grade distribution 
of the English young is shown in Table 1, Series 11. All of the 13, 
mothers except one ( 9 16F) produced darker offspring in the Series II 
than in the Series I matings. The lowest average grade was shown by 
the young of 9 17G, viz. 2.44. For Series I matings, the lowest average 
was 2.15. The highest average grade in the Series II matings was given 
by the young of 9I6E, viz. 3.50. For Series I matings, the highest 
average was 2.78. Consequently both maximum and minimum aver- 
ages were higher in the Series II than in the Series I matings. The 
grand average of all the 189 Series II offspring was 2.92, as compared 
with 2.43, the average grade of the Series I young. The modal grade 
of the Series II young is 3.25. The modal grade for Series I was 2.00. 
Since the mothers were identical in both series, the difference in the 
young can be attributed only to the difference in the fathers. The 
male used in the Series II matings differed genetically as well as somati- 
cally from his father who sired the Series I young. Not only was he 
darker, but he also produced darker English young. Yet the father 
contained only a single dose (one gamete) of English pattern and the 
son derived his English pattern exclusively from this same source. 
Hence the English unit-character had changed quantitatively in trans- 
mission from father to son. This seems to us conclusive evidence 
against the idea of unit-character constancy, or "gametic purity.'^ If 
unit-characters are not constant, selection reacquires much of the impor- 
tance which it was regarded as possessing in Darwin's scheme of evolu- 
tion, an importance which many have recently denied to it. 
The question whether an imaginary ' unit-factor' for EngKsh pattern 
has or has not changed in correlation with the visibly changed English 
unit-character is not here discussed. We recognize that it has an aca- 
demic interest, which however scarcely affects the practical question 
whether the visible Mendelizing characters of animals are subject to 
change through crossing or through selection or both. 
