GENETICS: W. E. CASTLE 
129 
A first cross of the plus race (table 1) lowered the grade of the hooded char- 
acter from about +3.73 to +3.17. A second cross brought a slight rise in the 
mean grade of the extracted hooded young to +3.34, and a small group of 19 
hooded young extracted from a third cross had a mean grade of +3.04. It 
will be observed that the hooded character was lowered not over three-fourths 
of a grade by three successive crosses. This fact led me to conclude provision- 
ally in 1916'* that the hooded gene proper had really changed in the course of 
our selection experiments, since after the crosses it remained different from 
what it had been originally. This view is obviously erroneous in the light of 
the results obtained from the minus crosses subsequently studied. 
TABLE 1 
Results of Crossing the Plus Selected Race with a Wild Race 
MEAN GRADE 
STANDARD DEVIATION 
NUMBER OF HOODED 
YOUNG 
Control, uncrossed plus race, gen- 
eration 10 
Once extracted hooded F2 young . . . 
Twice extracted hooded F2 young . . 
Thrice extracted hooded F2 young. . 
+3.73 
+ 3.17 
+3.34 
+3.04 
0.36 
0.73 
0.50 
0.64 
776 
73 
256 
19 
TABLE 2 
Results of Crossing the Minus Selected Race with a Wild Race 
MEAN GRADE 
STANDARD DEVIATION 
NUMBER ON HOODED 
YOUNG 
Control, uncrossed minus race, 
Once extracted hooded F2 young . . . 
Twice extracted hooded F2 young . . 
Thrice extracted hooded F2 young. . 
-2.63 
-0.38 
+ 1.01 
+2.55 
0.27 
1.25 
0.92 
0.66 
1,980 
121 
49 
104 
The crosses of the minus selected race were started some six generations 
later in the history of our selection experiments than were those with the plus 
selected race. They gave results much more striking than those of the plus 
crosses. (See table 2.) The mean grade of the minus race, when the crosses 
were started, was —2.63. The F2 hooded young from a first cross with the 
wild race were of mean grade —.38, a change of over two grades. A second 
cross produced hooded F2 young chiefly plus in character, mean grade +1.01. 
A third cross produced F2 hooded young exclusively plus in character, mean 
+2.55. One family in this lot of thrice extracted hooded young, consisted of 
14 hooded individuals of mean grade +3.05, almost exactly identical in mean 
grade with the thrice extracted hooded young of the plus series (table 1). 
This result indicates that three crosses with a third race had sufliced prac- 
tically to eliminate whatever differences had been produced in the minus and 
