136 W. J. MOENKHAUS 
3, it will be seen that the pair marked A produced offspring out of 
which nine of twelve pairs tested were infertile; pair'B produced 
offspring of which four pairs out of fourteen tested were infertile 
and pair C threw offspring with five pairs out of fifteen infertile. 
We have here, then, three pairs showing a wide variation in the 
degree of fertility of their offspring. Pair A showed 75 per cent of 
the pairs infertile and pairs B and C approximately the reverse 
ratio. In the further progress of the experiment pair C was dis- 
TABLE 4 
Strain A 
| NUMBER | NUMBER NUMBER PAIRS lpr CENT PAIRS PER CENT PAIRS 
| PAIRS TESTED | PAIRS FERTILE INFERTILE | FERTILE | INFERTILE 
LS OD )ee | 52 27 25 51 49 
18 (2)... 51 37 14 | 72 | 28 
1Sa(3)e.. 52 37 15 71 | 29 
18 (4)... rl 56 45 | 11 | 80 | 20 
1305) ae =| 28 19 9 | 69 | 31 
Average for 238 pairs 69 per cent. 
Strain B 
i ; —_ he - A 
HSL) eh trae es 15 Spi 0 100 0 
18 (2). 4 Cane 0 100 0 
1S 5(3) tectess seesee 2 19 19 | 0 100 0 
18 (4).. 22 15 7 | 68 32 
Gee 23 23 0 | 100 0 
Average for 93 pairs 92.5 per cent. 
continued so that only pairs A and B were used. I shall in the 
further description of the experiment refer to the descendants of 
A as strain A and of B as strain B. 
Before entering upon the experiment of selection it was neces- 
sary to ascertain whether, without selection, the descendants of 
pairs A and B continued to show a low and high fertility respec- 
tively. Accordingly, a single one of the fertile pairs of the 15th 
inbred generation of strain A and B was tested. Reference to 
the table shows that in strain A 27 pairs or 57 per cent of the forty 
seven pairs tested were infertile, while in strain B none of the thirty- 
