INBREEDING AND SELECTION IN DROSOPHILA AMPELOPHILA 137 
seven pairs tested were infertile. The same process was repeated 
with a pair of the sixteenth generation of the two strains. Strain 
A showed twenty-seven or sixty-four per cent of the forty-two 
pairs tested infertile and strain B one or three per cent of the 
thirty-six pairs tested. . 
Up to this point in the experiment only a single pair in each 
generation was tested as to the fertility of its offspring. It might 
well be that by chance in each case a pair of low fertility was taken 
in strain A and a pair of high fertility in strain B. Toeliminate 
this possible error five pairs were taken in each strain and the 
fertility of their offspring determined. It was further desirable to 
obtain an estimate of the variability in the fertility of the pairs in 
the two strains as well as to get a more correct estimate of the 
average fertility of both. In the diagram these five pairs are 
designated as 18 (1), 18 (2), ete. Table 4 shows the number of 
pairs of offspring tested for each pair and the number and per- 
centage of pairs fertile and infertile. 
The fertility thus varied in strain A from 51 per cent in 
18 (1) to 80 per cent in 18 (4), with an average fertility of 69 
per cent. In strain B the fertility was much less variable in the 
different pairs, the only exceptions being 18 (4), the average fertil- 
ity being 92.5 per cent. 
We now have definitely established two strains, one of low 
and another of high fertility. The important part to be empha- 
sized here is that this was produced by the process of selection from 
among the variable offspring of generation fourteen of the inbred 
strain. To make the experiment more complete it was now neces- 
sary to obtain a highly fertile strain out of the one with low fertility. 
Accordingly strain B was discontinued at this point and attention 
restricted to strain A. Five pairs, 19 (1), 19 (2), 19 (3), ete., were 
taken from among the offspring of 18 (4) because this showed the 
highest percentage of fertility. These were tested in the same way 
as in the preceding generation. Table 5 gives the details. 
By selection it will be seen that the average fertility was raised 
from 69 per cent in the 18th generation to 75 per cent in the 19th 
generation. Among the five pairs used one 19 (2) showed an 
unusually high fertility (96 per cent). This pair was accordingly 
JOURNAL OF MORPHOLOGY, VOL. 22, NO. 1 
