INBREEDING AND SELECTION IN DROSOPHILA AMPELOPHILA 131 
that the pairs that had not completely lost their fertility, in so 
far as hatching their eggs is concerned, had suffered no deteriora- 
tion whatever as a result of seventeen generations of closest in- 
breeding. 
A fact of further importance brought out by table 1 is that of 
the percentage of eggs that successfully produced imagos. This 
does not differ essentially in the two groups of inbreds nor do 
these differ essentially from the normals. Castle used as his 
measure ‘productiveness,’ meaning thereby the number of pupae 
that were successfully produced. Making allowance for some 
pupae which do not emerge, the imagos produced in my experi- 
ments were an approximation to his ‘productiveness.’ Inbreed- 
ing, consequently, does not affect adversely the productiveness of 
pairs that show any fertility at all. 
Castle found that his strains showed an annual fluctuation in 
productiveness, the period of least productiveness falling in the 
late autumn and early winter. My own experiments extended 
over about four and one half years and, although I have been on 
the lookout for this, I have never observed it. As Castle himself 
suggests, this fluctuation was probably a function of the tempera- 
ture of the room. My flies were kept in a room which varied from 
60 to 80 degrees and, when this was not possible, they were placed 
in an incubator kept at about the same range of temperature. It 
may also be that the productiveness of his strain ran low at this 
time of the year because they were placed in new hands at the open- 
ing of the college year. My observation has been that it takes 
some time for a new man to learn all the conditions that make for 
a favorable rearing of these creatures so that Castle’s low produc- 
tive periods may be merely a measure of the training period of the 
experimentor. 
3. Inbreeding and vigor 
At the outset of the experiments it was the expectation of the 
writer that such rigorous inbreeding would early and violently 
show itself in the vigor and fertility of the animals. In this, how- 
ever, he was largely disappointed. In the strain that is here under 
consideration no untoward results could be detected during the 
