INBREEDING AND SELECTION IN DROSOPHILA AMPELOPHILA 139 
tested. One of these possessed a high degree of sterility, while the 
two other pairs showed alow degree. The descendants of thelatter 
constituting strain B, retained this low degree of infertility through- 
out. Similarly the descendants of the former, constituting strain 
A, retained their high degree of infertility up to the time when 
selection away from this condition was introduced. In the latter 
process the transmissibility of the character is again emphatically 
revealed. In the eighteenth generation, pair 4 showed a lower 
degree of sterility than any of the remaining four pairs of brothers 
and sisters. Breeding from this pair at once showed offspring 
with a decided decrease in sterility, compared with the eighteenth 
generation, the average of the nineteenth generation being 75 per 
cent of the pairs fertile as compared to 69 per cent of the latter. 
Again, in the nineteenth generation, pair 19 (2) showed a much 
lower degree of infertility than the other pairs. Continuing the 
strain from this pair, this character is faithfully reproduced in 
the offspring in that they average fertility of the latter is raised 
to 93.8 per cent. 
It is important to note in this connection that Castle, in his 
experiments upon Drosophila, found that productiveness (which 
as previously noted is quite a different thing from the sterility 
here considered) was similarly transmissible and amenable to 
selection. Furthermore, Castle’s experiments would seem to indi- 
cate that this character of productiveness behaves, in inheritance, 
after the Mendelian fashion, low productiveness acting as the 
recessive character. We have evidently to do here, both in the pro- 
ductiveness in Castle’s experiments and in the sterility in my own, 
with characters that are germinal for they behave as such. In 
the strain upon which my experiments were made we have the 
further remarkable condition that the infertility is inherited 
only by the males. 
It is clear that whatever the causal factor or factors to which the 
sterility may be attributed, it is relatively insignificant compared 
to the effect of selection upon it. Furthermore, the modification 
is a germinal one. That inbreeding may be responsible for its 
prevalance in the strain seems probable, but that it is responsible 
