350 J. A DETLEFSEN AND E. ROBERTS 
While it is possible that this chromosomal region may fail to 
show the same phenomenon which we found in the sex chromo- 
some, we are rather inclined to believe that the difference between 
our results and Gowen's is more likely due to differences in the 
method of procedure, for Gowen states that his "chief difficulty 
lies in the few individuals that it was possible to include in a 
given generation." Gowen gives only the mean total crossing 
over in each generation, and we do not know how rigid his selec- 
tion may have been, for he does not state how many pairs were 
included in each generation nor does he give the frequency dis- 
tribution for crossover values. We suspect that he found the 
same impediments in using strict brothet-and-sister matings 
which we found and which prompted us to use en-masse matings 
in alternate generations to increase our numbers. We are carry- 
ing on selection experiments in other regions of the sex chromo- 
some and in the autosomes, which should decide whether other 
regions and chromosomes are similarly affected by selection. 
We have no reason to suppose that they will not be. 
The effects of selection upon crossover values may be due to 
one or a number of causes, some of which suggest themselves 
ahnost immediately. It would hardly be profitable to expatiate 
on these, since we are making tests, which we hope may indicate 
what has really happened in the course of selection. Briefly 
stated, we think of the following possible causes which may have 
been operative in modifying our crossover values: 
1. We may perhaps have dropped out a large part of the 
chromosome between white and miniature, thus bringing these 
two genes closer together. We can probably disregard this as a 
cause, for although 'deficiency' reduces crossing over (Biidges, 
'17) nevertheless the lethal action of deficiency would be seen in 
a disturbed sex ratio. We found no such disturbance. 
2. Is it possible that we may have moved the locus of the 
genes on the chromosome? This would mean that the locus of a 
gene is not permanently fixed, but that a given gene is found 
in a characteristic position in the majority of cases. If we have 
done this, and at the same time have not moved other genes, 
then linkage tests should disclose this fact, for the order of the 
genes would be changed. 
