34 AN ANALYSIS OF THE EFFECT OF SELECTION. 
Matings of intermediates by intermediates gave both intermediates 
and normals, showing that the character was either dominant or irreg- 
ular in appearance. 
Matings of intermediate to specks and to black purples of other 
stocks gave only normals, showing the character to be recessive. 
Mating together the F; normals from the last type of matings gave 
a few intermediates; but these were in no case speck or black or purple. 
This is the usual behavior of a second-chromosome recessive, due to 
no crossing over in the F, male. Hence ‘‘intermediate”’ is a recessive 
character, and lying in the second chromosome. Its occurrence in 
the Star Dichet must have been only a coincidence, and can have had 
nothing to do with the presence of Dichet in that stock. The differ- 
ence between this character and Extended is a striking illustration 
of the danger of arguments as to the identity of characters based on 
similarity of appearance. 
NOT-DICHATS FROM SELECTED LINES. 
As has already been pointed out, Dichet flies almost always have 
fewer bristles than have normals. All Dichets are heterozygous for 
the normal allelomorph. Therefore, in such an experiment as this 
one, in which Dichets are repeatedly mated together, one obtains 
normal flies the not-Dichzt genes in which have been associated with 
Dichet genes for many generations. The experiment is, then, suited 
for a study of the question as to whether or not factors “contaminate” 
their allelomorphs. If this contamination occurs, one might expect 
the not-Dichet flies to show a tendency to have fewer bristles than 
they normally have, and the Dichets to have more. That Dichets 
tend to increase in bristle number is very improbable. The stock 
has now been kept, always of necessity in heterozygous condition, for 
more than 40 generations. There is no evidence that any progressive 
change has. occurred, though no selection has been used in keeping 
the stock cultures. The modal class at present (5 bristles) is actually 
lower than the class (6) of the original mutant. 
There are some data regarding the bristles of the not-Dichets pro- 
duced by selected Dichets. Counts of these bristles have been taken 
only occasionally (see table 24), but whenever a bristle number other 
than 8 has been observed in such flies it has been noted on the record 
sheet. Examination of these notes shows that in the minus-selected 
series there are several records of 6 and 7 bristled not-Dichets, but 
none of numbers higher than 8. In the plus selected lines there are a 
number of records of nines and tens, but no sixes and only 1 seven 
(from 1190, an F, of the crossbred plus series). The complete counts 
taken of bristle numbers are given in table 24. 
It may be pointed out that the familiar yellow mouse and several similar cases in Drosophila 
afford evidence of the same sort against contamination. 
