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THE AMERICAN NATURALIST [Vol.L 



and is dominant to it, though not completely; club is 

 recessive; bar is dominant (somewhat incompletely). 



The reason that club was not put into the series with the 

 other recessives is that it was discovered (by Bridges) 

 after this series had already been put together, and so it 

 would have required taking the stock apart again, or else 

 obtaining a rare double cross-over, to wedge club into this 

 series. It was a valuable factor to have in the experiment, 

 however, since it lay in a region of the chromosome where 

 there were no other mutant factors to give data as to 

 crossing-over. Accordingly, it was inserted in the other 

 series. It will be observed, however, that, in spite of this, 

 one chromosome contains 7 more dominants than the other. 



The order of the above factors is y, w or c, A, b, c x , v, 

 m, s, r, f, Br. In making up the first stock the factors were 

 put together as follows (omitting from consideration all 

 trial or discarded combinations) : 



Of course the putting together of factors from two stocks, 

 although shown above as only one step in each case, always 

 requires several generations. Moreover, as will be seen 

 below, these steps do not usually consist in getting the 

 ordinary F 2 or back-cross, in the case of the complicated 

 combinations. This is partly because of the serious ob- 

 stacle which the poor viability of flies having many mu- 

 tant characters presents to the making up of multiple 

 stock, just as it does to the securing of counts from it ; 

 moreover, in the making up of stock, the sterility of such 

 flies is an equally important difficulty. 



These difficulties were overcome here in much the same 

 \v,iv as they were in making the counts— namely, by keep- 

 ing the stock, so far as possible, heterozygous. For ex- 



