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THE AMERICAN NATURALIST [Vol. XLVIII 



tained, but occasionally a strain of Beaded flies is met 

 with that gives only low percentages of Beaded-winged 

 offspring. See, for instance, Family 4, Table VII. Pos- 

 sibly such a stock would not be recognized at once, espe- 

 cially if it were so affected by environmental conditions 

 that even flies homozygous for the factor B'B' sometimes 

 had normal wings. Normal-winged flies, as will be 

 pointed out in a later section of this paper, do very fre- 

 quently appear in Beaded stock, but these flies when 

 mated to each other appear to throw as many Beaded- 

 winged offspring as do the Beaded-winged flies of the 

 stock, and often 100 per cent, of their offspring have 

 Beaded wings. 



In this connection it will be of interest to recall that 

 Chart 1, and Table I gave results that might be inter- 

 preted as evidence of the bimodal curve that should be 

 expected if the above hypothesis is correct. 



Normal females from the second generation of Family 

 2 were also back-crossed to Wild males. The results are 

 given in Table VIII. Most of these normal females gave 

 very few or no Beaded offspring (Type X) while two of 

 them gave a considerable number of Beaded offspring 

 (Type Y). The explanation here is perhaps that the type 

 Y females were genetically like most of the Beaded 

 females of an F 1 generation (on our hypothesis, B' L b' 1) 

 while the females of Type N were genetically lacking in 

 the factors that are usually present in Beaded Fj flies 

 (i. e., they were B' L 1/ L). That such an occurrence is 

 not infrequent in Drosopliila is seen in Table IV in which 

 three broods out of fifteen raised from normal F a flies 

 gave 25 per cent, or more of Beaded offspring though the 

 other twelve broods gave less than fifteen per cent., and 

 eight broods less than five per cent, of Beaded offspring. 

 It seems certain therefore that there are two types of 

 normal-winged offspring in the F, generation of the cross, 

 Beaded by Wild; one of these is genetically like the 

 Beaded flies of the same generation and the other is 

 genetically different from its Beaded brothers and sisters. 



Types X and Y have been found to occur in all of the 



