BRISTLE INHERITANCE IN DROSOPHILA 143 



heterozygosity of these accessory factors was regained in Fi, 

 and in Fo the segregation of these factors resulted in the lowered 

 bristle averages, and again offered a chance for selection to 

 modify the means. The differences between germ cells had 

 become great enough to influence the somas in spite of the 

 uncontrollable variations in the environment. 



If a single Mendelian factor that varied in potency be assumed, 

 the production of a high and a low race would be the expected 

 result of selection. Furthermore, it would be expected that the 

 transformation of the high race into a low race would be just as 

 easy. Since this was not the fact, it becomes necessary to as- 

 sume further that, because the factor had varied a certain amount 

 in one direction, it was hindered from varying in the opposite 

 direction — a supposition for which one would have difficulty in 

 devising a mechanism. If one varying factor be made the 

 basis of the explanation, a special hypothesis must be made 

 to explain why the advance shown in the early generations of 

 selecting was not continued. The physiological limit in bristle 

 numbers had evidently not been reached; some limitation on the 

 higher variations of this factor must be assumed. The last two 

 hypotheses leave a variable factor that, after a series of selections 

 is limited in its further variations in both directions. It becomes 

 a matter of the number of generations before the variability of 

 this factor no longer exists, before the end of the cul de sac is 

 reached. Further subsidiary hypotheses would have to be 

 added to account for the results of crosses with normals. It 

 would be necessary to assume that the variations found in the 

 germ cells of an individual heterozygous for this factor, are dif- 

 ferent from those in the germ cells of a homozygous individual. 

 The 'Contamination theory' affords an explanation for this. 

 The cross brought this factor and its allelomorphic mate to- 

 gether in the same nucleus. In order to explain any difference 

 due to this heterozygosity, it must be assumed that there is an 

 intimate fusion between these two members of the pair, and 

 then that this fusion weakens the power of that factor for form- 

 ing extra bristles. When the high race was selected longer, the 

 modification of the means and modes found in Fo was greater, 



