MULTIPLE FACTORS 237 



productivity depend on multiple factors. For ex- 

 ample, East crossed the strain Tom Thumb (having 

 short ears) to black Mexican sweet (having long 

 ears). The relative length of ear in these two races 

 is shown in the upper line of Fig. 59, to the left and 

 to the right. A sample of the Fi ears is shown in 

 Fig. 59, the middle of the figure, while the variability 

 of the F2 ears is shown in the lowest line. It is 

 evident not only that the original types reappear, 

 but that there are all intermediate lengths of ear 

 in F,. 



These are only a few typical illustrations, se- 

 lected from among many similar cases in which a 

 small variability in Fi and a larger, continuous 

 variability in F2 have been described. It should, 

 however, be noted that these criteria taken by them- 

 selves do not constitute decisive proof of the ex- 

 istence of multiple factors in any particular cross. 

 For even if only a single factor-difference is present, 

 the disturbing action of fluctuation in the somatic 

 manifestation of the factors may produce effects 

 superficially very similar to those described above, 

 provided the fluctuation is great enough to make 

 the different types overlap in appearance. This is 

 true especially in cases in which the dominance is 

 incomplete. A good example of this kind is Morgan 

 and Bridges' cross of flies with and without a trident 

 pattern on the thorax (see Fig. 59A, I to X). 

 When the flies of the two parent races were classified 

 according to the different grades of intensity of 

 the marking (Fig. 59^4), 1,614 of the lighter, or 



