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 F, 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 F,; and a larger, continuous 
variability in F, 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. 59A), 1,614 of the lighter, or 
