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progeny of the unselected eighth generation population lay much nearer 
to the mode of the original population. Within three generations the 
unselected strains had moved back to the mode of the species. Now 
it is to be remembered that Tower was dealing with an obligatory allo- 
gamous species. Moreover, what is now known concerning body 
pigmentation in Drosophila makes it altogether likely that quite a large 
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Fic. 109.—Diagrammatic representation of the results obtained by the creation of 
albinic and melanic strains from a mixed population of beetles. (From Tower.) 
number of genetic factors are concerned in the degree of pigmentation 
of these beetles. Hence selection of phenotypes for a number of genera- 
tions did not isolate genotypes, 7z.e., the plus and minus strains were not 
homozygous. As this is an allogamous species undoubtedly most of the 
individuals in the original population were heterozygous for many factors. 
Furthermore, Tower did not select single pairs but always took several 
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