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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 



Mode 



Fia. 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, i.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 



