Abstract
In 20 possible crosses of 5 geographic strains of D. sturtevanti, all tended to prefer homogamic matings. The extent of the incipient sexual isolation is variable, being statistically significant in 10 cases, and is not strictly correlated with the geogr. origin of the strains; some strains coming from remote localities show less aversion to interbreeding than strains from less remote localities. In 2 strains of D. nebulosa there is a onesided mating preference, [male][male] of one strain only showing a homogamic preference. In crosses of this type also there would be a tendency to reduce the rate of gene interchange.

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