Abstract
Evidence so far available indicates that in the Culex pipiens complex there exists a genetic system with unique properties not comparable with those of the chromosomal gene system and probably involved in a mode of speciation peculiar to this complex. In a long series of backcrosses it has been shown that probably every Culex pipiens population contains a factor that is inherited through the cytoplasm, the only phenotypic expression of which is the crossability or non-crossability (incompatibility) of a given strain with other strains. That there is a whole series of such cytoplasmic factors is indicated by the twenty or so crossing types of Culex strains found throughout the world.
In closed populations it seems likely that, as a result of genic diversification, a modification occurs that will eventually lead to differences in traits determined by genes.
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