 Darlington argued that the genetic system of a species is connected to its preference for inbreeding or outbreeding, since together they affect the frequency of heterozygosity (Darlington 1939; Darlington &amp; Mather 1949). He noted that Mendelian inheritance establishes a cycle between free and potential (latent) genetic variability (Darlington &amp; Mather 1949: 276): Potential variability is contained by heterozygotes, while free variability is exhibited by the phenotypes of their offspring as a result of segregation. 