 There must be a relationship between the hereditary materials, and their behaviour, throughout the whole group or species; and it is on this relationship the genetic system depends for its character. Hybridity optimum, segregation, and recombination must all be related throughout the group. Furthermore, they must be related not merely amongst themselves, but also to the mating habits of the individuals which compose the group, and to the barriers which make that group by separating or isolating it from others. (Darlington and Mather 1949: 237; their italics).