I shall deal only with the heterosis concerned with the genetic balance-mechanisms underlying morphism (balanced polymorphism as defined by E. B. Ford). All morphisms found in nature must involve a selective balance between the morphs (R. A. Fisher), and most are either extrinsically adaptive (in adapting the species to a number of widely distinct conditions or habitats), or long-estabhshed (as in Primate blood-groups, Cepaea pattern, etc.), or both. Accordingly, they provide special opportunities for studying the origin, function and evolution of any heterotic mechanisms concerned.