Introduction. By Darwinism is meant the idea that evolution is the result of natural selection. Neo-Darwinism adds to this idea a theory of heredity. In its most general form, the theory of heredity is Weismannism, i.e. it is the theory that changes in the hereditary material are in some sense independent of changes in the body or 'soma'. In particular. the theory of heredity is Mendelian, i.e. it assumes that heredity is atomic, and obeys either Mendel's laws or some modification of them explicable in terms of the behaviour of chromosomes ( e.g. linkage, polyploidy).