If systems of different complexity are encoded by genotypes of different length, then the genetic operators must allow changes in genotype length to take place in the process of going from parent to offspring. How this is done will necessarily depend to a large extent on the form of genotype encoding. One lesson spelt out in [4] is that any genetic operator which allows a change in genotype length should be restricted to small changes only -- to be precise, to changes which in general can be expected to produce a small phenotypic change. This last qualification allows for genetic operators such as gene duplication where the encoding used means that such duplications are neutral. Neutral gene duplication, followed by mutations in one of the copies, is a potentially powerful method for creating variants on useful substructures.