the current surgance of interest in cytodemes stems from the realisation that membership of the cytodeme is not necessarily restricted to the members of one species: frequently two or more species are in fact of the same cytodeme. although fully intercompatible and yielding fertile hybrids when they do cross these species never or rarely cross-breed in the natural environment because they are spatially isolated, geographically and/or ecologically. typically the species involved are members of the same genus. there are now a substantial number of cases in which two or more species of the same cytodeme are clearly different and even in different genera. thus all five diploid species recognised in the genus "brassicella" and both species recognised in the genus "hutera" (7spp. in total) are in the same cytodeme. perhaps the most extreme case on record concerns the wild grass, teosinte, "euchlaena mexicana" and the strikingly different maize, or indian corn, "zea mays", both 2n=20, fully interfertile and yielding fertile hybrids. there is perhaps a remote possibility that two such very different species could have evolved independently from distinct sources and converged in their chromosomal ideotype until they became members of the same cytodeme (including the capacity to cross-breed). it is more likely, though, that the cytodeme arose first complete with its suite of chromosomes and breeding patterns all intact and then, remaining constant in its fundamentals, it diversified into species sometimes so different as to merit generic distinction. thus, in what may be termed the "cytodeme adjunct" to darwinian theory, evolution becomes a two-stage process - first, the establishment of distinct cytodemes reproductively isolated both from one another and from all previously existing cytodemes; second, diversification within cytodeme to yield taxonomically recognisable (but not reproductively isolated) species.