as yet no mechanism is known which could account for the origin of new diploid cytodemes - nor is there any proof that there are any diploid cytodemes of recent origin. indeed, the evidence is that cytodemes are remarkably long lived. presumably the "platanus" cytodeme existed on pangaea before that land fragmented. the present day forms of "platanus" - "p. orientalis" in europe and "p. occidentalis" in america - are fully intercompatible when brought together artificially in botanic gardens yielding fertile hybrids like the london plane, establishing not only that they are in the same cytodeme as one another but also that they are both in the same cytodeme as their common ancestor of approximately 200 million years ago.