as a general rule for most species all of its members are of the same cytodeme excepting only the infrequent aneuploid aberrants. however some species are known to include several chromosome races which must necessarily belong to different cytodemes. although it is not intrinsic in the formal definition it is a matter of common observation that members of different cytodemes are essentially cross-incompatible and any hybrids that do arise are usually highly infertile.