An isomorphic relation between two structural domains prevails if and when their corresponding parts can be mapped onto each other, i.e. that each part of the first structural domain corresponds with a part from the second domain. In order that a part A from one domain corresponds with a part A' from the other domain, it is NOT necessary that A and A' should be identical in species nor that they should be similar. That what should correspond is their function and/or position within the respective structural domains.