An abundant literature of hybridisation data is in existence, particularly for the birds and mammals. At least 1 in 10 bird species is known to hybridise. As well as interspecific, intergeneric and even intertribal crosses have been found. To these can be added a great number of hybrids obtained in captivity. Scherer and colleagues have seized the opportunity to harness this data by constructing cross-breeding matrices, and in some cases have added supplementary criteria.
Further details are in the table at the end.
An overview of the Anatidae relationships is shown in the diagram below. One potential problem is that there are hierarchies of similarity, and it can be difficult to demarcate exactly where the discontinuity between kinds may lie. Scherer emphasised the objectivity of the hybridisation criterion. Basic Types will only be possible among organisms sharing similar embryological pathways, ie, having a common morphogenetic machinery. Scherer and colleagues have recently brought these ideas together into a publication, Typen des Lebens , which contains an overview, then a chapter focusing on each Basic Type.