One endemic limitation of the tree model is the very founding presumption on which it is based: it requires a classification based on languages.or, more generally, on language varieties. Since a variety represents an abstraction from the totality of linguistic features, there is the possibility for information loss during the translation of data (from a map of isoglosses) into a tree. For example, there is the issue of dialect continua. They provide varieties that are not unequivocally one language or another but contain features characteristic of more than one. The issue of how they are to be classified is similar to the issue presented by ring species to the concept of species classification in biology.