A taxon is a particular complete part of the phylogenetic tree (branch or monophyletic group or clade), which the relevant professional – taxonomist – defines and names. Thus, a taxon can be a single species, such as a chimpanzee, or perhaps the family of canine carnivores. At the present time, it is required that each taxon be monophyletic, i.e. that it include only a single species (common ancestor), whose ancestor was, itself, not a member of the particular taxon. A large number of experts (cladists) also require that the taxon include all the descendants of a particular common ancestor. Thus, cladists declared that a number of former taxons were invalid, including such ones as fish and reptiles. (It must be admitted that they had quite good reasons for this; however, it is probably better not to discuss this here.) A taxonomist can define and name any branch of the phylogenetic tree; however, in actual fact, he defines only those taxons whose members differ substantially in some way from the members of other taxons.