Unlike previous methods, cladistics attempts to consider all the characteristics of an organism, rather than those subjectively chosen as most important. As a result, it gives less significance to unique or bizarre characteristics than to those that are shared, since only the latter can demonstrate relationships. Cladistics also emphasises the concept of a monophyletic group, in other words one that consists only of a common ancestor and all its descendants – for example it regards the traditional term "reptile" as useless, since mammals and birds are descendants of different groups of "reptiles", but are not considered "reptiles".[1] The concepts of crown groups and stem groups, first presented in English in 1979, are consequences of this approach.