▁compar ative ▁method , ▁a ▁group ▁of ▁languages ▁that ▁exclus ively ▁shares ▁a ▁set ▁of ▁innov ations ▁constit utes ▁a ▁"( g ene alog ical ) ▁subgroup ". ▁A ▁link age ▁is ▁thus ▁usually ▁character ised ▁by ▁the ▁presence ▁of ▁intersect ing ▁sub groups . ▁The ▁tree ▁model ▁does ▁not ▁allow ▁for ▁the ▁existence ▁of ▁intersect ing ▁sub groups ▁and ▁so ▁is ▁ill - su ited ▁to ▁represent ▁link ages , ▁which ▁are ▁better ▁approached ▁using ▁the ▁wave ▁model . ▁ ▁The ▁cl ad istic ▁approach ▁underlying ▁the ▁tree ▁model ▁requires ▁the ▁common ▁ancest or ▁of ▁each ▁subgroup ▁to ▁be ▁dis cont igu ous ▁from ▁other ▁related ▁languages ▁and ▁unable ▁to ▁share ▁any ▁innov ation ▁with ▁them ▁after ▁their ▁" separ ation ". ▁That ▁assumption ▁is ▁absent ▁from ▁Ross ▁and ▁François ' s ▁approach ▁to ▁link ages . ▁Their ▁gene alog ical ▁sub groups ▁also ▁have ▁languages ▁desc ended ▁from ▁a ▁common ▁ancest or , ▁as ▁defined ▁by ▁a ▁set ▁of ▁exclus ively - shared ▁innov ations ), ▁but ▁whose ▁common ▁ancest or ▁may ▁not ▁have ▁been ▁dis cret ely ▁separated ▁from ▁its ▁neighb ours . ▁For ▁example , ▁a ▁chain ▁of ▁dialect s ▁{ A ▁B ▁C ▁D ▁E ▁F } ▁may ▁under go ▁a ▁number ▁of ▁lingu istic ▁innov ations , ▁some ▁affect ing ▁{ BC D }, ▁others ▁{ C DE }, ▁still ▁others ▁{ DEF }. ▁In so far ▁as ▁each ▁set ▁of ▁dialect s ▁was ▁mut ually ▁intellig ible ▁at ▁the ▁time ▁of ▁the ▁innov ations , ▁all