, ▁great - grand par ents ▁and ▁so ▁on . ▁The ▁same ▁approach ▁is ▁often ▁taken ▁when ▁trac ing ▁descend ants ▁via ▁children ▁and ▁grand children . ▁Daw kins ▁shows ▁that ▁this ▁approach ▁is ▁mis gu ided , ▁as ▁the ▁numbers ▁of ▁ancest ors ▁and ▁descend ants ▁seem ▁to ▁grow ▁expon entially ▁as ▁gener ations ▁are ▁added ▁to ▁the ▁line age ▁tree . ▁In ▁just ▁ 8 0 ▁gener ations , ▁the ▁number ▁of ▁ancest ors ▁can ▁exceed ▁a ▁tr ill ion ▁tr ill ion . ▁ ▁This ▁simple ▁calculation ▁does ▁not ▁take ▁into ▁account ▁the ▁fact ▁that ▁every ▁marriage ▁is ▁really ▁a ▁marriage ▁between ▁distant ▁c ous ins ▁which ▁include ▁second ▁c ous ins , ▁fourth ▁c ous ins , ▁six teenth ▁c ous ins ▁and ▁so ▁on . ▁The ▁ancest ry ▁tree ▁is ▁not ▁really ▁a ▁tree , ▁but ▁a ▁graph . ▁ ▁Daw kins ▁pre fers ▁to ▁model ▁ancest ry ▁in ▁terms ▁of ▁gen es ▁flow ing ▁through ▁a ▁river ▁of ▁time . ▁An ▁ancest or ▁gene ▁flows ▁down ▁the ▁river ▁either ▁as ▁perfect ▁rep lic as ▁of ▁itself ▁or ▁as ▁slightly ▁mut ated ▁desc endant ▁gen es . ▁Daw kins ▁fails ▁to ▁explicitly ▁contrast ▁ancest or ▁organ ism ▁and ▁desc endant ▁organ isms ▁against ▁ancest or ▁gen es ▁and ▁desc endant ▁gen es ▁in ▁this ▁chapter . ▁But ▁the ▁first ▁half ▁of ▁the ▁chapter ▁is ▁really ▁about ▁differences ▁between ▁these ▁two ▁models ▁of ▁line age . ▁While ▁organ isms ▁have ▁ancest ry ▁graphs ▁and ▁pro gen y ▁graphs ▁via