, ▁species ▁B , C , D , E , G , H , K ▁and ▁L ▁have ▁gone ▁ext inct . ▁In ▁Dar win ' s ▁own ▁words : ▁" Th us ▁the ▁small ▁differences ▁distingu ishing ▁vari eties ▁of ▁the ▁same ▁species , ▁will ▁stead ily ▁tend ▁to ▁increase ▁till ▁they ▁come ▁to ▁equal ▁the ▁greater ▁differences ▁between ▁species ▁of ▁the ▁same ▁genus , ▁or ▁even ▁of ▁distinct ▁gener a ." . ▁This ▁is ▁a ▁branch ing ▁pattern ▁with ▁no ▁names ▁given ▁to ▁species , ▁unlike ▁the ▁more ▁linear ▁tree ▁Ernst ▁H ae ck el ▁made ▁years ▁later ▁( figure ▁below ) ▁which ▁includes ▁the ▁names ▁of ▁species ▁and ▁shows ▁a ▁more ▁linear ▁development ▁from ▁" lower " ▁to ▁" high er " ▁species . ▁In ▁his ▁summary ▁to ▁the ▁section , ▁Dar win ▁put ▁his ▁concept ▁in ▁terms ▁of ▁the ▁met ap hor ▁of ▁the ▁tree ▁of ▁life : ▁ ▁The ▁meaning ▁and ▁importance ▁of ▁Dar win ' s ▁use ▁of ▁the ▁tree ▁of ▁life ▁met ap hor ▁have ▁been ▁extens ively ▁discussed . ▁Stephen ▁Jay ▁G ould , ▁for ▁example , ▁says ▁that ▁Dar win ▁placed ▁the ▁famous ▁passage ▁quoted ▁above ▁" at ▁a ▁cru cial ▁spot ▁in ▁his ▁text ", ▁where ▁it ▁marked ▁the ▁conclusion ▁of ▁his ▁argument ▁for ▁natural ▁selection , ▁illustr ating ▁both ▁the ▁inter connected ness ▁by ▁descent ▁of ▁organ isms ▁as ▁well ▁as ▁their ▁success ▁and ▁failure ▁in ▁the ▁history ▁of ▁life . ▁David ▁Pen ny ▁has ▁written ▁that ▁Dar win ▁did ▁not ▁use