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DIFFICULTIES OF THE THEORY
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according action adapted admit advantage allied America amount animals appear become believe belonging birds breeds called cause certain characters climate closely colour common complex considered continuous crossed descendants developed difficulty distinct domestic doubt effects existing extinct extremely fact families favourable fertility flowers follow formation forms genera genus give given groups habits hand Hence hybrids important increase individuals inhabitants inherited insects instance instincts intermediate islands kind known laws less living look male manner means modified namely natural selection naturalists nearly observed occasionally occur offspring organs origin parent perfect period plants points present preserved principle probably produced ranked reason remarked resemblance respect seeds seems seen separated shown side similar single slight sometimes species stage sterility structure successive supposed tend theory variability variations varieties various vary whole widely young