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LAWS OF VARIATION
DIFFICULTIES ON THEORY
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accumulated action adapted advantage allied America amount ancient animals appear become believe birds breeds cause chapter characters climate closely common considered continuous crossed descended developed difficulty distinct domestic doubt effects existing explained extinct extremely facts families favourable fertility flowers follow formation forms genera genus geological give given greater groups habits hand Hence hybrids important increase individuals inhabitants inherited insects instance instincts intermediate islands kind known land laws less living look male manner means migration modification namely natural selection naturalists nearly never occasionally occur offspring organic origin parent perfect period plants points present principle probably produced range ranked reason remarked seeds seems seen separated single slight sometimes species sterility structure struggle successive supposed tend theory tion variability variation varieties vary whole widely