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witen on board h.m.s. beagle, as naturalist, lwas much struck with certainfacts in the distribution of the organic beings inhabiting south america. and in thegeological relations of the present to the past inhabitants of that continent. thesefacts, as will be seen in the latter chapters of this volume, seemed to throw somelight on the origin of species--that mystery of mysteries, as it h~as been called byone of our greatest philosophers. on my return home. it occurred to me. in i837,that something might perhaps be made out on this question by'pxatientlyaccumulating and reflecting on all sorts of facts which could possibly have anybearing on it. after five years' work 1 allowed myself to speculate on the subject.and drew up some short notes; these i enlarged in i844 into a sketch of theconclusions, which then seemed to me probable: from that period to the presentday i have steadily pursued the same object. i hope that i may be excused forentering on these personal details, as i give them to show that i have not beenhasty in coming to a decision.