CLASSIFICATION, groups subordinate to groups--Natural system--Rules and difficulties in classification, explained on the theory of descent with modification--Classification of varieties--Descent always used in classification--Analogical or adaptive characters--Affinities, general, complex and radiating--Extinction separates and defines groups--MORPHOLOGY, between members of the same class, between parts of the same individual--EMBRYOLOGY, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age--RUDIMENTARY ORGANS; their origin explained--Summary 411-458 CHAPTER XIV. RECAPITULATION AND CONCLUSION. Recapitulation of the difficulties on the theory of Natural Selection--Recapitulation of the general and special circumstances in its favour--Causes of the general belief in the immutability of species--How far the theory of natural selection may be extended--Effects of its adoption on the study of Natural history--Concluding remarks 459-490 * * * * * {1} ON THE ORIGIN OF SPECIES. * * * * * INTRODUCTION. When on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species--that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years' work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. I hope that I may be excused for entering on these personal details, as I give them to show that I have not been hasty in coming to a decision.