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In the "Conclusion" to the Darwin sums up his views in these words: "I have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified during a long course of descent. This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of use and disuse of parts; and in an unimportant manner that is in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously".

The of organic evolution is no longer disputed; the many different lines of evidence so ably marshalled by Darwin in his great work were sufficient in themselves to bring most biologists to share his view that "species have been modified, during a long course of descent"; and since his day further proofs have accumulated. A general agreement on this matter now prevails. But no such unanimity yet exists concerning the means by which such modifications have been effected. During the past twenty—five years a great controversy has been carried on regarding the which may have been concerned in the process of development and no certain conclusions have yet been reached.