374 THE GROWTH OF THE 'ORIGIN OF SPECIES.' 



chapter on variation under domestication and on artificial 

 selection. This is followed, in both essays, by discussions on 

 variation under nature, on natural selection, and on the 

 struggle for life. Here, any close resemblance between the 

 two essays with regard to arrangement ceases. Chapter III. 

 of the Sketch, which concludes the first part, treats of the 

 variations which occur in the instincts and habits of animals, 

 and thus corresponds to some extent with Chapter VII. of 

 the 'Origin' (ist edit.). It thus forms a complement to the 

 chapters which deal with variation in structure. It seems to 

 have been placed thus early in the Essay to prevent the hasty 

 rejection of the whole theory by a reader to whom the idea of 

 natural selection acting on instincts might seem impossible. 

 This is the more probable, as the Chapter on Instinct in the 

 'Origin* is specially mentioned (Introduction, p. 5) as one of 

 the "most apparent and gravest difficulties on the theory." 

 Moreover the chapter in the Sketch ends with a discussion, 



" whether any particular corporeal structures are so 



wonderful as to justify the rejection prima facie of our the- 

 ory." Under this heading comes the discussion of the eye, 

 which in the 'Origin' finds its place in Chapter VI. under 

 "Difficulties on Theory," The second part seems to have 

 been planned in accordance with his favourite point of view 

 with regard to his theory. This is briefly given in a letter to 

 Dr. Asa Gray, November nth, 1859 : "I cannot possibly be- 

 lieve that a false theory would explain so many classes of 

 facts, as I think it certainly does explain. On these grounds 

 I drop my anchor, and believe that the difficulties will 

 slowly disappear." On this principle, having stated the 

 theory in the first part, he proceeds to show to what ex- 

 tent various wide series of facts can be explained by its 

 means. 



Thus the second part of the Sketch corresponds roughly 

 to the nine concluding Chapters of the First Edition of the 

 ' Origin.' But we must exclude Chapter VII. (' Origin ') 

 on Instinct, which forms a chapter in the first part of the 

 Sketch, and Chapter VIII. (' Origin ') on Hybridism, a subject 



