The sections into which the Essay of 1842 is divided are in the original merely indicated by a gap in the MS. or by a line drawn across the page. No titles are given except in the case of Sec. VIII.; and Sec. II. is the only section which has a number in the original. I might equally well have made sections of what are now subsections, _e.g. Natural Selection_ p. 7, or _Extermination_ p. 28. But since the present sketch is the germ of the Essay of 1844, it seemed best to preserve the identity between the two works, by using such of the author's divisions as correspond to the chapters of the enlarged version of 1844. The geological discussion with which Part II begins corresponds to two chapters (IV and V) of the 1844 Essay. I have therefore described it as Sec.Sec. IV. and V., although I cannot make sure of its having originally consisted of two sections. With this exception the ten sections of the Essay of 1842 correspond to the ten chapters of that of 1844. The _Origin of Species_ differs from the sketch of 1842 in not being divided into two parts. But the two volumes resemble each other in general structure. Both begin with a statement of what may be called the mechanism of evolution,--variation and selection: in both the argument proceeds from the study of domestic organisms to that of animals and plants in a state of nature. This is followed in both by a discussion of the _Difficulties on Theory_ and this by a section _Instinct_ which in both cases is treated as a special case of difficulty. If I had to divide the _Origin_ (first edition) into two parts without any knowledge of earlier MS., I should, I think, make Part II begin with Ch. VI, _Difficulties on Theory_. A possible reason why this part of the argument is given in Part I of the Essay of 1842 may be found in the Essay of 1844, where it is clear that the chapter on instinct is placed in Part I because the author thought it of importance to show that heredity and variation occur in mental attributes. The whole question is perhaps an instance of the sort of difficulty which made the author give up the division of his argument into two Parts when he wrote the _Origin_. As matters stand Sec.Sec. IV. and V. of the 1842 Essay correspond to the geological chapters, IX and X, in the _Origin_. From this point onwards the material is grouped in the same order in both works: geographical distribution; affinities and classification; unity of type and morphology; abortive or rudimentary organs; recapitulation and conclusion.