xx INTRODUCTION 



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 iv. and v., although I 

 cannot make sure of its having originally consisted 

 of two sections. With this exception the ten sections 

 of the Foundations correspond to the ten chapters 

 of the MS. of 1844. 



The Origin of Species differs from the Founda- 

 tions 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, varia- 

 tion 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 

 Foundations 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 iv. and v. of the Foundations corresponds 

 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 dis- 

 tribution ; affinities and classification ; unity of type 



