THE ORIGIN OF SPECIES. 101 



Existence ; in Chapter IV., which Darwin, in writing 

 to his publisher, called " the Keystone of my Arch," 

 the three preceding chapters are carried to their con- 

 clusion, and the operation of natural selection is 

 explained and discussecl. Hence, these four chapters 

 deal almost exclusively with this process. 



Chapter V. has for its subject the Laws of Varia- 

 tion, and explains causes of modification (external 

 conditions, use and disuse, correlation, reversion, etc.) 

 other than natural selection and the relation of the 

 latter to the former. 



In Chapter VI. difficulties are considered partly 

 those in the way of a belief in evolution and partly 

 those which, at first sight, seem to be incapable of 

 explanation on the theory of natural selection. 

 Chapter VII. deals with a special difficulty of the 

 latter kind, viz. Instinct, and shows how it can be 

 accounted for by natural selection acting upon varia- 

 tion, although allowing some weight to the inheritance 

 of habit. Chapter VIII. deals with Hybridism, the 

 sterility of first crosses and of hybrids being con- 

 sidered as an objection to the doctrine of Descent 

 with Modification. Chapter IX. treats of the Imper- 

 fection of the Geological Kecord as the explanation of 

 the apparently insufficient evidence of evolution during 

 past ages. Chapter X., on the Geological succession 

 of Organic Beings, shows that, allowing for this 

 Imperfection of Record, the facts brought to light by 

 Geology support a belief in evolution and in some 

 cases even in natural selection. Hence these five 



