INTRODUCTION. 5 



increase, will be treated of. This is the doctrine of 

 Malthus, applied to the whole animal and vegetable 

 kingdoms. As many more individuals of each species 

 are born than can possibly survive ; and as, conse- 

 quently, there is a frequently recurring struggle for 

 existence, it follows that any being, if it vary however 

 slightly in any manner profitable to itself, under the 

 complex and sometimes varying conditions of life, will 

 have a better chance of surviving, and thus be naturally 

 selected. From the strong principle of inheritance, any 

 selected variety will tend to propagate its new and 

 modified form. 



This fundamental subject of Natural Selection will 

 be treated at some length in the fourth chapter ; and we 

 shall then see how Natural Selection almost inevitably 

 causes much Extinction of the less improved forms of 

 life, and induces what I have called Divergence of 

 Character. In the next chapter I shall discuss the 

 complex and little known laws of variation and of corre- 

 lation of growth. In the four succeeding chapters, the 

 most apparent and gravest difficulties on the theory 

 will be given : namely, first, the difficulties of transi- 

 tions, or in understanding how a simple being or a simple 

 organ can be changed and perfected into a highly 

 developed being or elaborately constructed organ ; 

 secondly, the subject of Instinct, or the mental powers of 

 animals ; thirdly, Hybridism, or the infertility of species 

 and the fertility of varieties when intercrossed ; and 

 fourthly, the imperfection of the Geological Kecord. 

 In the next chapter I shall consider the geological 

 succession of organic beings throughout time ; in the 

 eleventh and twelfth, their geographical distribution 

 throughout space ; in the thirteenth, their classification 

 or mutual affinities, both when mature and in an em- 

 bryonic condition. In the last chapter I shall give a 



