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 leads to 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 Record. 
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 
