Chap. IV. 
SUMMAEY. 
127 
vary at all in the several parts of their organisation, and 
I think this cannot be disputed; if there be, owing to 
the high geometrical ratio of increase of each species, a 
severe struggle for life at some age, season, or year, and 
this certainly cannot be disputed; then, considering the 
infinite complexity of the relations of all organic beings 
to each other and to their conditions of existence, 
causing an infinite diversity in structure, constitution, 
and habits, to be advantageous to them, I think it would 
be a most extraordinary fact if no variation ever had 
occurred useful to each being’s own welfare, in the same 
manner as so many variations have occurred useful to 
man. But if variations useful to any organic being do 
occur, assuredly individuals thus characterised will have 
tlie best chance of being preserved in the struggle for 
life; and from the strong principle of inheritance they 
will tend to produce offspring similarly characterised. 
This principle of preservation, I have called, for the 
sake of brevity. Natural Selection; and it leads to the 
improvement of each creature in relation to its organic 
and inorganic conditions of life. 
Natural selection, on the principle of qualities being 
inherited at corresponding ages, can modify the egg, 
seed, or young, as easily as the adult. Amongst many 
animals, sexual selection will give its aid to ordinary 
selection, by assuring to the most vigorous and best 
adapted males the greatest number of offspring. Sexual 
selection will also give characters useful to the males 
alone, in their struggles with other males. 
Whether natural selection has really thus acted in 
nature, in modifying and adapting the various forms 
of life to their several conditions and stations, must be 
judged of by the general tenour and balance of evidence 
given in the following chapters. But we already see 
how it entails extinction; and how largely extinction 
