Chap. IV. SUMMARY. 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 powers of increase of each species, at 

 some age, season, or year, a severe struggle for life, 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, caus- 

 ing 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 

 way 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 the 

 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. Natural selection, on the 

 principle of qualities being inherited at corresponding 

 ages, can modify the egg y 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 

 has acted in the world's history, geology plainly de- 

 clares. Natural selection, also, leads to divergence of 



