242 CLASSICS OF MODERN SCIENCE 



SUMMARY OF CHAPTER 



If under changing conditions of life organic beings present in- 

 dividual differences in almost every part of their structure, and this 

 cannot be disputed; if there be, owing to their geometrical rate of 

 increase, a severe struggle for life at some age, season, or year, and 

 this certainly cannot be disputed ; then, considering the infinite com- 

 plexity of the relations of all organic beings to each other and to their 

 conditions of life, causing an infinite diversity in structure, constitu- 

 tion, and habits, to be advantageous to them, it would be a most extra- 

 ordinary fact if no variations had ever 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 ever 

 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, these will tend to produce offspring 

 similarly characterised. This principle of preservation, or the sur- 

 vival of the fittest, I have called Natural Selection. It leads to the 

 improvement of each creature in relation to its organic and inorganic 

 conditions of life; and consequently, in most cases, to what must be 

 regarded as an advance in organisation. Nevertheless, low and 

 simple forms will long endure if well fitted for their simple con- 

 ditions 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 have given 

 its aid to ordinary selection, by assuring to the most vigorous and 

 best adapted males the greatest number of offspring. Sexual selec- 

 tion will also give characters useful to the males alone, in their 

 struggles or rivalry with other males ; and these characters will be 

 transmitted to one sex or to both sexes, according to the form of in- 

 heritance which prevails. 



Whether natural selection has really thus acted in adapting the 

 various forms of life to their several conditions and stations, must be 

 judged by the general tenor and balance of evidence given in the fol- 

 lowing chapters. But we have already seen how it entails extinction ; 

 and how largely extinction has acted in the world's history, geology 

 plainly declares. Natural selection, also, leads to divergence of char- 



