NATURAL SELECTION 105 



structure, constitution, and habits, to be advantageous to them, it 

 would be a most extraordinary fact if no variations had ever oc- 

 curred 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 characterized 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 characterized. !This 

 principle of preservation, or the survival of the fittest, I have 

 called natural selection. It leads to the improvement of each crea- 

 ture in relation to its organic and inorganic conditions of life; and 

 consequently, in most cases, to what must be regarded as an ad- 

 vance in organization. Nevertheless, low and simple forms will long 

 endure if well fitted for their simple 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. Among 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 

 inheritance 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 following 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 character; for the more organic beings diverge in 

 structure, habits, and constitution, by so much the more can a 

 large number be supported on the area, of which we see proof by 

 looking to the inhabitants of any small spot, and to the produc- 

 tions naturalized in foreign lands. Therefore, during the modifica- 

 tion of the descendants of any one species, and during the inces- 

 sant struggle of all species to increase in numbers, the more diver- 

 sified the descendants become, the better will be their chance of 

 success in the battle for life. Thus the small differences distin- 

 guishing varieties of the same species, steadily tend to increase, 

 till they equal the greater differences between species of the same 

 genus, or even of distinct genera. 



We have seen that it is the common, the widely diffused, and 

 widely ranging species, belonging to the larger genera within each 



