234 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



extraordinary fact if no variations had ever occurred useful to eacl 

 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 im- 

 provement 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 organization. Nevertheless, low and simple 

 forms will long endure if well fitted for their simple conditions of life. 



Natural selection, on the principle of quahties 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 selection 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 follow- 

 ing 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 constitu- 

 tion, 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 productions naturalized in foreign lands. There- 

 fore, during the modification of the descendants of any one species, 

 and during the incessant struggle of all species to increase in number, 

 the more diversified the descendants become, the better will be their 

 chance of success in the battle for life. Thus the small differences dis- 

 tinguishing 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 



