186 THE ORIGIN OF SPECIES 



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 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 ordi- 

 nary 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 condi- 

 tions 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 productions naturalized in 

 foreign lands. Therefore, during the modification of the 

 descendants of any one species, and during the incessant 

 struggle of all species to increase in numbers, the more 

 diversified the descendants become, the better will be 

 their chance of success in the battle for life. Thus the 

 small differences distinguishing varieties of the same 

 species steadily tend to increase, till they equal the 



