116 SUMMARY. 



preservation, or the survival 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 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 in- 

 herited 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 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 adapt- 

 ing 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 s?en how it entails extinction ; and how largely ex- 

 tinction 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 pro- 

 ductions naturalized in foreign lands. Therefore, during the 

 modification of the descendants of any one species, and dur- 

 ing the incessant struggle of all species to increase in num- 

 bers, 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 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 class, which vary most ; and these tend to trans- 

 mit to their modified offspring that superiority which now 

 makes them dominant in their own countries. Natural se- 

 lection, as has just been remarked, leads to divergence of 

 fcharacter and to much extinction of the less improved and 



