Chap. IV.l SUMMARY. 103 



servation, 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 he regarded as an advance in organisation. 

 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. 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 

 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 same area, — of which we see proof by 

 looking to the inhabitants of any small spot, and to the productions 

 naturalised 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 de- 

 scendants 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 transmit to their modified 

 offspring that superiority which now makes them dominant in 

 their own countries. Natural selection, as has just been remarked,, 

 leads to divergence of character and to much extinction of the less 

 improved and intermediate forms of life. On these principles, the 

 nature of the affinities, and the generally well-defined distinctions 

 between the innumerable organic beings in each class throughout 



