SUMMARY. 123 



be disputed; then, considering the infinite complexity of 

 the relations of all organic beings to each other and to their 

 conditions of life, causing an infinite diversity in structure, 

 constitution and habits, to be advantageous to them, it 

 would be a most extraordinary fact if no variations had 

 ever occurred 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 fit- 

 test, I have called natural selection. "It leads to the im- 

 provement of each creature in relation to its organic and in- 

 organic 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 

 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. 

 Bat 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 descend- 



