134 SUMMARY. Chap. IV. 



Sum)) tart/ of Chcfpter. 



If iiiulcr changing conditions of life organic beings present 

 individual difiercnces in all parts of their structure, and this 

 cannot be disputed ; if there bo, owing to the high geometrical 

 ratio of increase of each species, a severe struggle for life at 

 some age, season, or year, and tliis certainly cannot 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 extraor- 

 dinary fact if no variations ever occurred useful to each be- 

 ing's OAvn welfare, in the same inanner as so many variations 

 have occurred useful to man. /But if variations useful to any 

 organic being do ever occur, assuredly individuals thus charac- 

 terized will have the best chance of being preserved in the 

 struggle for life ; and from the strong principle of inheritance, 

 they will tend to produce ofi'spring similarly characterized. 

 This principle of 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 in organic condi- 

 tions of life ; and consequent]}', 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 Avill give its aid to ordinary selection, by assuring to 

 the most vigorous and best adapted males the greatest num- 

 ber of offspring. Sexual selection will also give characters useful 

 only to the males, in their struggles with other males ; and 

 these characters Avill 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 of by the general tenor and balance of the evi- 

 dence given in the following chapters. But we already see 

 how it entails extinction ; and how largely extinction has acted 

 in the world's history, geology plainly tleclarcs. Natural selec- 

 tion, also, leads to divergence of character ; for the more organic 

 beings diverge in structure, habits, and constitution, by so much 

 can a greater number be supported on the same area — of which 



