480 THE WRITING OF THE ORIGIN OF SPECIES.' [185S. 



same species of shells at least have existed ; there must 

 have been during this period, millions on millions of gene- 

 rations. 



III. I think it can be shown that there is such an unerring 

 power at work, or Natural Selection (the title of my book), 

 which selects exclusively for the good of each organic being. 

 The elder De Candolle, W. Herbert, and Lyell, have written 

 strongly on the struggle for life ; but even they have not 

 written strongly enough. Reflect that every being (even the 

 elephant) breeds at such a rate that, in a few years, or at most 

 a few centuries or thousands of years, the surface of the earth 

 would not hold the progeny of any one species. I have found 

 it hard constantly to bear in mind that the increase of every 

 single species is checked during some part of its life, or dur- 

 ing some shortly recurrent generation. Only a few of those 

 annually born can live to propagate their kind. What a 

 trifling difference must often determine which shall survive 

 and which perish ! 



IV. Now take the case of a country undergoing some 

 change ; this will tend to cause some of its inhabitants to vary 

 slightly ; not but what I believe most beings vary at all times 

 enough for selection to act on. Some of its inhabitants will 

 be exterminated, and the remainder will be exposed to the 

 mutual action of a different set of inhabitants, which I believe 

 to be more important to the life of each being than mere 

 climate. Considering the infinitely various ways beings have 

 to obtain food by struggling with other beings, to escape 

 danger at various times of life, to have their eggs or seeds 

 disseminated, &c, &c, I cannot doubt that during millions 

 of generations individuals of a species will be born with some 

 slight variation profitable to some part of its economy ; such 

 will have a better chance of surviving, propagating this varia- 

 tion, which again will be slowly increased by the accumulative 

 action of natural selection ; and the variety thus formed will 

 either coexist with, or more commonly will exterminate its 

 parent form. An organic being like the woodpecker, or the 

 mistletoe, may thus come to be adapted to a score of contin- 



