36 Darwin-Wallace Celebration. 



2. Now suppose theio were a being who did not judge by 

 me're external appearances, but who could study the whole 

 internal organization, who was never capricious, and should 

 go on selecting for one object during millions of generations; 

 who will say what he might not effect ? In nature we have 

 some slight variation occasionally in all parts; and I think 

 it can be shown that changed conditions of existence is the 

 main cause of the child not exactly resembling its parents ; 

 and in nature geology shows us what changes have taken 

 place, and are taking place. We have almost unlimited 

 time; no one but % a practical geologist can fully appreciate 

 ihis. Think of the Glacial period, during the whole of 

 which the same species at least of shells have existed ; there 

 must have been during this period millions on millions of 

 generations. 



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

 power at work in 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 

 excellently 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, the surface of the earth would not 

 hold the progeny of one pair. I have found it hard con- 

 stantly to bear in mind that the increase of every single 

 species is checked during some part of its life, or during 

 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 ! 



4. Now take the case o a country undergoing some 

 change. This will tend to cause some of its inhabitants to 

 vary slightly not but that I believe most beings vary at all 

 times enough for selection to act on them. 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 far more important to the life of each 

 being than mere climate. Considering the infinitely various 

 methods which living beings follow to obtain food by 

 .struggling with other organisms, 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 occasionally born with some 

 slight variation, profitable to some part of their economy. 

 iSuch individuals will have a better chance of surviving, and 



