6298 



Tendency of Species 



sively for the good of each organic being. The elder DeCandolle, 

 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. 1 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 during some shortly recurrent genera- 

 tion. 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 of 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. Such individuals will have a better chance of surviving, 

 and of propagating their new and slightly different structure ; and the 

 modification may be slowly increased by the accumulative action of 

 natural selection to any profitable extent. The variety thus formed 

 will either coexist with, or, more commonly, will exterminate its parent 

 form. An organic being, like the woodpecker or misseltoe, may thus 

 come to be adapted to a score of contingencies — natural selection 

 accumulating those slight variations in all parts of its structure, which 

 are in any way useful to it during any part of its life. 



5. Multiform diflaculties will occur to every one, with respect to this 

 theory. Many can, I think, be satisfactorily answered. Naiura non 

 facit saltum answers some of the most obvious. The slowness of the 

 change, and only a very few individuals undergoing change at any 

 one time, answers others. The extreme imperfection of our geological 

 records answers others. 



6. Another principle, which may be called the principle of di- 

 vergence, plays, I believe, an important part in the origin of species. 

 The same spot will support more life if occupied by very diverse forms. 



