52 MESSES. C. DARWIN AND A. WALLACE ON THE 



not but that I believe most beings vary at all times enough for 

 selection to act on them. Some of its inhabitants will be extermi- 

 nated ; and the remainder will be exposed to the mutual action of 

 a different set of inhabitants, which I believe to be far more im- 

 portant 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 contingences — natural selection accu- 

 mulating 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 difficulties will occur to every one, with respect 

 to this theory. Many can, I think, be satisfactorily answered. 

 Natura non facit saltum answers some of the most obvious. The 

 slowness of the change, and only a very few individuals under- 

 going 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 

 divergence, plays, I believe, an important part in the origin of 

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

 diverse forms. "We see this in the many generic forms in a square 

 yard of turf, and in the plants or insects on any little uniform 

 islet, belonging almost invariably to as many genera and families 

 as species. We can understand the meaning of this fact amongst 

 the higher animals, whose habits we understand. We know that 

 it has been experimentally shown that a plot of land will yield a 

 greater weight if sown with several species and genera of grasses, 

 than if sown with only two or three species. Now, every organic 

 being, by propagating so rapidly, may be said to be striving its 

 utmost to increase in numbers. So it will be with the offspring 

 of any species after it has become diversified into varieties, or sub- 

 species, or true species. And it follows, I think, from the fore- 

 going facts, that the varying offspring of each species will try 



