12 POPULAR SCIENCE MONTHLY. 



them. Some of its inhabitants will be exterminated; and the re- 

 mainder will be exposed to the mutual action of a different set of in- 

 habitants, 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 or- 

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

 seeds disseminated, etc., etc., 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 or- 

 ganic 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 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 few individuals undergoing change at one time, 

 answers others. The extreme imperfection of our geological records 

 answers others. 



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

 gence, 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 invari- 

 ably 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 (only 

 few will succeed) to seize on as many and as diverse places in the 

 economy of nature as possible. Each new variety or species, when 

 formed, will generally take the place of, and thus exterminate its less 

 well-fitted parent. This I believe to be the origin of the classification 

 and affinities of organic beings at all times; for organic beings always 



