Chap. VII.] THEORY OF NATURAL SELECTION. 315 



gf a large number of individuals, which varied more or 

 less in any favourable direction, and of the destruction 

 of a large number which varied in an opposite manner. 



That many species have been evolved in an extremely 

 gradual manner, there can hardly be a doubt. The species 

 and even the genera of many large natural families are 

 so closely allied together, that it is difficult to distinguish 

 not a few of them. On every continent in proceeding 

 from north to south, from lowland to upland, &c, we 

 meet with a host of closely related or representative 

 species ; as we likewise do on certain distinct conti- 

 nents, which we have reason to believe were formerly 

 connected. But in making these and the following 

 remarks, I am compelled to allude to subjects hereafter 

 to be discussed. Look at the many outlying islands 

 round a continent, and see how many of their inhabitants 

 can be raised only to the rank of doubtful species. So 

 it is if we look to past times, and compare the species 

 which have just passed away with those still living 

 within the same areas; or if we compare the fossil species 

 embedded in the sub- stages of the same geological forma- 

 tion. It is indeed manifest that multitudes of species 

 are related in the closest manner to other species that 

 still exist, or have lately existed ; and it will hardly be 

 maintained that such species have been developed in an 

 abrupt or sudden manner. Nor should it be forgotten, 

 when we look to the special parts of allied species, 

 instead of to distinct species, that numerous and wonder- 

 fully fine gradations can be traced, connecting together 

 widely different structures. 



Many large groups of facts are intelligible only on the 

 principle that species have been evolved by very small 

 stepfl. For instance, the fact that the species included 



