CHAP. VII.] THEORY OF NATURAL SELECTION. 189 



analogy, that several wonderfully changed individuals appeared 

 simultaneously within the same district. This difficulty, as in 

 the case of unconscious selection by man, is avoided on the theory 

 of gradual evolution, through the preservation of 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 continents, 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 formation. 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 wonderfully 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 steps. For instance 

 the fact that the species included in the larger genera are more 

 closely related to each other, and present a greater number of 

 varieties than do the species in the smaller genera. The former 

 are also grouped in little clusters, like varieties round species; 

 and they present other analogies with varieties, as was shown in 

 our second chapter. On this same principle we can understand 

 how it is that specific characters are more variable than generic 

 characters ; and how the parts which are developed in an extra- 

 ordinary degree or manner are more variable than other parts of 

 the same species. Many analogous facts, all pointing in the same 

 direction, could be added. 



Although very many species have almost certainly been 

 produced by steps not greater than those separating fine varieties ; 

 yet it may be maintained that some have been developed in a 



