THEOR Y OF NA TURA L SF.L EOT ION, 239 



believe, in opposition to all analogy, that several wonder- 

 fully changed individuals appeared simultaneously within 

 the same district. This difficulty, as in the case of uncon- 

 scious selection by man, is avoided on the theory of gradual 

 evolution, through the preservation of a large number of 

 individuals, which varied more of less in any favorable 

 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, etc., 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 out- 

 lying 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 

 imbedded 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 main- 

 tained 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 dis- 

 tinct species, that numerous and wonderfully fine grada- 

 tions 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 pi-esent 

 other analogies with varieties, as was shown in our second 



