Chap. XV.] RECAPITULATION. 



281 



allied species also of the larger genera apparently have 

 restricted ranges, and in their affinities they are clustered 

 m little groups round other species — in both respects 

 resembling varieties. These are strange relations on 

 the view that each species was independently created, 

 but are intelligible if each existed first as a variety. 



As each species tends by its geometrical rate of repro- 

 duction to increase inordinately in number; and as the 

 modified descendants of each species will be enabled to 

 increase by as much as they become more diversified in 

 habits and structure, so as to be able to seize on many 

 and widely different places in the economy of nature, 

 there will be a constant tendency in natural selection 

 to preserve the most divergent offspring of any one spe- 

 cies. Hence, during a long-continued course of modi- 

 fication, the slight differences characteristic of varie- 

 ties of the same species, tend to be augmented into the 

 greater differences characteristic of the species of the 

 same genus. ISTew and improved varieties will inevitably 

 supplant and exterminate the older, less improved, and 

 intermediate varieties; and thus species are rendered to 

 a large extent defined and distinct objects. Dominant 

 species belonging to the larger groups within each class 

 tend to give birth to new and dominant forms; so that 

 each large group tends to become still larger, and at the 

 same time more divergent in character. But as all 

 groups cannot thus go on increasing in size, for the 

 world would not hold them, the more dominant groups 

 beat the less dominant. This tendency in the large 

 groups to go on increasing in size and diverging in char- 

 acter, together with the inevitable contingency of much 

 extinction, explains the arrangement of all the forms of 

 life in groups subordinate to groups, all within a few 



