RECAPITULATION AND CONCLUSION 423 



rule, to find it still in action ; and this is the case if 

 varieties be incipient species. Moreover, the species of 

 the larger genera, which afford the greater number of 

 varieties or incipient species, retain to a certain degree 

 the character of varieties ; for they differ from each 

 other by a less amount of difference than do the species 

 of smaller genera. The closely allied species also of 

 the larger genera apparently have restricted ranges, 

 and in their affinities they are clustered in little groups 

 round other species — in which respects they resemble 

 varieties. These are strange relations on the view of 

 each species having been independently created, but 

 are intelligible if all species first existed as varieties. 



As each species tends by its geometrical ratio of 

 reproduction to increase inordinately in number ; and 

 as the modified descendants of each species will be 

 enabled to increase by so much the more as they 

 become diversified in habits and structure, so as to be 

 enabled 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 species. Hence during 

 a long-continued course of modification, the slight 

 differences, characteristic of varieties of the same 

 species, tend to be augmented into the greater differ- 

 ences characteristic of species of the same genus. New 

 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 belong- 

 ing to the larger groups tend to give birth to new and 

 dominant forms ; so that each large group tends to be- 

 come still larger, and at the same time more divergent 

 in character. But as all groups cannot thus succeed 

 in 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 character, together with the 

 almost inevitable contingency of much extinction, ex- 

 plains the arrangement of all the forms of life, in 



