Masterpieces of Science 



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 a larger genera ap- 

 parently have restricted ranges, and in their 

 affinities they are clustered in 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 reproduction to increase inordinately in num- 

 ber; and as the modified descendants of each 

 species will be enabled to increase by as much as 

 they become more diversified in habits and struc- 

 ture, 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 species. Hence, during a long-continued 

 course of modification, the slight differences of 

 characteristic of varieties of the same species, 

 tend to be augmented into the greater differences 

 characteristic of the species of the same genus. 

 New and improved varieties will inevitably sup- 

 plant and exterminate the older, less improved, 

 and intermediate varieties; and thus species are 

 rendered to a large extent defined and distinct 

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