Chap. XV. Recapitulation. 413 



produced by secondary laws. On this same view we can understand 

 how it is that in a. region where many species of a genus have been 

 produced, and where they now nourish, these same species should 

 present many varieties ; for where the manufactory of species has 

 been active, we might expect, as a general rule, to find it still in 

 action ; and this is the case if varieties be incipient species. More- 

 over, 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 

 clrsely allied species also of the larger genera apparently have re- 

 stricted 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 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 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 differences characteristic of the 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 belonging to the larger 

 groups within each class tend to give birth to new and domi- 

 nant 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 character, 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 great classes, 

 which has prevailed throughout all time. This grand fact of the 

 grouping of all organic beings under what is called the Natural 

 System, is utterly inexplicable on the theory of creation. 



As natural selection acts solely by accumulating slight, successive, 

 favourable variations, it can produce no great or sudden modifica- 



