388 RECAPITULATION. [CHAP. XV. 



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 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 differ- 

 ences 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 inter- 

 mediate 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 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, succes- 

 sive, favourable variations, it can produce no great or sudden 

 modifications; it can act only by short and slow steps. Hence, 

 the canon of " Natura non facit saltum," which eveiy fresh addi- 

 tion to our knowledge tends toconfirm, is on this theory intelligible. 

 We can see why throughout nature the same general end is gained 

 by an almost infinite diversity of means, for every peculiarity 

 when once acquired is long inherited, and structures already modi- 

 fied in many different ways have to be adapted for the same 

 general purpose. We can, in short, see why nature is prodigal in 

 variety, though niggard in innovation. But why this should be a 

 law of nature if each species has been independently created no 

 man can explain. 



