RECAPITULATION. 457 



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 

 ha,bits 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 character- 

 istic 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 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, 

 successive, favorable 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 every fresh addition to our knowledge tends to con- 

 firm, is on this theory intelligible. We can see why through- 

 out 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 modified 

 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. 



Many other facts are, as it seems to me, explicable on thia 



