EFFECTS OF NATURAL SELECTIOx^ 127 



small genera; and the varying species of the large genera 

 present a greater number of varieties. We have, also, seen 

 that the species, which are the commonest and the most 

 widely diffused, vary more than do the rare and restricted 

 species. Let (A) be a common, widely-diffused, and varying 

 species, belonging to a genus large in its own country. The 

 branching and diverging dotted lines of unequal lengths pro- 

 ceeding from (A), may represent its varying offspring. The 

 variations are supposed to be extremely slight, but of the 

 most diversified nature; they are not supposed all to appear 

 simultaneously, but often after long intervals of time ; nor 

 are they all supposed to endure for equal periods. Only those 

 variations which are in some way profitable will be preserved 

 or naturally selected. And here the importance of the prin- 

 ciple of benefit derived from divergence of character comes 

 in ; for this will generally lead to the most different or di- 

 vergent variations (represented by the outer dotted lines) 

 being preserved and accumulated by natural selection. When 

 a dotted line reaches one of the horizontal lines, and is there 

 marked by a small numbered letter, a sufficient amount of 

 variation is supposed to have been accumulated to form it 

 into a fairly well-marked variety, such as would be thought 

 worthy of record in a systematic work. 



The intervals between the horizontal lines in the diagram, 

 may represent each a thousand or more generations. After a 

 thousand generations, species (A) is supposed to have pro- 

 duced two fairly well-marked varieties, namely a* and w\ 

 These two varieties will generally still be exposed to the 

 same conditions which made their parents variable, and the 

 tendency to variability is in itself hereditary ; consequently 

 they will likewise tend to vary, and commonly in nearly the 

 same manner as did their parents. Moreover, these two 

 varieties, being only slightly modified forms, will tend to 

 inherit those advantages which made their parent (A) more 

 numerous than most of the other inhabitants of the same 

 country; they will also partake of those more general advan- 

 tages which made the genus to which the parent-species 

 belonged, a large genus in its own country. And all 

 these circumstances are favorable to the production of new 

 varieties. 



