Chap. IV.] NATURAL SELECTION. 141 



represented in the diagram by the letters standing at 

 unequal distances. I have said a large genus, because as 

 we saw in the second chapter, on an average more species 

 vary in large genera than in 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, be- 

 longing to a genus large in its own country. The 

 branching and diverging dotted lines of unequal lengths 

 proceeding 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 principle, of benefit derived 

 from divergence of character comes in; for this will 

 generally lead to the most different or divergent varia- 

 tions (represented by the outer dotted lines) being pre- 

 served 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 accumu- 

 lated 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 dia- 

 gram, may represent each a thousand or more genera- 

 tions. After a thousand generations, species (A) is sup- 

 posed to have produced two fairly well-marked varieties, 

 namely a^ and m^- These two varieties will generally 



