CHAP. IV. DIVERGENCE OF CHAEACTEE. 117 



that on an average more of the species of large genera 

 vary than of 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 rare species with restricted ranges. Let (A) be a 

 common, widely-diffused, and varying species, belong- 

 ing to a genus large in its own country. The little fan 

 of 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 being- 

 derived from divergence of character comes in ; for this 

 will generally lead to the most different or divergent 

 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 accu- 

 mulated to have formed a fairly well-marked variety, 

 such as would be thought worthy of record in a sys- 

 tematic work. 



The intervals between the horizontal lines in the 

 diagram, may represent each a thousand generations ; 

 but it would have been better if each had represented 

 ten thousand generations. After a thousand genera- 

 tions, species (A) is supposed to have produced two 

 fairly well-marked varieties, namely a} and m\ These 

 two varieties will generally continue to be exposed to 

 the same conditions which made their parents variable, 



