I ] t RESULTS OF THE ACTION OF Chap. IV, 



Tlie Pi'ohdhle Results of the Action of Katural Selection 

 through Diverrjence of Character and JSxtinctiony vi the 

 Descendants of a Common Ancestor. 



After tlie forcg'oing discussion, which has been much com- 

 pressed, we may assume that the modified descendants of any 

 one species Avill succeed by so much the better as they become 

 more diversified in structure, and are thus enabled to encroach 

 on places occupied by other beings. Now let us see how this 

 principle of benefit being derived from divergence of character, 

 combined with the principles of i>atural selection and of ex- 

 tinction, tends to act. 



The accompanying diagram will aid us in understanding 

 this rather perplexing subject. Let A to L represent the spe- 

 cies of a genus large in its own country ; these species are 

 supposed to resemble each other in unequal degrees, as is so 

 generally the case in Nature, and as is represented in the 

 cliagram by the letters standing at unequal distances. I have 

 said a large genus, because Ave have seen in the second chap- 

 ter 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-diilused, 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 simul- 

 taneously, but often after long intervals of time ; nor are they 

 all supposed to endure for equal periods. Only those varia- 

 tions which are in some Avay 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 diver- 

 gent variations (represented by the outer dotted lines) being 

 preserved and accumulated by natural selection. When a dot- 

 ted line reaches one of the horizontal lines, and is there marked 

 by a small numbered letter, a sufficient ainount of variation is 

 supjiosed to have been accunuilated to have formed a fairly 

 well-marked variety, such as would l)e thought worthy of rec- 

 ord in a systematic work. 



