100 RESULT OF THE ACTION 



THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELEC- 

 TION THROUGH DIVERGENCE OF CHARACTER AND EXTINC- 

 TION, ON THE DESCENDANTS OF A COMMON ANCESTOR. 



After the foregoing discussion, which has been much 

 compressed, we may assume that the modified descendants 

 of any one species will succeed 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 

 natural selection and of extinction, tends to act. 



The accompanying diagram will aid us in understanding 

 this rather perplexing subject. Let A to L represent the 

 species 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 

 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 

 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 proceeding from (A) may represent its 

 varying offspring. The variations are supposed to be ex- 

 tremely 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 variations (repre- 

 sented by the outer dotted lines) being preserved and ac- 

 cumulated 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 



