NATURAL SELECTION 165 



principles of natural selection and of extinction, tends 

 to act. 



The accompanying diagram will aid us in understand- 

 ing 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 the most widely diffused, 

 1 ivary more than do the rare and restricted species. Let 

 J |(A) be a common, widely-diffused, and varying species, 

 • Ibelonging to a genus large in its own country. The 

 i Ibranching and diverging dotted lines of unequal lengths 

 IS Iproceeding from (A) may represent its varying offspring. 

 )t It he variations are supposed to be extremely slight, but 

 3f the most diversified nature; they are not supposed all 

 io appear simultaneously, but often after long intervals 

 3f time; nor are they all supposed to endure for equal 

 3eriods. Only those variations which are in some way 

 Drofitable will be preserved or naturally selected. And 

 ijl, lere the importance of the principle of benefit derived 

 rom divergence of character comes in; for this will 

 j5 generally lead to the most different or divergent varia- 

 [ions (represented by the outer dotted lines) being pre- 

 123, f^rved and accumulated by natural selection. When a 

 ,:^ Rotted line reaches one of the horizontal lines, and is 

 jIjj Ihere marked by a small numbered letter, a sufficient 



